Some elements possess more than one valence. Such elements are said to possess variable valency.
For example, iron has two electrons in its outermost shell (valence shell). On donating these electrons, its valency becomes 2. However, under specific experimental conditions, it can lose another electron from the shell next to valence shell. In such a case, the ion formed has 3 electropositive valencies. In short, iron exhibits two valencies 2 and 3. The Latin name of iron is Ferrum. For lower valency, it is written as Ferrous and the symbol is Fe2+. For higher valency, it is written as Ferric and the symbol is Fe3+ .
