3.2 Inorganic Chemistry · Year 12
3.2.1 Periodicity
Explain Period 3 trends using electron structure, bonding and the forces between particles.
What you need to know
Open a line for a quick recap. If it feels obvious, move straight to the linked practice.
3.2.1.1 Use proton number and periodic-table position to classify an element as belonging to the s, p, d or f block. Quick revision
The block tells you which subshell receives the highest-energy electron in the ground-state atom. Use the element’s proton number to locate it in the periodic table, then identify the s, p, d or f region.
For example, Na ends in 3s¹ so it is s-block, Cl ends in 3p⁵ so it is p-block, and Fe lies in the first-row d block. Do not classify an element from the charge on one particular ion.
3.2.1.2 Describe how atomic radius, first ionisation energy and melting point change across Period 3 from Na to Ar. Quick revision
Across Period 3, atomic radius generally decreases and first ionisation energy generally increases. The melting-point pattern is different: it rises through the metals to very high-melting Si, then drops sharply for the molecular/atomic elements.
Keep those three trends separate in your head. Radius and ionisation energy are mainly electronic trends; melting point follows the structure and bonding of the element.
3.2.1.2 Connect Period 3 trends with the changing structures and bonding of the elements. Quick revision
Na, Mg and Al have giant metallic structures, Si is giant covalent, P₄, S₈ and Cl₂ are simple molecular, and Ar is monatomic. That sequence of structures explains why the melting-point graph changes shape across the period.
When a question asks you to connect trend and structure, name the particles and the attraction being overcome. “Bonding gets weaker” is too vague across a period containing metallic, covalent and intermolecular attractions.
3.2.1.2 Account for Period 3 atomic-radius and first-ionisation-energy trends using nuclear attraction, shielding and electron arrangement. Quick revision
For the Period 3 radius and first-ionisation-energy trends, keep nuclear attraction, shielding and electron arrangement linked. Radius falls as proton number rises across the same main shell, so first ionisation energy generally rises as well.
Account for the two familiar dips as well: Al loses a higher-energy 3p electron than Mg loses from 3s, and S has a paired 3p electron with extra electron–electron repulsion compared with P.
3.2.1.2 Explain Period 3 melting-point changes by identifying the structure and bonding that must be overcome. Quick revision
Classify the structure before you explain the Period 3 melting points. Metals, giant covalent silicon and simple molecular or atomic substances melt for different reasons, so use the bonds or forces that are actually overcome in each part of the period.
Metallic bonding strengthens from Na to Al as ionic charge rises and radius falls; Si has many strong covalent bonds throughout a giant network. P₄, S₈ and Cl₂ melt by overcoming London forces, with S₈ highest because its larger electron cloud is more polarisable.