The Periodic Table
Solutions for Chemistry, Class 9, ICSE
Exercise 5A
16 questionsAnswer:
Reasons for classification of elements are:
- It helps in studying the elements in an organized manner.
- It helps in correlating the properties of elements with the fundamental properties of all states of matter.
- It helps in defining the relationship of one element with another.
Answer:
(a) When the atomic mass of A = 7, and that of C is = 39 then according to Dobereiner's triad, atomic mass of B should be mean value of atomic masses of A and C.
= = 23
Hence, atomic mass of B = 23
(b) Reasons for discarding Dobereiner's triads:
- Dobereiner failed to arrange all the then known elements in the form of triads.
- The law did not fully apply even within the same family.
Answer:
According to Newland's law of octaves, when elements are arranged by increasing atomic mass, the properties of every eighth element starting from any element are a repetition of the properties of the starting element.
Reasons for discarding the Newland's law of Octaves:
- This classification did not work with heavier elements i.e., those lying beyond Calcium. As more and more elements were discovered, they could not be fitted into Newland's Octaves.
- Newland adjusted two elements Cobalt (Co) and Nickel (Ni) in the same slot, and these were placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements.
- Iron, which resembles cobalt and nickel in properties has been placed far away from these elements.
Answer:
Yes, Dobereiner's triads also exist in the columns of Newland's octaves.
Example: Consider the elements lithium (Li), sodium (Na), and potassium (K) present in second column of Newland's classification of elements. If we start with lithium as the 1st element, then the 8th element from it is sodium, and according to Newlands' law of octaves, the properties of the 8th element, sodium should be similar to those of the 1st element, lithium.
Again, if we take sodium as the 1st element, then the 8th element from it is potassium, and according to Newland's law of octaves, the properties of the 8th element, potassium should be similar to those of the 1st element, sodium.
This means that according to Newland's law of octaves, the elements lithium, sodium, and potassium should have similar chemical properties. We also know that lithium, sodium, and potassium form a Dobereiner's triad having similar chemical properties. From this, we conclude that Dobereiner's triads also exist in the columns of Newland's Octaves.
Answer:
Similar properties of lithium, sodium and potassium were:
- They have one electron in the outermost shell, so they form unipositive ions.
- All these are soft metals.
- They are good reducing agents.
- These metals impart colour to the flame.
Answer:
Similar properties of calcium, strontium and barium were:
- All are metals.
- Their oxides are alkaline in nature.
- Each has valency 2.
Answer:
(a) Mendeleev classified elements according to Mendeleev's Periodic Law. Mendeleev's Periodic Law states that physical and chemical properties of elements are a periodic function of their atomic masses.
(b) As many features of the Mendeleev's periodic table were later used when Modern Periodic Table was made so we can say that Mendeleev's contributions to the concept of a periodic table laid the foundation for the Modern Periodic Table. For example:
- Mendeleev's periodic table contains vertical column called groups and horizontal rows called periods.
- They were all in eight groups.
- All elements in a group had similar properties and same valency.
- In a period elements gradually changed from metallic to non-metallic character.
- Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
- He predicted the properties of then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots.
- He was able to correct the atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.
Answer:
(a) Carbon is in group 4. Hydride of any element of group 4 is written as RH4. Hence, the hydride of carbon will have the formula CH4.
Silicon is also in Group 4. So, the hydride will be SiH4.
(b) K is in Group 1 and have general oxide formula R2O. So, the oxide will be K2O.
Al is in Group 3 and have general oxide formula R2O3. So, the oxide will be Al2O3.
Ba is in Group 2 and have general oxide formula RO. So, the oxide will be BaO.
Answer:
Merits of Mendeleev’s classification of elements:
- Grouping of elements: He generalized the study of the elements then known to a study of mere eight groups.
- Gaps of undiscovered elements: Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
- Prediction of properties of undiscovered elements: He predicted the properties of then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots.
- Incorrect atomic mass corrected: He was able to correct the atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.
Answer:
In order to make sure that elements having similar properties fell in the same vertical column or group, Mendeleev left some gaps in his periodic table. These gaps were left for subsequent inclusion of elements not known at that time.
For example,
(i) Eka-aluminium (means one place below aluminium in the group); its atomic mass and chemical properties are quite similar to those of the element gallium discovered later in 1876.
(ii) Properties of eka-silicon are the properties of germanium.
Answer:
Through an experiment, Henry Mosley found that number of positive charge present in the nucleus of an atom or atomic number is the most fundamental property of an atom.
According to him, if elements are arranged in order of their increasing atomic number, those with similar properties are repeated after regular intervals, i.e., periodicity in the periodic table occurs based on the atomic numbers (number of protons). Hence, the atomic number is more important to a chemist than its relative atomic mass.
Answer:
(a) Same group
- Li, Na, K belong to the same group IA.
- Be, Ca belong to the same group IIA.
(b) Same period
- Li and Be belong to Second period.
- K, Ca belong to Fourth period.
- Na belongs to Third period.
(a) Name an element whose properties were predicted on the basis of its position in Mendeleev's periodic table.
(b) Name two elements whose atomic weights were corrected on the basis of their positions in Mendeleev's periodic table.
(c) How many elements were known at the time of Mendeleev's classification of elements?
Answer:
(a) Gallium (Eka aluminium)
(b) Gold and Platinum
(c) 63
Exercise 5B
20 questionsAnswer:
(a) The Modern Periodic Law is stated as, 'the physical and chemical properties of elements are the periodic functions of their atomic number.'
(b) Groups - 18 and Periods - 07
Answer:
Main characteristic of the last element in each period of the Periodic Table are:
- They have valency equal to zero.
- They react under very special condition otherwise they don't react.
- They have stable gas electronic configuration.
General name of such elements is Inert Gases.
Answer:
(a) The groups are knows as:
- Group 1 — Alkali metals
- Group 17 — Halogens
- Group 18 — Noble gases
(b) Two elements of each group are:
- Group 1 — Lithium, Sodium
- Group 17 — Fluorine, Chlorine
- Group 18 — Neon, Argon
Answer:
(a) In the second period of the periodic table, the number of valence electrons increase from 1 to 8 as we move from left to right. The valency first increases from 1 to 4 from element Lithium (Li) to Carbon (C) and then reduces to zero from element Nitrogen (N) till Neon (Ne).
(b) In the third period of the periodic table, the number of valence electrons increases from 1 to 8 as we move from left to right. The valency first increases from 1 to 4 from element Sodium (Na) to Silicon (Si) and then reduces to zero from element Phosphorus (P) till Argon (Ar).
Answer:
As we move across a period from left to right, the number of shells remain the same and number of valence electron increases by one. The size of atom decreases because with increase in number of protons, nuclear pull increases, which decreases the size.
Answer:
The criteria used by Mendeleev in proposing his periodic table was the atomic masses of elements. He classified elements according to Mendeleev's Periodic Law. Mendeleev's Periodic Law states that physical and chemical properties of elements are a periodic function of their atomic masses.
Answer:
In Mendeleev's periodic table, Cobalt with atomic mass 58.9 preceded Nickel with atomic mass 58.6. This anomaly was resolved in Modern periodic table by arranging the elements based on their atomic number instead of atomic mass.
Cobalt with atomic number 27 is placed in group 9 and Nickel with atomic number 28 is placed in group 10 in the Modern periodic table.
Answer:
Since, all isotopes of chlorine i.e., Cl35 and Cl37 have the same number of protons so their atomic number is the same. Hence, they are placed at the same position in the periodic table in group 17, period 3.
Answer:
(a) Greatest metallic character is shown in the elements at the bottom of the group.
(b) Largest atomic size is shown in the elements at the bottom of the group.
Answer:
Number of valence electrons remain the same as we go down a group.
Example:
Element | Electronic configuration | valence electron |
---|---|---|
Lithium [Li] | [2,1] | 1 |
Sodium [Na] | [2, 8, 1] | 1 |
Potassium [K] | [2, 8, 8, 1] | 1 |
Sodium and aluminium have atomic numbers 11 and 13, respectively. They are separated by one element in the periodic table and have valencies 1 and 3 respectively. Chlorine and potassium are also separated by one element in the periodic table (their atomic numbers being 17 and 19, respectively) and yet both have valency 1. Explain.
Answer:
Sodium and aluminium both belong to period 3 of the periodic table. They are the first and the third elements of their period belonging to group IA and IIIA, respectively. As they lie towards the left in their period, they exhibit metallic character and being separated by one element exhibit positive valencies of +1 and +3, respectively.
On the other hand, even though chlorine and potassium are also separated by one element, chlorine belongs to period 3 and potassium to period 4 of the periodic table. Chlorine is the rightmost element of period 3 belonging to the Halogens group VIIA. So, it exhibits a negative valency of -1. Potassium is the leftmost element of period 4 belonging to the Alkali metals group IA. So, it exhibits a positive valency of +1.
This question refers to elements of the periodic table with atomic numbers from 3 to 18. In the table below, some elements are shown by letters, even though the letters are not the usual symbols of the elements.
3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 |
A | B | C | D | E | F | G | H |
11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 |
I | J | K | L | M | N | O | P |
(a) Which of these is
(i) a noble gas
(ii) a halogen
(iii) an alkali metal
(iv) an element with valency 4
(b) If A combines with F, what would be the formula of the resulting compound?
(c) What is the electronic configuration of G?
Answer:
(a) (i) H and P are noble gases, as they are in group zero and have their octet complete.
(ii) G and O are halogens, as they are in group VIIA and have 7 valence electrons.
(iii) A and I are alkali metals, as they are in group IA and have 1 valence electron.
(iv) D and L have valency 4, as they are in group IVA and have 4 valence electrons.
(b) Valency of A [2, 1] is +1, so it tries to donate an electron and attain a stable state.
Valency of F [2, 6] is -2, so it tries to gain two electrons and attain a stable state. Hence, one F atom takes electron from two A atoms. Therefore, the formula of their compound will be A2F
(c) Electronic configuration of G is [2, 7] as its atomic number is 9.
The position of elements A, B, C, D and E in the periodic table are shown below:
Group 1 | Group 2 | Group 17 | Group 18 |
---|---|---|---|
– | – | – | D |
– | B | C | – |
A | – | – | E |
(a) State which are metals, non-metals and noble gas in this table.
(b) State which is the most reactive (i) metal (ii) non-metal.
(c) Which type of ion will be formed by element A, B and C.
(d) Which is larger in size (i) D or E (ii) B or C.
Answer:
(a) Metals — A and B, as they belong to alkali metals group and alkaline earth metals group.
Non-metals — C, as it belongs to halogen group 17.
Noble gases — D and E, as they belong to group 18 or zero group.
(b) A is the most reactive metal as Group 1 (Alkali metals) elements are very reactive and the degree of reactivity further increases down the group. C is the most reactive non-metal as it is in Group 17 (Halogens) and the degree of reactivity of halogens decreases down the group.
(c) Type of ion formed by elements A, B and C will be:
- Element A will lose the one valence electron and become an electropositive ion [i.e., cation A+].
- Element B will lose two valence electrons and become an electropositive ion [i.e., cation B2+].
- Element C will gain one electron, complete its octet and become an electronegative ion [i.e., anion C-].
(d) Larger in size
- E is larger in size as the number of shells increases as we move down a group increasing the size of the atom.
- B is larger in size as atomic size decreases on moving from left to right across a period due to increase in nuclear charge.
Write the electronic configuration of the element 3517T.
(a) What is the group number of T?
(b) What is the period number of T?
(c) How many valence electrons are there in an atom of T?
(d) What is the valency of T?
(e) Is it a metal or a non-metal?
(f) State number of protons and neutrons in T.
Answer:
The electronic configuration of 3517T is [2, 8, 7]
(a) Group number of T — VIIA [∵ it has 7 valence electrons]
(b) Period number of T — Third period [∵ it has 3 shells]
(c) Valence electrons — Seven [from electronic configuration]
(d) Valency of T is -1. [∵ it needs to gain one electron to complete its octet]
(e) T is a non-metal [∵ it has a tendency to accept electrons]
(f) Protons = 17, Neutrons = 18
Exercise 5C Descriptive Type
8 questionsAnswer:
(a) (i) Group 17 elements (halogens) can react directly with metals to form metal halides (or salts).
(ii) Group 17 elements (halogens) react with non-metals to form halides like chlorides, bromides and iodides.
(b) Group 17 elements are highly reactive as they need only one electron to complete its octet and attain a stable state.
Answer:
(a) Group 17 elements are called halogens. The name halogen (Greek, halo = sea or salt + gen = producing) means 'salt former'. Group 17 elements form salts, hence, they are called Halogens.
(b) (i) Halogens are the most reactive non-metals. Their reactivity decreases down the group. For example: fluorine is the most reactive and iodine is the least reactive.
(ii) The intensity of the colour of the element increases from pale to dark. Fluorine is a pale yellow gas, chlorine is a greenish-yellow gas, bromine is a reddish-brown liquid and iodine is a violet solid.
(iii) The physical state of the elements changes from gas to solid as we move down the group. Fluorine is a gas, chlorine is a gas, bromine is liquid and iodine is solid.
Answer:
- Periodic table has been useful in predicting the existence of new elements.
- It has been useful in the past in correcting the position of elements in relation to their properties.
- Study of elements and their compounds has become systematic and easier to remember.
- Position of an element in the periodic table reveals its:
- atomic number
- electronic configuration
- number of valence electrons
- properties
- Nature of chemical bond, formula of compound formed and properties of that compound can all be predicted from the periodic table.
- Position of an element in the periodic table reveals:
- valency of the element
- whether the element is a metal or a non-metal — metals occupy the extreme left positions of the periodic table while non-metals are at the extreme right of the periodic table.
Answer:
Merits of Mendeleev’s periodic table :
- Grouping of elements — He generalized the study of the elements then known to a study of mere eight groups.
- Gaps of undiscovered elements — Mendeleev left some gaps in his periodic table for subsequent inclusion of elements not known at that time.
- Prediction of properties of undiscovered elements — He predicted the properties of then unknown elements on the basis of the properties of elements lying adjacent to the vacant slots.
- Incorrect atomic mass corrected — He was able to correct the atomic mass of elements like gold and platinum by placing these elements strictly on the basis of similarities in their properties.
Answer:
Defects of table proposed by Mendeleev:
- Anomalous pairs — The following elements did not follow Mendeleev's principles:
- Argon with atomic mass 39.9 precedes potassium with atomic mass 39.1
- Cobalt with atomic mass 58.9 precedes nickel with atomic mass 58.6
- Tellurium with atomic mass 127.6 precedes iodine with atomic mass 126.9
- Position of isotopes — According to Mendeleev's periodic table isotopes should be given separate place in the table since they have different atomic mass. But they were not assigned separate places.
- Grouping of chemically dissimilar elements — Elements such as copper and silver bear no resemblance to alkali metals (lithium, sodium etc.) but they have been placed together in the first group.
- Separation of chemically similar elements — Elements that are chemically similar, such as gold and platinum have been placed in separate groups.
- Electron arrangement — It does not explain the electron arrangement of elements.
- Position of hydrogen — Hydrogen was not given a fixed position. It was considered in group IA as well as in group VIIA because it forms forms both a positive ion, viz. in HCl and a negative ion, viz. in NaH.
Answer:
- They include both metals and non-metals. There is a regular gradation from metallic to non-metallic character as one moves from left to right across the period.
- They form electrovalent as well as covalent compounds with non-metals.
- Metallic nature increases on moving down any of these seven groups.
- Metals, which are good conductors of heat and electricity, are present in groups 1 and 2. Non-metals, which are present in groups 16 and 17, are poor conductors of heat and electricity.
- Some heavier elements, like tin and lead, exhibit variable valencies.
Answer:
(a) Halogens are the most reactive non-metals. Their reactivity decreases down the group. For example: fluorine is the most reactive and iodine is the least reactive.
(b) The intensity of the colour of the element increases from pale to dark. Fluorine is a pale yellow gas, chlorine is a greenish-yellow gas, bromine is a reddish-brown liquid and iodine is a violet solid.
(c) They all form negative ions carrying a single charge [Fluoride ions F-, Chloride ions Cl-, Bromide ions Br-, Iodide ions I-.]
Two elements 'P' and 'Q' belong to the same period of the modern periodic table and are in group 1 and group 2 respectively. Compare the following characteristics in tabular form.
(a) number of electrons in their valence shell.
(b) their tendency to lose electrons.
(c) their metallic characters.
(d) formation of their oxides.
(e) formulae of their chlorides.
Answer:
These elements belong to alkali metals and alkaline earth metals, respectively. Below is the comparison of their characteristics:
S. No. | Characteristic | Alkali metals (P) | Alkaline earth metals (Q) |
---|---|---|---|
(a) | Number of electrons in their valence shell | 1 | 2 |
(b) | Tendency to lose electrons. | Can easily lose electrons | Loses electrons but relatively less easily than alkali metals. |
(c) | Metallic character | Metallic | Less metallic than P |
(d) | Formation of their oxides | React rapidly with oxygen in the air | Less reactive than alkali metals |
(e) | Formulae of their chlorides | NaCl, KCl, CsCl | MgCl2, CaCl2, BaCl2 |
Exercise 5C Multiple Choice Type
19 questionsAnswer:
Number of valence electrons
Reason — The arrangement of elements based on their atomic weights in the Modern Periodic Table puts elements with same number of valence electrons in the same group.
Answer:
Each element belongs to the same period
Reason — Period 3, known as short period, contains 8 elements i.e., sodium (Na), magnesium (Mg), aluminium (Al), silicon (Si), phosphorous (P), sulphur (S) and chlorine (Cl).
Answer:
Transition elements, Inner transition elements, Representative elements.
Reason — Noble gases have a stable electronic configuration with their outermost orbits complete.
Answer:
3rd period and VI A group
Reason — The element with atomic number 16 has electronic configuration 2,8,6. It has 3 shells so it will be in period 3 and 6 valence electrons so it is placed in VI A group.
Answer:
One
Reason — As group 1 elements have 1 valence electrons hence they tend to donate it, attain a stable configuration and get oxidized. Therefore, they are the strongest reducing agents.
Answer:
Fluorine
Reason — As reactivity of halogens group decreases as we move downwards, hence fluorine is the most reactive non-metal. Reactivity of fluorine is more than oxygen as it has a stronger tendency to take electrons and complete its octet.
Look at the periodic table given below and answer the following questions.
Groups | I | II | III | IV | V | VI | VII | VIII |
---|---|---|---|---|---|---|---|---|
Oxide: Hydride: | R2O RH | RO RH2 | R2O3 RH2 | RO2 RH4 | R2O5 RH3 | RO3 RH2 | R2O7 RH | RO4 |
Periods ↓ | A B | A B | A B | A B | A B | A B | A B | Transition Series |
1 | H 1.008 | |||||||
2 | Li 6.939 | Be 9.012 | B 10.81 | C 12.011 | N 14.007 | O 15.999 | F 18.998 | |
3 | Na 22.99 | Mg 24.31 | Al 29.98 | Si 28.09 | P 30.974 | S 32.06 | Cl 35.453 |
(I) This table was given by:
- Newland
- Döbereiner
- Mendeleev
- Moseley
(II) According to this periodic table, the formula of the oxide of nitrogen is:
- N2O3
- NO
- N2O5
- N2O7
(III) The defect(s) of this periodic table is/are:
P — Anomalous pairs
Q — Separation of chemically similar elements.
R — Grouping of chemically dissimilar elements.
- Only P
- Both Q and R
- Both P and Q
- P, Q and R
Answer:
(I) Mendeleev
(II) N2O5
(III) P, Q and R
Reason —
(I) The above table was given by Mendeleev. It was published in German journal in 1872.
(II) Mendeleev used letter 'R' to represent the element of that particular group. Hence the oxide of nitrogen in the periodic table is R2O5. Here 'R' is nitrogen. Hence, oxide of nitrogen is N2O5.
(III) Mendeleev's periodic table had anomalous pair, which did not follow Mendeleev's principle. Major defects in Mendeleev's periodic table were grouping of chemically dissimilar elements and separation of chemically similar elements.
Answer:
Only Q
Reason — Group 17 has elements in all the three states i.e., solid, liquid, gas. Fluorine is a gas, chlorine is a gas, bromine is liquid and iodine is solid.
Answer:
Both Q and R
Reason — Modern periodic table can be used to predict nature of chemical bond, formula of compound formed and properties of that compound.
Assertion (A): Noble gases do not combine with other elements.
Reason (R): The outermost orbit of noble gases are complete, they have a stable electronic configuration.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true and R is the correct explanation of A.
Explanation — Noble Gases or Inert gases are Group 18 elements. They do not react with other elements because they have complete 8 electrons in their outermost orbits (except He, which has only 2), which is stable electronic configuration. Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
Assertion (A): Lithium, sodium and potassium form a Dobereiner's triad.
Reason (R): The atomic mass of the middle element is the average of the masses of the other two elements.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true and R is the correct explanation of A.
Explanation — The atomic masses of Lithium (Li) is 7 , Sodium (Na) is 23 and Potassium (K) is 39. The mean of atomic masses of Lithium and Potassium is
= = = 23
Therefore, the atomic mass of middle element sodium (23) is the average of the masses of the other two elements. Hence both assertion (A) and reason (R) are true and reason (R) is the correct explanation of assertion (A).
Assertion (A): Chlorine with atomic number 17 is placed in group 17 and period 3.
Reason (R): The number of shells signifies a period, while the number of valence electrons indicates the group.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true but R is not the correct explanation of A.
Explanation — Chlorine, a halogen with atomic number 17 has valency -1 so, it is placed in group 17. Chlorine has total three shells- K, L and M. So, it is placed in period 3. Hence, the assertion (A) is true.
The number of shell(s) in an atom signifies in which period it is placed and number of valence electrons indicates its group. Hence reason (R) is true.
But, reason (R) does not explain why chlorine is placed in group 17 and period 3. Hence reason (R) is not the correct explanation of assertion (A).
Assertion (A): 19Fluorine, 35Chlorine and 80Bromine do not constitute a Döbereiner's triad.
Reason (R): Atomic mass of chlorine is not the average mass of Fluorine and Bromine.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true and R is the correct explanation of A.
Explanation — The atomic masses of Fluorine is 19, Chlorine is 35 and Bromine is 80.
The mean of atomic masses of fluorine and bromine is
= = = 49.5
The obtained mean is not the atomic mass of chlorine. So, they do not constitute a Döbereiner's triad. Hence the assertion (A) is true.
Atomic mass of chlorine is not the average mass of Fluorine and Bromine. Hence the reason (R) is true.
Reason (R) justifies why florine, chlorine and bromine are not Döbereiner's triad. Hence reason (R) is the correct explanation of assertion (A).
Assertion (A): Hydrogen can be placed in two groups i.e., with Alkali metals and with halogens.
Reason (R): Hydrogen has two electrons in its valence shell.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
A is true but R is false.
Explanation — Hydrogen resembles the alkali metals of Group IA and the halogens of Group VIIA. This is due to the fact that it has only one electron in its valence shell, hence it behaves like the alkali metals which have one valence electron. At the same, it also behaves like halogens as it has just one electron less than the nearest inert gas. Hence the assertion (A) is true.
Hydrogen has only one electron in its valence shell. Hence reason (R) is false.
Assertion (A): Cobalt (atomic mass 58.94) is placed before Nickel (atomic mass 58.68) in Mendeleev's periodic table.
Reason (R): Physical and chemical properties of the elements are a periodic function of their atomic masses.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
Both A and R are true but R is not the correct explanation of A.
Explanation — Cobalt with atomic mass 58.94 is placed before Nickel with atomic mass 58.68 in Mendeleev's periodic table. They are anomalous pair which do not follow mendeleev's principle. Hence the assertion is true.
Mendeleev's periodic law states that physical and chemical properties of the elements are a periodic function of their atomic masses. Hence the reason (R) is true.
However, reason (R) does explain why cobalt is placed before nickel in mendeleev's periodic table. Hence reason (R) is not the correct explanation of assertion (A).
Assertion (A): Group 17 elements are known as alkaline earth metals.
Reason (R): Group 17 elements are salt formers.
- Both A and R are true and R is the correct explanation of A.
- Both A and R are true but R is not the correct explanation of A.
- A is true but R is false.
- A is false but R is true.
Answer:
A is false but R is true.
Explanation — Group 17 elements are known as halogens. Hence the assertion (A) is false.
Halogen name is derived from combination of greek word halo meaning sea or salt and gen meaning producing. Therefore, group 17 elements are salt formers. Hence the reason (R) is true.
Exercise 5C Short Answer Type
9 questionsAnswer:
(a) Li < Na < K < Rb < Cs
(b) F > Cl > Br > I
(c) Mg > Na > He
(d) Cl < Mg < Na
Answer:
(a) 3, as the electronic configuration of the element is 2,8,3.
(b) +3, as the element will try to donate three valence electrons and attain a stable octet configuration.
(c) Metal, as the element has 3 valence electrons and tries to donate it to attain a stable configuration.
(d) Aluminium (Al), as it is in 3rd period and Group IIIA (13)
Answer:
(a) Atomic number is 20.
(b) Its electronic configuration is 2, 8, 8, 2.
(c) +2, as the element tries to donate the two valence electrons, gain a stable octet configuration and become an electropositive ion.
(d) Group 2 Period 4
(e) Metal, as the element has 2 valence electrons and tries to donate it to attain a stable configuration.
(f) Reducing agent, as metals are strong reducing agents.
Answer:
(a) Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr) are alkali metals. They have 1 electron in their outermost orbit.
(b) (i) Alkali metal like sodium reacts rapidly with oxygen forming metal oxide
4Na + O2 ⟶ 2Na2O
(ii) Alkali metal like sodium reacts with water violently and produces its hydroxide and hydrogen
2Na + 2H2O ⟶ 2NaOH + H2
(iii) Alkali metal react violently with dil. HCl and dil. H2SO4 to produce hydrogen
2Na + 2HCl ⟶ 2NaCl + H2
Answer:
(a) Alkaline earth metals are reactive metals, hence they do not occur in free state in nature. Their oxides occur in the earth's crust.
(b) Electronic configuration of beryllium (Be) is [2, 2] and magnesium (Mg) is [2, 8, 2]
Answer:
(a) As alkali metals are very reactive metals, hence they are kept in inert solvents.
(b) As alkali metals and halogens are very reactive, hence, they do not occur free in nature.
(c) As alkali and alkaline earth metal have 1 and 2 valence electrons respectively hence, they tend to donate the valence electrons to attain a stable configuration and form electrovalent compounds.
(d) As the outermost orbits of inert gases are completely filled and they are in a stable state hence they do not form compounds.
Exercise 5C Structuredapplicationskill Type
2 questionsWestern music Do | Re | Me | Fa | So | La | Ti |
---|---|---|---|---|---|---|
Indian music Sa | Re | Ga | Ma | Pa | Dha | Nee |
1 | 2 | 3 | 4 | 5 | 6 | 7 |
H | Li | Be | B | C | N | O |
F | Na | Mg | Al | Si | P | S |
Cl | K | Ca | Cr | Ti | Mn | Fe |
(a) Name the scientist who gave this table.
(b) Which group of elements were not known at his time?
(c) This system worked quite well for the .............. (heavier/lighter) elements.
(d) The table shown above relates the properties of the elements to their .............. (atomic number / atomic mass)
(e) Why was this table discarded?
Answer:
(a) John Newland
(b) 18th group of inert elements
(c) This system worked quite well for the lighter elements.
(d) The table shown above relates the properties of the elements to their atomic mass.
(e) Reasons for discarding the law of octaves:
- This classification did not work for heavier elements i.e., those lying beyond calcium. As more and more of elements were discovered, they could not be fitted in this table.
- Newland adjusted two elements cobalt (Co) and nickel (Ni) in the same slot and these were placed in the same column as fluorine, chlorine and bromine which have very different properties than these elements.
- Iron, which resembles cobalt and nickle in properties, has been placed far away from these elements.
(a) Complete the table.
Atomic No | Element | Electronic configuration | Select element of the same group |
---|---|---|---|
11 | Sodium | ............... | (Ca/N/K) |
15 | Phosphorus | ............... | (Al/N/C) |
16 | Sulphur | ............... | (F/Cl/O) |
9 | Fluorine | ............... | (Ca/Cl/K) |
(b) Classify the elements given in the table as metals and non-metals.
(c) To which group does sodium belong? Why is Hydrogen not considered while describing the elements of this group?
Answer:
(a) The completed table is given below:
Atomic No | Element | Electronic configuration | Select element of the same group |
---|---|---|---|
11 | Sodium | 2, 8, 1 | K |
15 | Phosphorus | 2, 8, 5 | N |
16 | Sulphur | 2, 8, 6 | O |
9 | Fluorine | 2, 7 | Cl |
(b)
Element | Metal/Non-metal |
---|---|
Sodium (Na) | Metal |
Phosphorous (P) | Non-metal |
Sulphur (S) | Non-metal |
Fluorine (F) | Non-metal |
(c) Sodium belongs to group 1.
Position of hydrogen is not satisfactory. Hydrogen atom is very small in size and therefore many of its properties are different from group 1 elements (alkali metals). It is a gas at room temperature, while other elements of group 1 are solid metals. Secondly, it does not readily form cations, in most compounds, whereas group 1 metals readily form cations and form only ionic bonds. Due to these reasons Hydrogen is not considered while describing the elements of group 1.
Exercise 5C Very Short Answer Type
5 questionsMatch column A with column B
Column A | Column B |
---|---|
(a) Elements short by 1 electron in octet | (i) Transition elements |
(b) Highly reactive elements | (ii) Noble gases |
(c) Non-reactive elements | (iii) Alkali metals |
(d) Elements of group 3 to 12 | (iv) Alkaline earth metals |
(e) Radioactive elements | (v) Halogens |
(f) Elements with 2 electrons in the outermost orbit | (vi) Actinides |
Answer:
Column A | Column B |
---|---|
(a) Elements short by 1 electron in octet | (v) Halogens |
(b) Highly reactive elements | (iii) Alkali metals |
(c) Non-reactive elements | (ii) Noble gases |
(d) Elements of group 3 to 12 | (i) Transition elements |
(e) Radioactive elements | (v) Actinides |
(f) Elements with 2 electrons in the outermost orbit | (vi) Alkaline earth metals |
Write down the word that will correctly complete the following sentences:
(a) Relative atomic mass of a light element up to calcium is approximately ............... its atomic number.
(b) The horizontal rows in a periodic table are called ...............
(c) Going across a period left to right, atomic size ...............
(d) Moving left to right in the second period, number of valence electrons ...............
(e) Moving down in the second group, number of valence electrons ...............
Answer:
(a) Relative atomic mass of a light element up to calcium is approximately twice its atomic number.
(b) The horizontal rows in a periodic table are called periods.
(c) Going across a period left to right, atomic size decreases.
(d) Moving left to right in the second period, number of valence electrons increases
(e) Moving down in the second group, number of valence electrons remains same
Name or state the following with reference to the elements of the first three periods of the periodic table.
(a) Noble gas with duplet arrangement of electrons.
(b) Metalloid in Period 3.
(c) Valency of elements in Group 14 and 15.
(d) Noble gas having electronic configuration 2, 8, 8.
(e) Group whose elements have zero valency.
(f) A covalent compound formed by an element in period 2 and a halogen.
(g) Non-metallic element present in Period 3 of Groups 15 and 16.
(h) An electrovalent compound formed by an alkaline earth metal and a halogen.
(i) Bridge elements of Period 3 of Group 1, 2 and 3.
(j) Alkali metal in Period 3 that dissolves in water giving a strong alkali.
(k) Typical elements of Groups 14 and 15.
(l) Alkaline earth metal in Period 3.
Answer:
(a) Helium (He)
(b) Silicon (Si)
(c) 4, 3
(d) Argon (Ar)
(e) Group 18 — Noble gases
(f) Carbon tetrachloride (CCl4)
(g) Phosphorus and sulphur
(h) Sodium chloride (NaCl)
(i) Lithium (Li) and Magnesium (Mg);
Beryllium (Be) and Aluminium (Al);
Boron (B) and Silicon (Si)
(j) Sodium (Na)
(k) Silicon (Si) and phosphorus (P).
(l) Magnesium (Mg)