Learn all you should know about the Atomic Theory, an important topic in chemistry.
O + 2e- O2-
As explained in the previous example, the oxygen atom now became a dinegatively charged ion – O2-. It could also be called a divalent anion.
But as could be clearly seen, the oxygen atom could not gain all the three electrons lost by the aluminium atom. This is because oxygen would obtain a stable outer electronic by the gain of two electrons. But during a chemical reaction between elements all the electrons lost by one atom MUST be gained by the other element. For this condition to be satisfied, two atoms of aluminium must each lose three electrons, so that six electrons would be lost altogether. In turn three oxygen atoms would each gain two electrons, so that all of the six electrons lost would be gained. Hence two atoms of aluminium could react chemically with three atoms of oxygen by lose and gain of electrons.
The formula of the compound formed between aluminium and oxygen is therefore Al2O3 and its name is aluminium oxide. This is another ionic or electrovalent compound.
From these two examples, it follows that group 1, 2 and 3 elements could lose 1, 2 and 3 electrons respectively in their attempt to become stable:
Na - e- Na+ univalent cation