Question

What is the oxidation state of calcium in calcium carbonate( ${ Ca }{ CO }_{ 3 }$)?
  (A) $+6$
  (B) $+5$
  (C) $+4$
  (D) $+1$
  (E) $+2$

Answer 1

Hint: We need to know the number of electrons which is needed to make the bond, in order to find the oxidation state of calcium. Then by determining whether the element gains or loses electrons we can determine the oxidation state of calcium in calcium carbonate.

Complete step by step solution:
- Calcium is not usually found in isolated forms in nature. It's always found in the form of calcium carbonate or other compounds. Calcium carbonate is made up of calcium and carbonate ions.
- The oxidation state of an element can be defined as the positive or negative nature(character) of an atom of that element in a compound. For an uncombined element the oxidation number will be zero.
- Hydrogen generally has an oxidation number of $+1$ and oxygen has an oxidation number of $-2$.
- For neutral compounds, the sum of oxidation numbers of its constituent atoms will be zero. In the case of polyatomic ions, the sum of oxidation numbers of atoms will be equal to the net charge of the polyatomic ion.
- We are asked to find the oxidation state of calcium in ${ Ca }{ CO }_{ 3 }$.As we can see it’s a ternary ionic compound which means it is composed of atoms of three different elements. In the carbonate anion or $CO_{3}^{2-}$ oxygen has a $-2$ oxidation state and thus carbon has a $+4$ charge.
-Let's take the oxidation state of calcium as X. From the above definitions we can write as follows
\[X+4+3\times \left( -2 \right)=0\]
\[X+4-6=0\]
\[X=+2\]
Hence the oxidation state of calcium is $+2$

Therefore, the answer is option (E).

Note: Once we know the charges of common ions like carbonate, it's really not necessary to do the above calculations and we can find the oxidation state or charge by mental calculations. As we know calcium is a group 2 element or an alkaline earth metal. Keep in mind that the electro valency of alkaline earth metals will be $+2$ and hence the oxidation state of calcium will also be $+2$.