Periodic Trends and Chemical Reactivity

Reactivity across the table. Elements on the left side of the periodic table have low ionisation energy and so readily lose their valence electrons; elements on the right side have high electron affinity and so readily gain electrons. Because of this, elements at both extreme ends of a period are highly reactive, while elements in the middle of a period are comparatively less reactive. Noble gases, with their completely filled valence shell, neither lose nor gain electrons readily and so are chemically inert.

Ionisation energy and metallic character. Ionisation energy is directly tied to metallic character: elements in the lower-left of the table have low ionisation energy and show metallic character, while elements in the upper-right have very high ionisation energy and are non-metallic.

Oxide acidity/basicity, worked example. Comparing an alkali metal and a halogen from opposite sides of a period, both reacting with oxygen:

Sodium oxide reacts with water to give the strong base sodium hydroxide, $Na_2O + H_2O \rightarrow 2\,NaOH$, so $Na_2O$ is a basic oxide. Chlorine heptoxide instead reacts with water to give the strong acid perchloric acid, $Cl_2O_7 + H_2O \rightarrow 2\,HClO_4$, so $Cl_2O_7$ is an acidic oxide -- exactly the behaviour expected of elements from the two extreme ends of the periodic table. …