Periodic trends and chemical reactivity

Every chemical and physical property shows up the electronic configuration. Across a period radii shrink, ionization enthalpy mostly rises and electron gain enthalpy becomes more negative.

So reactivity is highest at the two ends of a period and lowest in the middle: on the left by losing an electron to form a cation (alkali metals), on the right by gaining one to form an anion (halogens). This links to reducing and oxidising behaviour.

Metallic character is greatest at the far left and gives way to non-metallic character towards the right.

Reaction with oxygen shows it: the oxide at the left end is the most basic (Na₂O) and at the right end the most acidic (Cl₂O₇). Oxides in the middle are amphoteric (Al₂O₃, As₂O₃) or neutral (CO, NO, N₂O).

Amphoteric oxides act as acids towards bases and as bases towards acids; neutral oxides do neither. With water: Na₂O + H₂O → 2NaOH (strong base) and Cl₂O₇ + H₂O → 2HClO₄ (strong acid); litmus confirms each.

In the 3d transition series, radii change much less across the period than for main group elements, and even less in the 4f series. Their ionization enthalpies lie between those of the s- and p-blocks, so they are less electropositive than groups 1 and 2.

Down a main group, radii grow, ionization enthalpy falls gradually and electron gain enthalpy becomes less negative (except that O and F are less negative than S and Cl). Metallic character rises and non-metallic character falls. Transition elements show the reverse trend down a group, explained by size and ionization enthalpy.