Question 1

Question 1Compare the properties of electrons, protons and neutrons.

Accepted Answer

PropertyElectronsProtonsNeutronsChargeNegatively chargedPositively chargedNo chargeLocationLocated outside the nucleusLocated within the nucleusLocated within the nucleusWeightMass is negligible1 a.m.u1 a.m.uAffinityAttracted towards positively chargedAttracted towards negatively chargedNot attracted towards any charged particle

Question 2

Question 2What are the limitations of J.J. Thomson's model of the atom?

Accepted Answer

Limitations of J.J. Thomson's model of the atom were:It failed to explain how the positive charge holds on the electrons inside the atom. It also failed to explain the stability of an atom.The concept of the nucleus was not present in his model.It failed to explain the results of Rutherford's ∝–particle scattering experiment.

Question 3

Question 3What are the limitations of Rutherford's model of the atom?

Accepted Answer

The revolution of the electron in a circular orbit is not expected to be stable.Any particle in circular orbit would undergo acceleration, and hence, charged particles would radiate energy. Thus, the revolving electron would lose energy and finally fall into the nucleus.Hence, atoms must be highly unstable and matter would not exist in its known form. We know that atoms are quite stable.

Question 4

Question 4Describe Bohr's model of the atom.

Accepted Answer

Neil Bohr put forward the following postulates about the model of an atom:Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.While revolving in discrete orbits the electrons do not radiate energy.

Question 5

Question 5Compare all the proposed models of an atom given in this chapter.

Accepted Answer

Thomson's ModelRutherford's ModelBohr's modelIt was proposed by J.J. Thomson.It was proposed by Ernst Rutherford.It was proposed by Neil Bohr.An atom consists of a positively charged sphere and the electrons are embedded in it.There is a positively charged centre in an atom called the nucleus. Nearly all the mass of an atom resides in the nucleus.Only certain special orbits known as discrete orbits of electrons are allowed inside the atom.The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.There is very large empty space between nucleus and electrons.While revolving in discrete orbits the electrons do not radiate energy.It could not explain the result of Rutherford's ∝–particle scattering experiment.It could not explain the stability of atom because revolving electron will lose energy and will ultimately fall into the nucleus.This model has overcome the limitations of the Rutherford model by defining energy levels for electrons.

Question 6

Question 6Summarise the rules for writing of distribution of electrons in various shells for the first eighteen elements.

Accepted Answer

The following rules are followed for writing the number of electrons in different energy levels or shells for the first eighteen elements:The maximum number of electrons in each shell or orbit is determined by the formula 2n2, where n is the shell number.K shell, n=1, no. of electrons = 2 x 12 = 2L shell, n=2, no. of electrons = 2 x 22 = 8M shell, n=3, no. of electrons = 2 x 32 = 18Electrons are not accommodated in a given shell, unless the inner shells are filled. Shells are filled in step-wise manner.The atom's outermost shell needs 2 electrons for stability (duplet rule) if it has only one shell. If it has multiple shells, it requires 8 electrons (octet rule) in the outermost shell for stability.

Question 7

Question 7Define valency by taking examples of silicon and oxygen.

Accepted Answer

The definite combining capacity of the atoms of each element, wherein electrons are lost, gained or shared to make the octet of electrons present in the outermost shell, is defined as valency.(i) The electronic configuration of silicon [14] is 2, 8, 4. It has 4 electrons in its outermost shell. So, the valency of silicon = 8 – the number of electrons in the outermost shell = 8 - 4 = 4.Hence, valency of silicon = 4.(ii) The electronic configuration of oxygen [8] is 2, 6. It has 6 (more than 4) electrons in its outermost shell. So, the valency of oxygen = 8 – the number of electrons in the outermost shell = 8 − 6 = 2.Hence, valency of oxygen = 2.

Question 8

Question 8Explain with examples(i) Atomic number,(ii) Mass number,(iii) Isotopes and(iv) Isobars.Give any two uses of isotopes.

Accepted Answer

(i) Atomic number — The number of protons present in the nucleus of an atom is defined as the atomic number and is denoted by Z.Example: Atomic number of hydrogen (Z) = 1 as only one proton is present in the nucleus of hydrogen atom.(ii) Mass number — The total number of protons and neutrons present in the nucleus of an atom is known as the mass number and is denoted by A.Example: Mass number of Carbon (A) = 12 as it has 6 protons and 6 neutrons.(iii) Isotopes — The atoms of the same element having the same atomic number but different mass number are known as isotopes.Example: Carbon has two isotopes 6C12 and 6C14(iv) Isobars — Atoms of different elements with different atomic numbers, which have same mass number are known as isobars.Example: Calcium [20Ca40] atomic number 20 and argon [18Ar40] atomic number 18 are isobars as both have same mass number i.e., 40 and different atomic numbers.Isotopes have special properties which makes them useful in various fields like:An isotope of uranium is used as a fuel in nuclear reactors.An isotope of cobalt is used in the treatment of cancer.

Question 9

Question 9Na+ has completely filled K and L shells. Explain.

Accepted Answer

The atomic number of sodium is 11. It has 11 electrons in its orbitals. So, its electronic configuration is K-2, L-8, M-1. The maximum electrons in K, L, M shells according to 2n2 rule are 2, 8 and 18 respectively.In case of Na+ (10), one electron from the M shell is lost, and it obtains a positive charge. The new electronic configuration is K-1 and L-8. Hence, it has completely filled shells.

Question 10

Question 10If the bromine atom is available in the form of, say, two isotopes 3579Br (49.7%) and 3581Br (50.3%), calculate the average atomic mass of bromine atom.

Accepted Answer

The atomic mass of an element is the mass of one atom of that element.Average atomic mass takes into account the isotopic abundance.Isotope of bromine with atomic mass 79 u = 49.7%Therefore, Contribution of 3579Br to atomic mass = 79×49.7100\dfrac{79 	imes 49.7}{100}10079×49.7​ = 39.263 uIsotope of bromine with atomic mass 81 u = 50.3%Contribution of 3581Br to the atomic mass of bromine = 81×50.3100\dfrac{81 	imes 50.3}{100}10081×50.3​ = 40.743uHence, the average atomic mass of the bromine atom = 39.263 + 40.743 = 80.006 u

Question 11

Question 11The average atomic mass of a sample of element X is 16.2 u. What are the percentages of isotopes 816X and 818X in the sample?

Accepted Answer

Let the percentage of 816X be a and that of 818X be 100 - a.16.2u = 16a100\dfrac{	ext{16a}}{100}10016a​ + 18(100-a)10018\dfrac{	ext{(100-a)}}{100}18100(100-a)​⇒ 1620 = 16a + 1800 – 18a⇒ 1620 = 1800 – 2a⇒ 2a = 1800 - 1620⇒ a = 1800−16202\dfrac{1800 - 1620}{2}21800−1620​⇒ a = 1802\dfrac{180}{2}2180​⇒ a = 90∴ Percentage of 816X = 90%andPercentage of 818X = 100 - a = 100 - 90 = 10%Hence, the percentage of the isotope 816X in the sample is 90% and that of 818X is 10%

Question 12

Question 12If Z=3, what would be the valency of the element? Also, name the element.

Accepted Answer

Given,Atomic number, Z = 3The electronic configuration of the element = 2,1. Hence, its valency = 1The element with atomic number 3 is Lithium.

Question 13

Question 13Composition of the nuclei of two atomic species, X and Y, are given as under XYProtons =66Neutrons =68Give the mass numbers of X and Y. What is the relation between the two species?

Accepted Answer

Mass number = No. of protons + No. of neutronsMass number of X = 6 + 6 = 12Mass number of Y = 6 + 8 = 14Number of protons is same in both so atomic number is same however as the number of neutrons is different so mass number is different. Hence, these two elements are isotopes.

Question 14

Question 14For the following statements, write T for True and F for False.(a) J.J. Thomson proposed that the nucleus of an atom contains only nucleons.(b) A neutron is formed by an electron and a proton combining together. Therefore, it is neutral.(c) The mass of an electron is about 12000\dfrac{1}{2000}20001​ times that of a proton.(d) An isotope of iodine is used for making tincture iodine, which is used as a medicine.

Accepted Answer

(a) False.Reason — Thomson proposed that:An atom consists of a positively charged sphere and the electrons are embedded in it.The negative and positive charges are equal in magnitude. So, the atom as a whole is electrically neutral.(b) False.Reason — Neutron is not formed by an electron and a proton combining together. It is present in the nucleus of an atom alongside protons.(c) True.Reason — The mass of an electron is about 12000\dfrac{1}{2000}20001​ times that of a proton.(d) False.Reason — Tincture iodine is an antiseptic that usually contains 2-7% elemental I-127 dissolved in a mixture of ethanol and water. Normal iodine is used in its preparation, not its isotope. ​

Question 15

Question 15Rutherford's alpha–particle scattering experiment was responsible for the discovery ofAtomic nucleusElectronProtonNeutron

Accepted Answer

Atomic nucleusReason — Rutherford's alpha–particle scattering experiment was responsible for the discovery of atomic nucleus.

Question 16

Question 16Isotopes of an element haveThe same physical propertiesDifferent chemical propertiesDifferent number of neutronsDifferent atomic numbers

Accepted Answer

Different number of neutronsReason — Isotopes of an element have same atomic number (i.e., number of protons) and different mass number (i.e., sum of protons + neutrons).

Question 17

Question 17Number of valence electrons in Cl- ion are1681718

Accepted Answer

8Reason — The electronic distribution of Cl is 2, 8, 7. There are 7 valence electrons in chlorine. It gains one electron and forms Cl-. Hence, its valency is 8.

Question 18

Question 18Which one of the following is a correct electronic configuration of Sodium?2, 88, 2, 12, 1, 82, 8, 1

Accepted Answer

2, 8, 1Reason — Sodium has atomic number 11 and it's electronic configuration is 2,8,1.

Question 19

Question 19Complete the following table.Atomic NumberMass NumberNumber of NeutronsNumber of ProtonsNumber of ElectronsName of the Atomic Species9  10      1632      Sulphur  2412      2  1      1010

Accepted Answer

Atomic NumberMass NumberNumber of NeutronsNumber of ProtonsNumber of ElectronsName of the Atomic Species9191099Fluorine1632161616Sulphur1224121212Magnesium12111Deuterium11010Protium

Question 20

Question 1What are the canal rays?

Accepted Answer

The canal rays are positively charged radiations present in a gas discharge. They helped in the discovery of another sub-atomic particle that was positively charged called the proton.

Structure of the Atom

Exercises

Intext Questions 1

Intext Questions 2

Intext Questions 3

Intext Questions 4

Intext Questions 5

Intext Questions 6

Intext Questions 7