Periodic Properties and Variations of Properties

ICSE Class 10 · Chemistry · Chapter 1

What this chapter covers

Periodicity on the basis of atomic number
Atomic size
Metallic and non-metallic character
Ionisation potential
Electron affinity
Electronegativity
Alkali metals: group 1
Halogens: group 17

Key points

The modern periodic law makes properties of elements periodic functions of their atomic numbers.
Periodicity is caused by the repetition of the same number of valence electrons at regular intervals.
Period number equals the number of shells; group number equals valence electrons (groups 1, 2) or valence electrons plus ten (groups 13 to 17).
Questions are set on elements up to calcium (period 4); explanations use nuclear charge and shells, not orbitals.
Across a period nuclear charge rises with the same number of shells; down a group a new shell is added at each step.
Atomic size and metallic character decrease across a period and increase down a group.
Ionisation potential, electron affinity, electronegativity and non-metallic character increase across a period and decrease down a group.
Oxides change from basic to amphoteric to acidic across a period.
Fluorine is the most electronegative element; chlorine has the highest electron affinity; helium has the highest ionisation potential.
Alkali metals have one valence electron, form +1 ions, are soft, light and very reactive, and grow more reactive down the group.
Halogens have seven valence electrons, form -1 ions and diatomic molecules, and grow less reactive down the group.
A more reactive halogen displaces a less reactive one from its salt solution.

Questions answered

State the modern periodic law.

The physical and chemical properties of elements are periodic functions of their atomic numbers.

What is the cause of periodicity in the properties of elements?

The repetition of the same number of valence electrons after definite intervals when elements are arranged by increasing atomic number.

An element has atomic number 12. Give its electronic configuration, period, group and valency.

2, 8, 2; period 3; group 2; valency 2. It is magnesium.

Define ionisation potential. How does it vary down a group, and why?

Ionisation potential is the energy required to remove an electron from the outermost shell of an isolated gaseous atom to form a positive ion. It decreases down a group because each new shell puts the outer electron farther from the nucleus and increases shielding.

Why does atomic size decrease across a period?

The number of shells stays the same while the nuclear charge increases, so the electrons are pulled closer to the nucleus.

Distinguish between electron affinity and electronegativity.

Electron affinity is the energy released when an isolated gaseous atom gains an electron. Electronegativity is the tendency of an atom in a covalent bond to attract the shared pair of electrons towards itself; it has no unit.

Name the most electronegative element and the element with the highest ionisation potential.

Fluorine is the most electronegative element; helium has the highest ionisation potential.

Arrange Li, K and Na in decreasing order of ionisation potential.

Li > Na > K.

Past board exam questions

ICSE 2027 sample paper · 1 mark

X and Y are diatomic elements. X is less reactive than Y. What are elements X and Y?

X - chlorine, Y - iodine
X - fluorine, Y - nitrogen
X - iodine, Y - bromine
X - oxygen, Y - nitrogen

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X - iodine, Y - bromine

ICSE 2027 sample paper · 1 mark

Non-metals like chlorine and iodine on reaction with hydrogen give:

acid
base
salt
metal

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acid

ICSE 2027 sample paper · 1 mark

Statement I: Halogens are very reactive. Statement II: Halogens have high electron affinity.

Both the statements are true.
Both the statements are false.
Statement I is true, and statement II is false.
Statement I is false, and statement II is true.

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Both the statements are true.

ICSE 2027 sample paper · 3 marks

An element ‘P’ has 3 valence electrons in its L - shell and an element ‘Q’ has 7 valence electrons in its M - shell. Using this information, answer the following questions: (a) Identify the exact position of element ‘Q’ in terms of group and period in the Modern Periodic Table. (b) The atomic radius of P is __________ (more/less) than that of Q.

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P: 2, 3 (boron, period 2); Q: 2, 8, 7 (chlorine). (a) Q is in Period 3, Group 17. (b) less

ICSE 2027 sample paper · 3 marks

In the reactivity series of metals, Z is a metal above hydrogen and its oxide has the formula ZO. It was found that ZO reacted with water and formed a hydroxide as the product. With reference to the above context, answer the following questions: (a) State the number of valence electrons present in element Z. (b) Name the group which Z belongs to. (c) What will be the formula of the hydroxide of Z?

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(a) 2 (Z has valency 2) (b) Group 2 (alkaline earth metals) (c) Z(OH)₂

ICSE 2026 sample paper · 1 mark

Which element amongst the following has the largest atomic radius?

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ICSE 2026 board paper · 3 marks

Elements P, Q, and R are in the same period of the modern periodic table. • P readily loses its one valence electron to form a stable ion. • Q shares its electrons in bonding but does not form ions easily. • R has high electronegativity. Answer the following questions based on the above information: (a) Which element would be most difficult to reduce among P, Q and R? (b) Which element is expected to have the smallest atomic radius among P, Q and R? (c) Arrange P, Q and R in order of decreasing ionization potential.

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P is a Group 1 metal (left of the period), Q lies in the middle (e.g. Group 14) and R is a halogen-like non-metal (right of the period). (a) P — reduction is gain of electrons; P has the strongest tendency to lose electrons and the least to gain them. (b) R — atomic radius decreases across a period, and R is furthest to the right. (c) R > Q > P

ICSE 2026 sample paper · 3 marks

L, M and N are three elements with atomic numbers 13, 7 and 10 respectively. Answer the following questions using only the alphabets given. Do not identify the elements. Which element: (a) can combine with hydrogen to form a gas which produces dense white fumes with concentrated HCl? (b) has zero electron affinity? (c) can form an ionic compound with oxygen?

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(a) M (2, 5 — forms MH₃, which gives dense white fumes with HCl) (b) N (2, 8 — noble gas with a complete octet) (c) L (2, 8, 3 — a metal, forms an ionic oxide)