Concept 03 / 05Classification of Elements and Periodicity
Valence, Oxidation States, Metallic Character, Anomalous Second-Period Elements and Diagonal Relationship
Valence and its periodicity, common oxidation states across groups, the variation of metallic and non-metallic character and reactivity, why second-period elements behave anomalously (small size, no d orbitals), and the diagonal relationships Li-Mg, Be-Al and B-Si.
Valence is the combining capacity of an element, commonly the number of valence electrons (for groups to and to ) or minus that number (for groups to when forming hydrides and anions). It is periodic. In period the valence towards hydrogen goes (NaH, , , , , , HCl), while the maximum valence towards oxygen rises to (). The maximum oxidation state of a main-group element usually equals its group number (for to : group number minus ).
Heavier p-block elements show the inert pair effect: the electrons become reluctant to participate, so oxidation states two lower than the group maximum become more stable down the group. Tl(I) is more stable than Tl(III), and Pb(II) more stable than Pb(IV), which makes a strong oxidising agent.
Metallic character (tendency to lose electrons) decreases across a period and increases down a group, following ionisation enthalpy. Non-metallic character does the reverse. The boundary between metals and non-metals runs diagonally across the p-block, with metalloids (B, Si, Ge, As, Sb, Te) along it. Reactivity is highest at the two extremes of each period: the alkali metals (strongest reducing agents) and the halogens (strongest oxidising agents).
Anomalous behaviour of second-period elements. The first element of each group (Li, Be, B, C, N, O, F) differs markedly from the rest of its group because of
very small size and high charge density,
high electronegativity,
absence of d orbitals in the valence shell, so the maximum covalency is (using and only), and
the ability to form strong - multiple bonds (C=C, C=O, N≡N, O=O).
Examples: nitrogen exists as with a triple bond while phosphorus is ; nitrogen forms no although exists; boron forms only while aluminium forms ; oxygen forms hydrogen-bonded water, a liquid, while is a gas.
Diagonal relationship. Some second-period elements resemble the third-period element of the next group (diagonally below and to the right): Li with Mg, Be with Al, B with Si. Moving right increases charge and decreases size; moving down increases size. The two effects roughly cancel, giving similar charge-to-size ratios (ionic potential) and electronegativity.
Li and Mg: both form nitrides (, ) by direct combination; carbonates and nitrates decompose on heating to oxides; their chlorides are deliquescent and hydrated; both form normal oxides rather than peroxides.
Be and Al: both oxides and hydroxides are amphoteric; both are rendered passive by nitric acid; their chlorides are covalent, Lewis acidic and form bridged dimers.
B and Si: both are metalloids, form covalent hydrides and acidic oxides, and form borates and silicates.
Key Takeaways:
- Maximum oxidation state usually equals the group number (minus for the p-block).
- Inert pair effect: lower oxidation states stable for heavy p-block elements (Tl(I), Pb(II), Bi(III)).
- Metallic character: decreases across, increases down.
- Second-period anomalies: small size, no d orbitals (covalency ≤ 4), strong - bonds.
- Diagonal pairs Li-Mg, Be-Al, B-Si from similar charge-to-size ratios.
- Pitfall: expecting nitrogen to form by analogy with .
Brain-Teaser 1: Why does lithium form on burning in air while sodium does not form a nitride?
Solution: The small, highly polarising ion gives a very high lattice energy, enough to compensate for breaking the triple bond and forming . The larger gives much smaller lattice energy, so the nitride is not stable. Magnesium, diagonally related to lithium, also burns to a nitride, which is the diagonal relationship at work.
Brain-Teaser 2: Explain why is unstable and decomposes to , while and are stable.
Solution: Down group the inert pair effect grows: the pair of lead is held tightly (poor shielding by and electrons increases its effective nuclear charge, and relativistic effects contract it), so using all four electrons becomes energetically unfavourable. Pb(II) is more stable than Pb(IV), and . For C and Si the state is the stable one.
Brain-Teaser 3: Beryllium hydroxide dissolves in both HCl and NaOH. Write the reactions and name the property. Which other element behaves similarly?
Solution: and . It is amphoteric. Aluminium hydroxide behaves the same way (forming in base), reflecting the Be-Al diagonal relationship. Other group hydroxides are purely basic.
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