Question

What is the oxidation number of oxygen in water?

Answer 1

Hint: Oxidation number of an element: It is defined as the total number of electrons that an atom accepts or loses in order to make a chemical bond which results in the formation of a chemical compound.

Complete step by step solution:
Let us first talk about the oxidation number.
Oxidation number of an element: It is defined as the total number of electrons that an atom accepts or loses in order to make a chemical bond which results in the formation of a chemical compound.
Here we are given with the compound water. And we know that generally hydrogen shows the oxidation number as $ + 1$ because it belongs to the group $1$ in periodic table. The atomic number of hydrogen is $1$ so it has electronic configuration as: $1{s^1}$. So it can lose electrons and become free.
Now the compound water is a neutral compound. So the sum of the oxidation numbers of all the elements should be zero. Here in this compound the elements are hydrogen and oxygen. And we already know the oxidation number of hydrogen i.e. $ + 1$. Now let us assume that the oxidation number of oxygen is $x$. So the equation of sum of the oxidation numbers will be as:
$
  2( + 1) + x = 0 \\
  x = - 2 \\
 $
So the oxidation number of oxygen in this compound i.e. water, will be $ - 2$.
If a compound has net charge either positive or negative then the sum of all charges in the compound will be equal to that net charge on the compound. By using this concept we can calculate the oxidation number of elements in charged species.

Note:
Those elements are stable which have eight electrons in their outermost orbitals. For example: Noble gases are stable because they have eight electrons in their outermost orbits. Every element wants to achieve this octet (to be stable) hence elements react with each other to form the compounds.