Periodic Trends in Chemical Reactivity

An element's chemical reactivity is governed by how easily it can attain the stable electronic configuration of its nearest noble (inert) gas neighbour, either by gaining or by losing electrons. Elements that come BEFORE a noble gas in the periodic table tend to react by GAINING electrons into their outermost shell (to reach that noble gas's configuration by addition); elements that come immediately AFTER a noble gas tend to react by LOSING their valence electrons (to reach the PREVIOUS noble gas's configuration by subtraction). So chemical reactivity, in the end, is governed by both electron gain enthalpy and ionization enthalpy values — which are themselves governed by effective nuclear charge and, ultimately, by atomic size. Since ionization enthalpy is smallest for the element at a period's extreme LEFT, and electron gain enthalpy is most negative for the element second from a period's extreme RIGHT (i.e., the element immediately before the noble gas that ends the period), the MOST REACTIVE elements of any period sit at its two extremes, excluding the unreactive noble gas itself. This same reactivity pattern shows up clearly in how vigorously an element combines with oxygen, and in the acid-base character of the resulting oxide: reactive elements on the extreme left (the alkali metals) combine vigorously with oxygen to form oxides (e.g. Na2O) that react with water to give STRONG BASES (e.g. NaOH); reactive elements on the right (the halogens) form oxides (e.g. Cl2O7) that react with water to give STRONG ACIDS (e.g. HClO4); the oxides of elements nearer the centre of the main-group block are instead AMPHOTERIC (e.g. Al2O3, reacting with both acids and bases), NEUTRAL (e.g. CO, NO), or WEAKLY ACIDIC (e.g. CO2). The transition metals and inner-transitio …