What is the formula for calcium oxide?

Hint: Calcium oxide, commonly called quicklime or burnt lime, is a white, alkaline, caustic and crystalline solid at room temperature. It is generally prepared by thermal decomposition of limestone or seashells which comprise calcium carbonate in a lime kiln. Similar to other metal oxides, calcium oxide is a basic oxide as it readily dissolves in water in order to form calcium hydroxide.

Complete step by step answer:
As the name suggests, the given compound calcium oxide consists of a metal calcium (\[Ca\]) and a non-metal oxygen (\[O\]). We know that any compound containing a metal and a non-metal is considered to be an ionic compound. In ionic compounds, we have to take into account the charges of each of the elements present in the compound. Now let us identify the formula for the given compound i.e. calcium oxide. From the periodic table, it is clear that calcium is a metal and it belongs to group 2 thus, it possesses +2 cationic charge and oxygen is a non-metal and it possesses -2 anionic charge. It will look like $C{a^{2 + }}{O^{2 - }}$. As calcium oxide is neutral, the net charge on the compound should be zero. So in the present case, $( + 2) + ( - 2) = 0$. Thus, this will be the formula for calcium oxide as one to one ratio of the charges makes this formula for calcium oxide.
Hence, the formula for calcium oxide is $CaO$.

Note: Calcium oxide is formed by heating the calcium carbonate to above \[825\;^\circ C{\text{ }}\left( {1,517\;^\circ F} \right)\] by following a process known as calcination or called as lime burning liberating a molecule of carbon dioxide and a mole of calcium oxide.
\[CaC{O_3}\left( s \right) \to CaO\left( s \right) + C{O_2}\left( g \right)\]