Explain that elements of the periodic table show trends in chemical and physical properties across periods and down groups as exemplified by groups 1, 2, 13 –18 and period 3.

A student measures the atomic radii of elements in period 2 and finds that fluorine (atomic number 9) has a smaller atomic radius than carbon (atomic number 6). Explain this observation in terms of nuclear charge and electron attraction.

Explain why the first ionisation energy of sulfur (999 kJ mol⁻¹) is lower than that of phosphorus (1,012 kJ mol⁻¹), even though sulfur is positioned to the right of phosphorus in period 3.