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ICSE Class 9 Chemistry Question 3 of 20

The Periodic Table — Question 17

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Question 16

Write short notes on the following types of elements –

(a) alkali metals

(b) alkaline earth metals

(c) halogens

(d) noble gases

(e) transition and inner transition elements

(f) normal elements

Answer

(a) Alkali metals —

  1. The elements of group 1 [IA] from 3Li [Lithium] to 87Fr [Francium] are called alkali metals as they react with water forming strong alkalis.
  2. They are light metals and are univalent with one valence electron in outer shell. This makes them strong reducing agents as they readily donate the one electron to achieve stable electronic configuration.
  3. They are highly reactive electropositive metals, which are soft and can be cut with a knife.
  4. They are good conductors of heat & electricity.
  5. They form electrovalent compounds with non-metals. e.g. KCl, NaBr.

(b) Alkaline earth metals —

  1. The elements of group 2 [IIA] from 4Be [Beryllium] to 88Ra [Radium] are called Alkaline earth metals.
  2. They are light metals and are divalent with two valence electrons in their outer shell. They are reducing agents as they can donate the two electrons to achieve stable electronic configuration.
  3. They have properties similar to alkali metals but are slightly less reactive than alkali metals.
  4. They are good conductors of heat & electricity.
  5. They form electrovalent compounds with non-metals. e.g. CaCl2, MgO.

(c) Halogens —

  1. The elements of group 17 [VIIA] from 9F [Fluorine] to 53I [Iodine] are called Halogens. They are called Halogens as Halogens means salt producer and these elements react with metals to form salts.
  2. They are non-metals and are univalent with seven valence electrons in their outer shell. This makes them strong oxidising agents as they readily accept one electron to achieve stable electronic configuration.
  3. They are highly reactive, electronegative non-metals.
  4. They are bad conductors of heat & electricity.
  5. They form covalent compounds with non-metals. e.g. HCl, PCl3.

(d) Noble gases —

  1. The elements of group 18 [0] from 9He [Helium] to 53Rn [Radon] are called Noble gases.
  2. They are non-metals and have eight valence elections in outer shell [except Helium (He) which has only two]. They have stable electronic configuration hence their valency or combining capacity is Zero.
  3. Noble gases do not react with other elements & are therefore inert or unreactive. They are all monatomic gases.

(e) Transition and inner transition elements —

Transition elements —

  1. The elements of group 3 to 12 [IB to VIIB & VIII] are called Transition elements.
  2. They are all metals.
  3. They lie at the centre of the periodic table between metals [highly electropositive] on the left and non-metals [highly electronegative] on the right.

Inner transition elements —

  1. Lathanide [Rare earths] and Actinides series are called inner transition elements.
  2. They lie at the bottom of the periodic table in two horizontal rows [14 elements each].
  3. Lanthanide series : from Cerium 58Ce to Lutetium 71Lu : [Period - 6]
  4. Actinide series : from Thorium 90Th to Lawrencium 103Lr: [Period - 7]
  5. They are all metals and chemically very reactive [actinides are radioactive elements].

(f) Normal elements —

  1. Elements of groups 1, 2, 13, 14, 15, 16, 17 are called normal elements.
  2. Atoms of these elements have all inner shells complete except outermost shell.
  3. They consist of metals, metalloids and non-metals.
  4. The number of electrons in their outermost shell signifies the group to which they belong.

Chapter Overview: The Periodic Table

The Periodic Table organises all known elements based on their properties. Dobereiner grouped elements into triads, Newlands proposed the Law of Octaves, and Mendeleev arranged elements by increasing atomic mass, predicting undiscovered elements. The Modern Periodic Table (Moseley) arranges elements by increasing atomic number. It has 7 periods and 18 groups. Periods represent the number of electron shells, while groups contain elements with similar valence electron configurations and chemical properties. The Class IX syllabus introduces students to the basic structure of the periodic table, the distinction between metals (left side), non-metals (right side), and metalloids (along the staircase line). Students learn periodic properties at an introductory level: atomic size, metallic character, and their general trends. The table is divided into s-block (Groups 1-2), p-block (Groups 13-18), d-block (Groups 3-12, transition metals), and f-block (lanthanides and actinides). Understanding the periodic table helps predict element behaviour and forms the basis for Class X chemistry.

Key Definitions

Term Definition
Dobereiner's TriadsGroups of three elements where the middle element's atomic mass is the average of the other two
Newlands' OctavesEvery eighth element has properties similar to the first (like musical octaves)
Mendeleev's Periodic LawProperties of elements are periodic functions of their atomic masses
Modern Periodic LawProperties of elements are periodic functions of their atomic numbers
MetalsElements that are shiny, malleable, ductile, good conductors; tend to lose electrons
Non-metalsElements that are dull, brittle, poor conductors; tend to gain electrons
MetalloidsElements with properties intermediate between metals and non-metals (e.g., Si, Ge)

Must-Know Concepts

  • Alkali metals (Group 1): Li, Na, K - very reactive, 1 valence electron, stored in oil
  • Halogens (Group 17): F, Cl, Br, I - very reactive non-metals, 7 valence electrons
  • Noble gases (Group 18): He, Ne, Ar - inert, fully filled outer shells
  • Metals are on the left and centre; non-metals on the upper right of the table
  • Mendeleev left gaps for undiscovered elements: eka-boron (Scandium), eka-aluminium (Gallium), eka-silicon (Germanium)
  • Limitations of Mendeleev's table: position of hydrogen, no place for isotopes, some elements out of order by mass

Metals vs Non-metals

Property Metals Non-metals
StateMostly solids (except Hg)Solids, liquids, or gases
LustreShiny (lustrous)Dull (except diamond, iodine)
ConductivityGood conductorsPoor conductors (except graphite)
Electron tendencyLose electrons (form cations)Gain electrons (form anions)

Important Diagrams to Practice

  • Layout of the Modern Periodic Table showing s, p, d, f blocks
  • Position of metals, non-metals, and metalloids on the table
  • Staircase line separating metals and non-metals

Common Mistakes

  • Confusing Mendeleev's law (atomic mass) with the Modern law (atomic number)
  • Thinking noble gases have 8 electrons in outermost shell (He has only 2)
  • Placing hydrogen definitively in Group 1 or Group 17 (its position is debatable)
  • Forgetting exceptions: Hg is a liquid metal, graphite conducts electricity despite being non-metal

Scoring Tips

  • Know Mendeleev's predictions and how they were validated (eka-aluminium = Gallium)
  • For comparison questions, always present in tabular form for clarity
  • Remember the limitations of each classification attempt (triads, octaves, Mendeleev)
  • Group names to memorise: 1 = Alkali metals, 2 = Alkaline earth, 17 = Halogens, 18 = Noble gases

Frequently Asked Questions

Why is hydrogen's position in the periodic table controversial?

Hydrogen has 1 valence electron like alkali metals (Group 1), but it is a non-metal and can gain 1 electron like halogens (Group 17). It does not fit perfectly in either group, so its position remains special.

What were the limitations of Newlands' Law of Octaves?

It worked only up to calcium (Z=20). After that, elements did not fit the pattern. He assumed all elements had been discovered and left no gaps for new elements. Also, he sometimes placed two elements in the same slot.

Why are noble gases called inert gases?

Noble gases have completely filled outermost electron shells (2 for He, 8 for others), making them extremely stable. They have no tendency to gain, lose, or share electrons, so they rarely participate in chemical reactions. However, some compounds of heavier noble gases (Xe, Kr) have been synthesised.