9

Periodic Trends

The periodic table organizes and rationalizes the diverse physical and chemical properties of the elements through systematic variations with atomic number

Introduction to Periodicity

The periodic table helps show that the properties of elements vary reasonably systematically with atomic number. Once these trends and patterns are recognized, the detailed properties of elements no longer seem like random, unrelated facts.

Fundamental Principle Almost all trends in elemental properties can be traced to electronic configuration and atomic radii, and their variation with atomic number.

The Periodic Table Structure

Block Classification

s-block Groups 1-2: ns¹⁻²

p-block Groups 13-18: ns²np¹⁻⁶

d-block Groups 3-12: (n-1)d¹⁻¹⁰ns⁰⁻²

f-block Lanthanoids/Actinoids: (n-2)f¹⁻¹⁴

Major Trends Summary

→ Across period: ↑ Zeff, ↓ radius, ↑ IE, ↑ EN
↓ Down group: ↑ radius, ↓ IE, ↓ EN (usually)

9.1 Valence Electron Configurations

Key Point The valence electron configuration can be predicted from position in the periodic table. The (n−1) orbitals fill in the d block; (n−2) orbitals fill in the f block.

Main Group (s and p blocks)

Group12131415161718
Config ns¹ns²ns²np¹ns²np²ns²np³ns²np⁴ns²np⁵ns²np⁶

d-Block (Period 4 example)

Group3456789101112
ElementScTiVCrMnFeCoNiCuZn
Config 3d¹4s²3d²4s²3d³4s²3d⁵4s¹3d⁵4s²3d⁶4s²3d⁷4s²3d⁸4s²3d¹⁰4s¹3d¹⁰4s²
Half-filled and Full Subshell Stability

Note Cr (3d⁵4s¹) and Cu (3d¹⁰4s¹) — half-filled and full d subshells are energetically favored due to exchange energy (spin correlation).

9.2 Atomic Parameters

Atomic Radii
Ionization Energy
Electronegativity
Atomization

Atomic Radii

Key Point Atomic radii increase down a group and decrease left to right across a period (due to increasing Zeff).

Main Group Trends

  • Li (152 pm) → Na (154 pm) → K (227 pm)
  • C (77 pm) → Si (117 pm) → Ge (122 pm)
  • Small increase C→Si→Ge due to d-block intervention

d-Block Trends

  • Decrease across due to poor d-electron shielding
  • 5d radii ≈ 4d radii (lanthanoid contraction)
  • Hf smaller than Zr despite being in Period 6
Lanthanoid Contraction

The filling of poorly shielding 4f orbitals causes a cumulative increase in Zeff, making 5d elements much smaller than expected. This results in similar radii for 4d and 5d congeners (e.g., Zr = 159 pm, Hf = 156 pm).

Ionization Energy

Key Point Ionization energy increases across a period and decreases down a group. Correlates strongly with atomic radius.
ElementIE₁ (kJ/mol)Notes
Li513Low — easily loses 2s¹
Be899Higher — full 2s²
B801Lower than Be — loses 2p¹
N1402High — half-filled 2p³
O1314Lower than N — paired electron
F1681Highest Period 2 (except Ne)
Alternation Effect

In Group 13: IE(Ga) > IE(Al) due to intervention of 3d subshell increasing Zeff. Similar effects in Groups 14 and 15.

Electronegativity (Pauling Scale)

Key Point Electronegativity increases across a period and decreases down a group. F is most electronegative (3.98).
Elementχ (Pauling)Elementχ (Pauling)
F3.98Cl3.16
O3.44N3.04
C2.55H2.20
Na0.93Cs0.79

Alternation Effect Examples

Electronegativities showing anomalies:

  • Al (1.61) < Ga (1.81) — 3d intervention
  • In (1.78) < Tl (2.04) — 4f intervention

Enthalpies of Atomization

Key Point Atomization enthalpies increase then decrease across periods (maximum at C, Si in p-block; Groups 5-6 in d-block). They decrease down groups in s/p blocks but increase in d block.
ElementΔaH° (kJ/mol)ElementΔaH° (kJ/mol)
C715Si439
N473P315
Cr398Mo651
W844Re791
d-Block Maximum

Maximum atomization enthalpy occurs at Groups 5-6 where maximum unpaired electrons are available for bonding. W has the highest melting point (3410°C) among elements except C.

9.3 Occurrence

Key Point Hard-hard and soft-soft interactions help systematize the terrestrial distribution of elements (Goldschmidt classification).

Goldschmidt Classification

Lithophiles

Found in Earth's crust (lithosphere) in silicate minerals

Examples: Li, Mg, Ti, Al, Cr

Hard cations + hard O²⁻

Chalcophiles

Found with sulfide, selenide, telluride minerals

Examples: Cd, Pb, Sb, Bi, Zn

Soft cations + soft S²⁻

Siderophiles

Occur mainly in elemental state

Examples: Pt, Pd, Ru, Rh, Os

Intermediate hardness

Atmophiles

Occur as gases

Examples: H, N, noble gases

Volatile elements

9.4 Metallic Character

Key Point Metallic character decreases across a period and increases down a group. Related to ability to lose electrons and form metallic bonding.
→ Across period: Metallic → Metalloid → Nonmetallic
↓ Down group: Increasing metallic character

Group 15 Example

ElementCharacterProperties
NNonmetalDiatomic gas N₂
PNonmetalSeveral allotropes (white, red, black)
AsMetalloidMetal, metalloid, and nonmetal allotropes
SbMetalloid/MetalBrittle, poor conductor
BiMetalLow melting, poor conductor

p-Block Allotropes

ElementAllotropes
CDiamond, graphite, fullerenes, amorphous
ODioxygen (O₂), ozone (O₃)
PWhite, red, black
SMany catenated rings, chains, amorphous
SnGrey (α), white (β)

9.5 Oxidation States

Main Group Elements

Key Point Group oxidation number is achieved for Groups 1-3 by electron loss. Groups 15-17 can achieve negative oxidation states by electron gain. The inert-pair effect stabilizes oxidation states 2 less than group number for heavier p-block elements.
GroupGroup Ox. StateInert-Pair StateExamples
13+3+1Tl(I) most common for Tl
14+4+2Pb(II) more stable than Pb(IV)
15+5+3Bi(III) dominant; BiF₅ unknown
Inert-Pair Effect

The relative stability of an oxidation state 2 less than the group number increases down a group. Attributed to relativistic stabilization of ns² electrons and weak M–X bonds for heavier elements.

d-Block Elements

Key Point Group oxidation state achieved only for Groups 3-8. Requires highly oxidizing F or O. Higher oxidation states become more stable descending a group.

3d-Series Oxidation States

ElementConfigCommon Oxidation States
Scd¹s²+3
Tid²s²+4 +3 +2
Vd³s²+5 +4 +3 +2
Crd⁵s¹+3 +6 +2
Mnd⁵s²+2 +7 +4
Fed⁶s²+3 +2
Cod⁷s²+2 +3
Nid⁸s²+2
Cud¹⁰s¹+2 +1
Znd¹⁰s²+2

Highest Halides by Group

Group3d4d5d
4TiI₄ZrI₄HfI₄
5VF₅NbI₅TaI₅
6CrF₅MoCl₆WBr₆
7MnF₄TcCl₆ReF₇

9.6-9.8 Periodic Characteristics of Compounds

Coordination Numbers

Key Point Coordination numbers increase down a group as central atom radius increases. 4d/5d elements often have higher CN than 3d congeners.
Group 15 Coordination Numbers

N: CN = 3 (NF₃), 4 (NH₄⁺)

P: CN = 3 (PF₃), 4, 5 (PF₅), 6 (PF₆⁻)

Higher CN for P due to larger radius allowing more atoms around central atom

Bond Enthalpy Trends

p-Block: Lone Pair Effect
N–N163 kJ/mol
P–P201 kJ/mol
As–As180 kJ/mol

Period 2 single bonds weaker due to lone pair repulsion

d-Block: Increases Down Group
Cr–H258 kJ/mol
Mo–H282 kJ/mol
W–H339 kJ/mol

d orbitals become more effective for bonding

Binary Compounds

Hydrides
Oxides
Halides

Classification of Hydrides

Saline

Groups 1, 2 (except Be)

Ionic, high mp

Molecular

Groups 13-17

Covalent, gases

Metallic

Groups 3-5, f-block

Nonstoichiometric

Oxide Acid-Base Character

→ Across period: Basic → Amphoteric → Acidic
TypeCharacterExamples
Metal oxidesBasicNa₂O, BaO, MgO
Metalloid oxidesAmphotericAl₂O₃, ZnO
Nonmetal oxidesAcidicSO₃, P₄O₁₀, CO₂

High oxidation state d-metal oxides (OsO₄, RuO₄) are molecular and covalent, used as selective oxidizing agents.

Halide Character

  • s-block halides: predominantly ionic (except Li, Be)
  • p-block fluorides: predominantly covalent
  • d-block: low ox. state → ionic; high ox. state → covalent

Example: TiF₄ (mp 284°C) vs TiCl₄ (bp 136°C) — covalent character increases with heavier halogens

9.10 Anomalous Nature of First Members

Key Point First members of each group show different behavior due to: small atomic radius, high ionization energy, high electronegativity, lack of low-lying d orbitals, and limited coordination numbers.

Period 2 Anomalies

PropertyPeriod 2Lower Periods
Max CN4 (octet rule)5, 6+ (hypervalent)
Multiple bondingStrong (C=C, N≡N)Weak (Si=Si rare)
CatenationExtensive (C)Limited
H-bondingStrong (N, O, F)Weak

Diagonal Relationships

Elements at the head of groups show diagonal relationships with elements to their lower right, due to similar atomic radii and electronegativities.

Li ↘ Mg

Both form covalent compounds

Be ↘ Al

Covalent hydrides/halides

B ↘ Si

Flammable gaseous hydrides

Noble Character (Group 11 and Pt Metals)

Platinum and Coinage Metals

Cu, Ag, Au and the platinum metals (Ru, Rh, Pd, Os, Ir, Pt) are resistant to oxidation due to strong intermetallic bonding and high ionization energies.

  • Not oxidized by H⁺ under standard conditions
  • Aqua regia (3:1 HCl:HNO₃) oxidizes Au and Pt
  • Au: Au + NO₃⁻ + 4Cl⁻ + 4H⁺ → [AuCl₄]⁻ + NO + 2H₂O

Z + 8 Relationships

Similarities exist between p-block elements (Z) and d-block elements (Z+8) with same number of valence electrons:

Zp-blockZ+8d-blockSimilarity
13Al21Sc3 valence e⁻, similar E°
16S24CrSO₄²⁻ and CrO₄²⁻
17Cl25MnClO₄⁻ and MnO₄⁻

Chapter Summary

Across a Period
  • ↑ Effective nuclear charge
  • ↓ Atomic radius
  • ↑ Ionization energy
  • ↑ Electronegativity
  • ↓ Metallic character
  • Basic → acidic oxides
Down a Group
  • ↑ Atomic radius
  • ↓ Ionization energy
  • ↓ Electronegativity (usually)
  • ↑ Metallic character
  • ↑ Coordination number
  • ↑ Stability of low ox. states (inert pair)

Special Effects

EffectCauseConsequence
Lanthanoid contractionPoor 4f shielding5d radii ≈ 4d radii
Alternation effect3d/4f interventionAnomalous χ, IE trends
Inert-pair effectRelativistic ns² stabilization+n-2 states stable for heavy p-block
Diagonal relationshipsSimilar radii/χLi~Mg, Be~Al, B~Si