Metals tend to form positive ions because they contain fewer than four electrons in their outermost shells, making it energetically favorable for them to lose these electrons and gain the electronic structure of the nearest noble gas. Larger atoms with fewer electrons in the outermost shell lose electrons more easily.
Group IA is the most metallic group in the periodic table, containing the alkali metals. These elements lose their single, outermost electrons easily, changing into positive ions with the electronic configuration of the preceding noble gas. Moving downwards in group IA is accompanied by an increase in the reactivity of the elements as the number of electrons in shells increases, causing the atoms to grow larger. The valence electrons of atoms with elements having larger atomic radii and more inner shells are less attracted by nuclear charge. This enables them to leave the atom more easily than their smaller counterparts.
The number of outermost electrons increases from the left to the right in the periodic table. Group IIA is the group adjacent to IA in the periodic table. It contains the alkali earth metals, which contain two electrons in their outermost shells. As the number of electrons increases, the ease of losing these electrons to become positive ions also increases.