An atom of an element A has three electrons in its outer shell and B has six electrons in outermost shell. The formula of the compound formed between these two willbe

In this case the valence electrons in the atom A is three and hence its valency is generally 3. In the atom B, the number of valence electrons is six. Hence its valency is usually 2. Hence, the formula of the molecule formed from A and B could be A2B3. A, example of two such elements are A1 and O and the formula of aluminum oxide is Al2O3.