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6 JEE (XI) Module-1 PW
CHAPTER
Classification of Elements and
Periodicity in Properties
Trends at a Glance
Fig.: The periodic trends of elements in the periodic table
Periodic Variation in lonisation Energy
For representative (s- and p- block) elements-Trend across a period: General trend -the Zeff increases in a period, hence IE increases. Table: Trend in ionisation energy across periods
Period No. Trend in ionisation energy Comments
2 Expected: Actual: Li < B < Be < C < O < N < F < Ne^ Li < Be < B < C < N < O < F < Ne
3 Expected Actual: Na < Al < Mg < Si < S < P < Cl < Ar^ Na < Mg < Al < Si < P < S < Cl < Ar Irregular Trend
4 Expected: Actual: K < Ga < Ca < Ge < Se < As < Br < Kr^ K < Ca < Ga < Ge < As < Se < Br < Kr
5 Rb < Sr < ln < Sn < Sb < Te < l < Xe Regular Trend 6 Cs < Ba < Tl < Pb < Bi < Po < At < Rn
For representative (s-and p-block) elements-Trend across a period:
PW Classification of Elements and Periodicity in Properties 7
Table: Exceptions in the trend of variation of ionisation energy across periods
Exceptions to the trend across periods
Expected Actual Explanation
Be < B Mg < Al Ca < Ga
Be > B Mg > Al Ca > Ga
Be, Mg and Ca have fully-filled ns^2 orbitals
N < O
P < S
As < Se
N > O
P > S
As > Se
N, P and As have half-filled electronic configuration
For representative (s-and p-block) elements - Trend down the group: General trend-the size increases down a group, hence IE decreases. Table: Exceptions in the trend of variation of ionisation energy down the group
Exceptions to the trend across groups
Group No. Expected Actual Explanatin Overall order in group
Al > Ga Al ≈ Ga Poor shielding ofd-electrons in Ga B > Tl > Ga ≥ Al > ln ln > Tl ln < Tl Lanthanoidcontraction in Tl
14 Sn > Pb Sn < Pb Lanthanoidcontraction in Pb C > Si > Ge > Pb > Sn
Factors Affecting Electronegativity
Table: Effect of Atomic radius, Zeff, oxidation state and Hybridisation on Electro negativity
Factor Effect on EN
Atomic Radius Atomic radius ↑, EN↓
Li (r = 133 pm, EN = 1.0) Na (r = 155 pm, EN = 0.9)
Z (^) eff Zeff ↑, EN ↑
O(Z (^) eff = 4.55, EN = 3.5 F(Zeff = 5.20, EN = 4.0)
Oxidation State For the atoms of same element in different compunds, as ON↑, EN↑
EN of Fe3+^ > Fe2+, EN of O 2–^ < O –
Hybridisation Greater the %s chracter, greater is the attraction on the electrons, greater is the EN.
sp > sp^2 > sp 3
