Periodic trends and the s block

MDCAT Chemistry · Chemistry · s and p block elements. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

In the modern periodic table the elements are arranged in order of proton (atomic) number. The period number equals the number of electron shells. The group is fixed by the outer electrons, and the subshell being filled gives the block: s, p, d or f.

Most trends come from one idea: how strongly the nucleus holds the outer electrons. Across a period the nuclear charge rises while the shielding stays almost the same, so the hold gets stronger. Down a group there are more shells and more shielding, so the hold gets weaker.

The s block metals (groups 1 and 2) lose their outer electrons easily. They are strong reducing agents, and they become more reactive down the group.

Rules to remember

Across a period: atomic radius decreases; ionisation energy, electronegativity and the energy released on gaining an electron generally increase. Down a group: the opposite.
Exceptions: first ionisation energy Al < Mg (3p electron, shielded by 3s) and S < P (paired 3p electron is repelled). Cl releases more energy than F on gaining an electron. Adding a second electron (O⁻ → O²⁻) absorbs energy.
Period 3 melting points: rise Na < Mg < Al (more delocalised electrons), highest at Si (giant covalent), then low for the small molecules: S₈ > P₄ > Cl₂ > Ar.
A positive ion is smaller than its atom and a negative ion is larger. For ions with the same number of electrons, more protons means a smaller ion.
Burning in excess oxygen: Li gives Li₂O, Na gives Na₂O₂ (peroxide), K gives KO₂ (superoxide). Only Li in group 1 forms a nitride, Li₃N.
Down group 2: hydroxides become more soluble and more basic; sulphates become less soluble (BaSO₄ least); carbonates, nitrates and hydroxides need stronger heating to decompose. BeO is amphoteric.
Flame colours: Li crimson, Na golden yellow, K lilac, Ca brick red, Sr crimson, Ba apple green. Be and Mg give no colour.

Common mistake

Students say atoms get bigger across a period because electrons are added. The electrons go into the same shell while the nuclear charge rises, so the atoms get smaller.

Worked example

Arrange Na, Mg, Al and Si in order of increasing first ionisation energy.

All four are in period 3, so the general trend is an increase from Na to Si as the nuclear charge rises.
Exception: Al loses a 3p electron, which is higher in energy and shielded by the 3s pair, so Al is below Mg.
Si has one more proton than Al and also loses a 3p electron, so Si is above Al, and it is above Mg too.
Values in kJ mol⁻¹ confirm this: Na 496, Al 578, Mg 738, Si 786.

Answer: Na < Al < Mg < Si.

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