Classification of Elements and Periodicity - Quick Revision
Genesis of classification
- Dobereiner's triads: middle element's atomic weight ~ average of the other two (e.g., Li/Na/K).
- Newlands' Law of Octaves: every 8th element resembles the first (valid only up to Ca).
- Mendeleev's Periodic Law: properties are a periodic function of atomic weights. Left gaps (Eka-aluminium = Ga, Eka-silicon = Ge) and predicted their properties.
- Moseley (1913): atomic number, not atomic mass, is the fundamental property.
- Modern Periodic Law: physical and chemical properties are periodic functions of atomic numbers.
Structure of the long form table
- 7 periods (rows), 18 groups (columns, IUPAC 1-18). Period number = highest principal quantum number n.
- Elements per period: 2, 8, 8, 18, 18, 32, (32). Number per period = 2 x orbitals being filled.
- Blocks: s (Gp 1-2, ns1-2), p (Gp 13-18, ns2np1-6), d (Gp 3-12, (n-1)d1-10 ns0-2), f (lanthanoids Ce-Lu, actinoids Th-Lr, (n-2)f1-14 (n-1)d0-1 ns2).
- s+p = representative/main group elements; d = transition; f = inner-transition. Metals > 78% of elements. Metalloids (Si, Ge, As, Sb, Te) border the zig-zag line.
IUPAC nomenclature for Z > 100
- Roots: 0 nil, 1 un, 2 bi, 3 tri, 4 quad, 5 pent, 6 hex, 7 sept, 8 oct, 9 enn; add 'ium'. e.g., 120 = unbinilium (Ubn).
Periodic trends (across period -> ; down group v)
- Atomic/ionic radius: decreases ->, increases v. Cation < parent atom; anion > parent atom.
- Ionization enthalpy: increases ->, decreases v. Always positive; IE2 > IE1.
- Electron gain enthalpy: more negative ->, less negative v. Halogens most negative; noble gases positive.
- Electronegativity: increases ->, decreases v (Pauling: F = 4.0, highest).
- Valence: = no. of outermost electrons or 8 minus it.
- Chemical reactivity highest at the two extremes of a period, lowest at centre. Oxides: basic (left) -> amphoteric/neutral (centre) -> acidic (right).