Question

What is the oxidation state of Zinc in $ZnO_2^{2 - }$ ?

Answer 1

The combining capacity of an element to form a compound is determined by valency. To form a compound, the number of electrons an element can gain or lose is determined by valency whereas, after the formation of a particular compound, the number of electrons an element gained or lost is the oxidation state. According to IUPAC oxidation state of an atom is the charge of the respective atom after ionic approximation of its heteronuclear bonds.

Complete Step By Step Answer:
Zinc is a metal. Zinc has the maximum oxidation state +2. Electronic configuration of zinc is $[Ar]3{d^{10}}4{s^2}$ . Zinc has no unpaired electron present in its ground state as well as in oxidation state.
Let $x$ be the oxidation number of Zn in $Zn{O_2}^{2 - }$ . $- 2$ is the overall charge on the complex. So $- 2$ will be the sum of oxidation states of elements.
$x + 2( - 2) = - 2$
$x + ( - 4) = - 2$
$x = + 2$
Therefore the oxidation state of Zn in $ZnO_2^{2 - }$ is $+ 2$ .
Losing 2 electrons from $4s$ i.e. the outermost shell can be done but losing more than 2 electrons is difficult because it will have the need of freeing up the quantum state in the 3d outer core orbit.

Note:
The oxidation state of a free element is zero. Chemical compounds containing one or more oxygen atoms combined with another element are oxides. Due to the electronegativity, oxygen sets up a stable chemical bond with many of the elements to give corresponding oxides. Compounds having the oxidation number of oxygen $- \dfrac{1}{2}$ are superoxides. Compounds composed of oxygen-oxygen single bonds are peroxide. Hydrogen peroxide is the most common peroxide.