Periodicity of atomic radius, ionisation energy and melting temperature across Periods 2 and 3, the s, p and d blocks, and the trends explained by electronic structure and nuclear charge.

What this topic is asking

Eduqas topic C1.6 covers periodicity: the repeating pattern of properties across periods, focusing on atomic radius, first ionisation energy and melting temperature across Periods 2 and 3. You must describe each trend and, crucially, explain it in terms of electronic structure, nuclear charge, shielding and the type of structure and bonding.

Blocks of the periodic table

Trend in atomic radius

Across a period, atomic radius decreases. Each successive element has one more proton, so the nuclear charge increases, but the added electron enters the same outer shell, so shielding barely changes. The stronger net attraction pulls the outer shell closer to the nucleus, so the atoms get smaller from left to right.

Trend in first ionisation energy

First ionisation energy rises across a period for the same reason: greater nuclear charge and smaller radius hold the outer electrons more tightly. The trend is not smooth, however; Eduqas expects the two characteristic dips to be explained.

Trend in melting temperature

Melting temperature across Period 3 reflects the structure of each element. The metals (Na, Mg, Al) have rising melting points as the metallic bonding strengthens (more delocalised electrons, smaller ions). Silicon, a giant covalent solid, has the highest melting point of all because covalent bonds must be broken. Then the simple molecular elements ($\text{P}_4$, $\text{S}_8$, $\text{Cl}_2$) and monatomic argon have low melting points set only by weak van der Waals forces.

Why periodicity matters

Examples in context

Example 1. Reactivity trends follow ionisation energy. Group 1 metals become more reactive down the group because the outer electron is further from the nucleus and more shielded, so the first ionisation energy falls and the electron is lost more readily.

Example 2. The diagonal high point of silicon. Across every short period the melting-temperature maximum sits at the Group 4 element, because that is where the giant covalent structure appears (carbon in Period 2, silicon in Period 3), a clear illustration of structure-controlled periodicity.

Try this

Q1. State and explain the trend in atomic radius down Group 2. [2 marks]

• Cue. Atomic radius increases down the group because each element has an additional electron shell, increasing shielding and distance from the nucleus, which outweighs the rising nuclear charge.

Q2. Identify the block of the periodic table for an element with configuration $[\text{Ar}]3\text{d}^6 4\text{s}^2$. [1 mark]

• Cue. The d-block (its highest-energy electrons are in the $3\text{d}$ sub-shell); this element is iron.