Periodic Table blocks and periodic trendsEdexcel International A Level Chemistry: Revision notes

The Periodic Table is divided into blocks according to the sub-shell that holds the highest-energy electron:

• s block: Groups 1 and 2 (and helium)
• p block: Groups 3 to 7 and 0
• d block: the transition elements between Groups 2 and 3, e.g. Sc to Zn

Sub-shell capacities: s holds 2 electrons, p holds 6, d holds 10. So shell 1 holds up to 2, shell 2 up to 8, shell 3 up to 18 (3s, 3p, 3d), and shell 4 starts with 4s and 4p before 4d and 4f.

A periodic property is one that shows a repeating pattern across each period as atomic number increases. First ionisation energy, atomic radius and melting temperature are periodic properties.

When the first ionisation energies of elements 1 to 36 are plotted, the pattern repeats: a peak at each noble gas and a minimum at each Group 1 element.

Successive ionisation energies cover a huge range, so graphs plot log₁₀(ionisation energy). For magnesium, log₁₀ values are 2.87, 3.16, 3.89, and so on; the large jump after the 2nd electron shows two outer electrons, and the jump after the 10th shows eight electrons in the second shell, so the structure is 2,8,2.

The trend follows the structure and bonding:

• Na, Mg, Al are metals. Melting temperature rises because the cation charge increases (1+, 2+, 3+) and there are more delocalised electrons, so metallic bonding is stronger.
• Si is a giant covalent lattice. Many strong covalent bonds must be broken, so it has the highest melting temperature.
• P₄, S₈, Cl₂ are simple molecules. Only weak London forces between molecules are overcome, so melting temperatures are low. S₈ is the largest molecule, so it has the highest value of the three.
• Ar is monatomic with very weak London forces: the lowest value.

The same ideas apply in Period 2:

• Li and Be are metals; Be melts much higher because Be²⁺ ions are smaller and two electrons per atom are delocalised.
• Carbon (diamond or graphite) is giant covalent, with the highest melting temperature.
• N₂, O₂, F₂ are simple molecules with low boiling temperatures.
• Ne is monatomic and has the lowest boiling temperature.

The first ionisation energy generally increases across a period: nuclear charge increases, electrons are added to the same shell, so the atomic radius decreases and the outer electron is attracted more strongly.

Two dips occur in each of Periods 2 and 3:

• Group 3 lower than Group 2 (B lower than Be, Al lower than Mg): the outer electron is in a p sub-shell, higher in energy than s.
• Group 6 lower than Group 5 (O lower than N, S lower than P): the outer electron comes from a p orbital containing two electrons, so repulsion between the pair makes it easier to remove.

Down a group the first ionisation energy decreases. The outer electron is in a shell further from the nucleus, and there is more shielding from the inner shells. These two effects outweigh the increased nuclear charge, so the attraction on the outer electron is weaker.

Example: Group 2 values fall from Be 900 to Ba 503 kJ mol⁻¹.