Atomic and physical trends

From Na to Cl the nuclear charge increases. Added electrons enter the third shell, so the shielding of these outer electrons changes relatively little. Stronger effective attraction generally draws the outer shell closer: atomic radius decreases, while first ionisation energy and electronegativity generally increase.

Explain the two first-ionisation-energy exceptions using orbitals. Al loses a higher-energy 3p electron rather than Mg's 3s electron. S loses a paired 3p electron, with extra electron-electron repulsion, whereas P has three singly occupied 3p orbitals. The resulting dips do not mean the general nuclear-charge trend is wrong.

Down Cl to I, the outer shell is farther from the nucleus and is more shielded. These effects outweigh the increased nuclear charge: atomic radius increases, first ionisation energy decreases and electronegativity decreases. The number of outer electrons stays seven.

For ionic radii, compare species with similar electron arrangements. Na⁺, Mg²⁺ and Al³⁺ each have ten electrons: increasing nuclear charge contracts this isoelectronic series. P^3-, S^2- and Cl^- each have eighteen; their radii also decrease in that order. Crossing from a cation series to an anion series introduces a large jump, so ionic radius does not form one smooth decreasing line across Period 3. Down Cl^-, Br^-, I^-, more occupied shells give larger ions.

The molecular comparison uses white phosphorus, P₄. S₈ has a larger, more easily polarised electron cloud than P₄, so sulfur has stronger intermolecular attractions and a higher melting point. Cl₂ is much smaller and melts much lower. Other allotropes can have different structures; an element name alone does not specify an allotrope.

Na, Mg and Al conduct electricity through mobile delocalised electrons. Conductivity generally increases from Na to Al: each atom supplies one, two and three valence electrons respectively, giving a greater density of mobile charge carriers. The ions remain in the solid lattice. Silicon's covalent network has far fewer mobile carriers at room temperature, so it is a semiconductor with much lower conductivity than the metals. P₄, S₈ and Cl₂ lack mobile charged particles and are electrical insulators in their pure forms.

Down the group, the molecules have more electrons and more polarisable electron clouds. Instantaneous dipole-induced dipole attractions become stronger, so more energy is needed to separate molecules and volatility decreases. This is an intermolecular explanation; do not use the X-X covalent bond strength to explain boiling.