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Modern Periodic Table

Ch. 7Std 11

Easy Overview

The periodic table isn't just a chart — it's the ultimate chemistry cheat sheet. Once you understand its organization, you can predict almost anything about an element: size, reactivity, bond type, and compound formulas. Mendeleev (1869) arranged elements by atomic mass, leaving gaps for undiscovered elements. Modern periodic law (Moseley): properties are periodic functions of atomic number. The table has 18 groups, 7 periods, and 4 blocks (s, p, d, f). Periodic trends — atomic radius, ionization energy, electronegativity, electron affinity — are the real power. Across a period: radius decreases, IE increases, EN increases. Down a group: radius increases, IE decreases, EN decreases. These trends predict chemical behavior.

Mendeleev vs Modern Periodic Law

Mendeleev (1869): arranged 63 elements by atomic mass, left gaps for undiscovered, predicted properties (eka-Al=Ga, eka-Si=Ge). Anomalies: Te and I, Ar and K placed out of mass order. Modern (Moseley, 1913): properties periodic function of atomic number. Moseley's X-ray studies (√ν ∝ Z) proved Z is fundamental. Fixed all anomalies.

Structure of the Modern Periodic Table

18 groups, 7 periods. s-block (1-2): last e⁻ in s. p-block (13-18): last e⁻ in p. d-block (3-12): last e⁻ in d (transition metals). f-block: lanthanides and actinides below. Periods: 1 (2 elements), 2-3 (8), 4-5 (18), 6 (32 + lanthanides), 7 (incomplete + actinides). Group # = valence e⁻ for s,p blocks.

Blocks of the Periodic Table

s-block (Gr 1-2): ns¹ or ns², highly reactive metals (except H, He). p-block (Gr 13-18): ns²np¹⁻⁶, includes metals, metalloids, non-metals. d-block (Gr 3-12): (n-1)d¹⁻¹⁰ns⁰⁻², transition metals — variable oxidation states, colored compounds, catalysts. f-block: (n-2)f¹⁻¹⁴, lanthanides + actinides (all radioactive).

Atomic Radius — How Big Is an Atom?

Half the distance between nuclei of identical bonded atoms. Decreases across period (increasing Z_eff pulls e⁻ tighter): Na (186) > Mg (160) > Al (143) > Si (117) > P (110) > S (104) > Cl (99 pm). Increases down group (new shells): Li (152) < Na (186) < K (231) < Rb (244) < Cs (262 pm).

Ionization Energy — Removing an Electron

Energy to remove most loosely bound e⁻ from gaseous atom. Increases across period (higher nuclear charge): Na (496) < Mg (738) > Al (578) < Si (786) < P (1012) > S (1000) < Cl (1251) < Ar (1520 kJ/mol). Drops: Mg>Al (p e⁻ easier), P>S (pairing repulsion). Decreases down group: Li (520) > Na (496) > K (419) > Rb (403) > Cs (376).

Electronegativity — The Electron Attraction

Tendency to attract shared e⁻ in covalent bond. Pauling scale. Increases across period, decreases down group. F (4.0) > O (3.5) > N (3.0) > Cl (3.2) > Br (3.0) > I (2.7). ΔEN > 1.7 = ionic, 0.4-1.7 = polar covalent, <0.4 = nonpolar covalent. Least EN: Cs, Fr (0.7). EN predicts bond type and reactivity.

Electron Affinity — Gaining an Electron

Energy change when e⁻ added to neutral gaseous atom. Most exothermic (negative) for halogens (Cl: -349 kJ/mol) — achieve noble gas config. More negative EA across period, less negative down group. Anomalies: Group 2 (positive EA — e⁻ goes to higher p orbital), N (positive — half-filled p³), O (less negative than expected — e⁻-e⁻ repulsion in compact 2p).

Periodic Trends in Metallic Character

Metallic character ↓ across period (Na metal → Mg, Al → Si metalloid → P, S, Cl non-metals), ↑ down group (B metalloid → Al metal; C non-metal → Si, Ge metalloids → Sn, Pb metals). Correlates with low IE, low EN, large radius. Diagonal relationship: Li-Mg, Be-Al, B-Si (similar charge/size).

Trends in Chemical Reactivity

Metals: reactivity ↑ down group (Cs explodes in water, Li reacts slowly). Reactivity ∝ low IE. Non-metals: reactivity ↓ down group (F₂ terrifying, I₂ mild). Displacement: F₂ displaces all; Cl₂ displaces Br⁻, I⁻; Br₂ displaces I⁻. Across Period 3: Na (very reactive) → Mg, Al → Si → P, S, Cl (reactive non-metals) → Ar (inert).

Effective Nuclear Charge and Shielding

Z_eff = Z - σ (nuclear charge minus shielding constant). Across period: Z_eff increases significantly (same shielding, more protons) → smaller radius, higher IE. Down group: Z_eff increases slightly but new shells dominate → larger radius. s orbital penetrates more than p, which penetrates more than d → s electrons experience higher Z_eff.

Anomalous Trends — Why Exceptions Exist

Group 2 IE > Group 13 (Mg > Al — filled s² stable vs p¹ easier). Group 15 IE > 16 (P > S — half-filled p³ stable). O's EA less negative than S (2p electron repulsion in compact O). Lanthanide contraction: poor f-orbital shielding causes gradual radius decrease across lanthanides → post-lanthanides (Hf, Ta, W) similar to Period 5 counterparts (Zr, Nb, Mo).

Periodicity in Valence and Oxidation States

Max positive OS = group number for representative elements. Gr 1: +1. Gr 2: +2. Gr 13: +3. Gr 14: +4 (and +2, increasing down — inert pair effect). Gr 15: +5, +3, -3. Gr 16: +6, +4, -2. Gr 17: +7, +5, +3, +1, -1. Inert pair effect: heavier elements (Tl⁺, Pb²⁺, Bi³⁺) prefer lower OS — ns² e⁻ reluctant to participate.

Periodicity in Physical Properties

MP across Period 3: Na (98°C) < Mg (650) < Al (660) < Si (1414) > P (44) < S (115) > Cl (-102) > Ar (-189). Reflects bonding: metallic → network covalent (Si, high) → molecular covalent (P, S, Cl, Ar, low). Density: highest near center (Os, Ir). Conductivity: metals (Ag, Cu, Au best).

Prediction of Properties Using Periodic Trends

Position → size, IE, EN, metallic character, OS, formula of oxide, nature of oxide. Oxides: basic (left, Gr 1-2) → amphoteric (center, Al₂O₃) → acidic (right, SO₃, Cl₂O₇). Hydrides: Gr 1 (MH), Gr 14 (MH₄), Gr 15 (MH₃), Gr 16 (MH₂), Gr 17 (MH). Periodic trends predict properties of unknown elements.

d-Block Elements — Transition Metals

Last e⁻ in d orbital. Properties: variable OS (Fe²⁺/Fe³⁺, Mn²⁺→Mn⁷⁺), colored compounds (Cu²⁺ blue, Cr³⁺ green, MnO₄⁻ purple — d-d transitions), paramagnetism (unpaired d e⁻), catalytic (Fe in Haber, V₂O₅ in Contact, Pt in converters), complex formation ([Cu(NH₃)₄]²⁺). Atomic radii decrease slowly (d contraction). High MPs (W: 3422°C).

f-Block Elements and Lanthanide Contraction

Last e⁻ in f orbital. Lanthanides (Ce-Lu): similar properties (deep 4f orbitals), +3 OS predominantly. Lanthanide contraction: gradual radius decrease due to poor f shielding. Consequence: post-lanthanides (Hf, Ta, W, etc.) nearly same size as Period 5 counterparts (Zr, Nb, Mo) → they're very difficult to separate. Actinides (Th-Lr): all radioactive, only Th, Pa, U natural.

Key Points

  • •Modern periodic law: properties periodic function of atomic number
  • •18 groups, 7 periods; s, p, d, f blocks based on last electron orbital
  • •Moseley: √ν ∝ Z — Z is fundamental, fixed Mendeleev's anomalies
  • •Atomic radius: decreases across period, increases down group
  • •IE: increases across period, decreases down group
  • •EN: increases across period, decreases down group (F=4.0 highest)
  • •EA: most negative for halogens; anomalies at Gr 2, 15, and O
  • •Metallic character: decreases left→right, increases top→bottom
  • •Z_eff explains all trends: higher Z_eff = smaller radius, higher IE
  • •Inert pair effect: heavier p-block prefer lower OS
  • •Diagonal relationship: Li-Mg, Be-Al, B-Si
  • •d-block: variable OS, colored, paramagnetic, catalytic
  • •Lanthanide contraction: poor f shielding → post-lanthanides same size as Period 5
  • •Oxide nature: basic (left) → amphoteric → acidic (right)
  • •Noble gases: full shell, very high IE, nearly inert

Practice Questions

  • State modern periodic law. How is it different from Mendeleev's?
  • Explain trends in atomic radius and IE across Period 3 and down Group 1.
  • Why is IE of N higher than O? Why is Mg's IE higher than Al's?
  • Arrange F, Cl, Br, I, O, N in decreasing EN.
  • Explain inert pair effect for Tl, Pb, Bi.
  • What are s, p, d, f blocks? Give two examples each.
  • What is lanthanide contraction? Consequences on post-lanthanide elements?
  • Predict formula and nature of oxides for Groups 1, 2, 13, 14, 16.
  • Explain: (i) EA of O less negative than S (ii) IE of Al lower than Mg.