Periodicity - Class 11 Chemistry

Classification of Elements

Overview: Modern Periodic Law, Groups, Periods, and Periodic Trends in properties.

1. Periodic Laws

Mendeleev's Law: Properties are periodic function of Atomic Mass.
Modern Periodic Law (Moseley): Properties are periodic function of Atomic Number (Z).

Long form of Periodic Table: Based on Electronic Configuration. 7 Periods, 18 Groups.

2. Blocks of Elements

s-block (Gp 1, 2): Representative elements. Metals. ns^1-2.
p-block (Gp 13-18): Representative elements. Metals, Non-metals, Metalloids. ns²np^1-6.
d-block (Gp 3-12): Transition elements. (n-1)d^1-10ns^1-2.
f-block (Lanthanides, Actinides): Inner Transition elements. (n-2)f^1-14(n-1)d^0-1ns².

3. Periodic Trends

Property	Left to Right (Period)	Top to Bottom (Group)
Atomic Radius	Decreases	Increases
Ionization Enthalpy	Increases	Decreases
Electron Gain Enthalpy	Becomes more -ve	Becomes less -ve
Electronegativity	Increases	Decreases
Metallic Character	Decreases	Increases

4. Key Properties

Ionization Enthalpy (IE)

Energy required to remove electron from isolated gaseous atom. Endothermic.

Successive IE: IE₃ > IE₂ > IE₁.

Electron Gain Enthalpy (Δ_egH)

Energy change when electron is added. Can be negative (exothermic) or positive (noble gases).

Electronegativity

Tendency to attract shared pair of electrons. Fluorine (4.0) is most electronegative.

Numericals - Periodicity

Numericals

IUPAC Naming

Q1. Write IUPAC symbol for element with Z = 119.

1 = un, 1 = un, 9 = enn.

Suffix = ium.

Name: Ununennium. Symbol: Uue.

Ionic Radius

Q2. Arrange in increasing radius: Mg2+, Na+, Al3+.

All are isoelectronic (10 e-).

Higher Z means smaller radius due to stronger pull.

Al3+ (Z=13) < Mg2+ (Z=12) < Na+ (Z=11).

Groups & Periods

Q3. Locate element with Z=17 in Periodic Table.

Config: [Ne] 3s2 3p5.

Period = n = 3.

Group = 10 + 2 + 5 = 17 (Halogens).

Ionization Enthalpy

Q4. Why is IE1 of N > O?

N (2p3) has stable half-filled configuration.

O (2p4) loses e- easily to gain stability (2p3).

So removal of e- from N is harder.

Electron Gain Enthalpy

Q5. Why does Cl have more negative EgH than F?

F is very small, leading to e- e- repulsion for incoming electron in 2p orbital.

Cl has larger 3p orbital, accommodating e- easier.

Screening Effect

Q6. Order of screening effect of orbitals?

s > p > d > f

s orbitals are closer to nucleus, screen effectively.

f orbitals are diffused, poor shielding.

Magnetic Moment

Q7. Calculate magnetic moment of Mn2+ (Z=25).

Mn: [Ar] 3d5 4s2. Mn2+: [Ar] 3d5.

Unpaired electrons n = 5.

μ = √(5(7)) = √35 ≈ 5.92 BM.

Effective Nuclear Charge

Q8. Trend of Zeff across a period?

Z increases by 1, shielding increases slightly (same shell).

Net result: Zeff increases across period.

Cause of decreasing radius.

Valency

Q9. Valency of Group 13 and Group 16?

Gp 13: ns2 np1. Valency = 3.

Gp 16: ns2 np4. Valency = 2 (8-6) or 6.

Second Ionization

Q10. Why is IE2 of Na very high?

Na: [Ne] 3s1. Na+: [Ne] (Stable noble gas config).

Removing 2nd electron breaks stable octet.

Requires huge energy.

Formulas & Facts - Periodicity

Equations & Formulas

Concept	Trend/Formula
General Config s-block	ns^1-2
General Config p-block	ns² np^1-6
General Config d-block	(n-1)d^1-10 ns^1-2
Zeff	Z - S (Screening Const)
Magnetic Moment	μ = √[n(n+2)]
Radius Trend	Anion > Neutral > Cation
Electronegativity (Mulliken)	X = (IE + EA) / 2
IE vs Size	IE ∝ 1/Size

50 NEET Facts

Key points for Periodic Table.

1. Most Abundant Element (Universe) Hydrogen.

2. Most Abundant (Earth Crust) Oxygen > Silicon > Aluminum.

3. Most Abundant Metal Aluminum.

4. Most Electronegative Fluorine (4.0).

5. Highest Electron Affinity Chlorine (Not Fluorine due to small size).

6. Highest Ionization Enthalpy Helium (Noble gas, small size).

7. Lowest Ionization Enthalpy Cesium (Largest stable alkali metal).

8. Strongest Oxidizing Agent Fluorine.

9. Strongest Reducing Agent Lithium (in sol) / Cs (in gas).

10. Lightest Metal Lithium.

11. Heaviest natural element Uranium.

12. Liquid Metal Mercury (Hg).

13. Liquid Non-metal Bromine (Br).

14. Metalloids B, Si, Ge, As, Sb, Te, Po.

15. Coinage Metals Cu, Ag, Au (Group 11).

16. Noble Metals Au, Pt (Unreactive).

17. Rare Earth Elements Lanthanides + Sc + Y.

18. Transuranic Elements Elements after Uranium (Z > 92). All synthetic/radioactive.

19. Diagonal Relationship Li-Mg, Be-Al, B-Si. Similar properties due to similar charge/size ratio (ionic potential).

20. Anomalous behavior First member of each group differs due to small size, high EN, absence of d-orbitals.

21. Amphoteric Oxides Al2O3, ZnO, BeO, SnO2, PbO2. React with both acid and base.

22. Neutral Oxides CO, NO, N2O, H2O.

23. Acidic Oxides Non-metal oxides (CO2, SO3). Acidity increases with oxidation state.

24. Basic Oxides Metal oxides (Na2O, CaO).

25. Lanthanoid Contraction Decrease in size of Lanthanides due to poor shielding of f-electrons.

26. Consequence of Lan Contraction Zr and Hf have almost same size. Hard to separate.

27. Group 18 Valency Zero valency (usually).

28. Most Electropositive Group 1 (Alkali Metals).

29. Best Conductor Silver (Ag).

30. Highest Melting Point Tungsten (W) among metals. Carbon (Diamond) overall.

31. Lowest Melting Point Mercury (Hg).

32. Most Reactive Non-metal Fluorine.

33. Bridge Elements Period 3 elements.

34. Representative Elements s and p block (except noble gases).

35. Transition Elements d block (Incomplete d subshell). Zn, Cd, Hg are not considered transition elements.

36. Inner Transition Elements f block. Properties very similar.

37. Atomic Size Unit Picometer (pm) or Angstrom. 1 A = 100 pm.

38. Covalent Radius Half distance between two similar atoms in covalent bond.

39. Van der Waals Radius Half distance between non-bonded atoms in solid state. Always larger than covalent.

40. Isoelectronic Species Same number of electrons. Size decreases as Z increases.

41. Penetration Effect s > p > d > f. Energy to remove s electron is highest.

42. Shielding Effect s > p > d > f.

43. Successive IE Always increases as positive charge increases.

44. Zero Electron Affinity Noble Gases (stable octet). Be, Mg, N (stable config).

45. EA vs EgH EA is potential to gain e (positive sign conv). EgH is energy released (negative sign conv).

46. Hydration Energy Increases with charge/size ratio. Li+ has highest hydration.

47. Mobility in Solution Li+ (hydrated) is largest, so slowest. Cs+ (hydrated) is smallest, fastest.

48. Nature of Hydrides L to R: Basic -> Neutral -> Acidic.

49. Nature of Oxides L to R: Basic -> Amphoteric -> Acidic.

50. Triads Dobereiner's Triads (Li, Na, K). Historic attempt.

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