Part 5: Periodicity across Periods 2 and 3

Connect repeated outer configurations to radius and ionisation-energy patterns, then use structure and bonding to explain melting and boiling data.

A repeating electronic pattern creates periodicity

Periodicity means a repeating pattern in properties when elements are ordered by atomic number. Period 2 runs Li, Be, B, C, N, O, F, Ne; Period 3 runs Na, Mg, Al, Si, P, S, Cl, Ar. Across each, the outer configuration progresses from ns¹ through ns²np⁶. The next period begins with an electron in a new shell.

Li and Na therefore share the outer pattern ns¹; Be and Mg share ns². Their reactions are related even though the shell number, atomic size and attraction differ. Similar configurations explain repeated chemical behaviour, while the change in principal shell explains why repetitions are not numerically identical.

Compare like quantities across a period

Atomic radius generally decreases across Periods 2 and 3 because nuclear charge increases and the outer electrons remain in the same principal shell with broadly similar inner-shell shielding. Across a whole period, first ionisation energy generally increases for the same reason, with the two subshell/pairing dips taught in Part 4.

At the start of the next period, the outer electron enters a new shell. Radius increases sharply and first ionisation energy falls. Do not use ionic radii in an atomic-radius graph or compare a covalent radius with a van der Waals radius as though they were measured identically. Radius data depend on the definition; use the type supplied in the question.

To sketch the first-ionisation pattern from memory, start with a low alkali-metal value, a broad rise to the noble gas, a dip from Be to B or Mg to Al, and a dip from N to O or P to S. Label element/atomic-number and energy axes. A smooth steadily rising line would omit evidence for subshell structure.

A melting trend requires the actual structure

Melting is not electron removal. First identify the particles and interactions holding the solid together. Sodium, magnesium and aluminium are giant metallic structures; silicon is a giant covalent structure; white phosphorus, sulfur and chlorine are molecular; argon is monatomic. Different interactions must be overcome, so no single ‘more protons’ explanation fits the entire period.

From Na to Mg to Al, the metal ions have greater charge and there are more delocalised electrons per atom; smaller cation size also increases attraction. Metallic bonding is generally stronger and the melting temperatures increase overall. The detailed values depend on crystal structure, so Mg and Al are close; do not invent a perfectly smooth trend.

Silicon has many strong covalent bonds extending throughout a giant network. Substantial energy is needed to disrupt that network, giving a high melting temperature. On moving to molecular phosphorus there is a large fall: melting overcomes intermolecular attractions between P₄ molecules, while the covalent bonds within each molecule remain intact.

Explain differences within the molecular region

S₈ has 128 electrons per molecule, P₄ has 60 and Cl₂ has 34. Larger, more polarisable electron clouds generally give stronger London attractions, so sulfur’s melting and boiling temperatures exceed those of white phosphorus, and chlorine has much lower values. Argon’s small individual atoms have very weak London attractions. Packing also affects melting, so inspect supplied data rather than treating electron count as a universal numerical law.

Boiling molecular substances also separates molecules and overcomes intermolecular attractions; it does not normally break their internal covalent bonds. Giant-network and metallic boiling involves much more extensive disruption of strong bonding. State which process is being discussed and which particles separate.

Transfer the explanation to Period 2 and unfamiliar data

Lithium and beryllium are metals. Boron and carbon have giant covalent structures and very high temperatures for disruption of their networks. Nitrogen, oxygen and fluorine are small diatomic molecules with weak London attractions and low boiling temperatures; neon is monatomic. Carbon allotropes and pressure affect the precise transition observed, so use the form and data given rather than claiming that every carbon sample has one simple melting point.

Worked interpretation: a supplied table gives a solid a very high melting temperature, no conductivity as a solid and no conductivity when molten. A giant covalent network is plausible because strong bonds resist disruption and there are no freely mobile charged particles. A high-melting material that is non-conducting when solid but conducts when molten instead suggests a giant ionic lattice. One property alone rarely identifies a structure uniquely.

For a graph, describe the actual rise, fall or anomaly before explaining it. Quote appropriate elements or a numerical comparison from the data, then identify the structure, relevant bond/force and energy needed. Pearson 9CH0/01 June 2023 Q4 and its report distinguish this reasoning from discussing ionisation energy or calling sodium a molecule.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official Edexcel mark allocations.

Sources