Question

The sum of oxidation number of all the atoms in a neutral molecule must be zero.
                   (A) True
                   (B) False
                   (C) Ambiguous
                   (D) None of these

Answer 1

Hint: The word neutral means zero (no charge) and for any type of ion the sum of the oxidation numbers of its constituent atoms (according to their stoichiometry) will be equal to the total charge on the ion.

Complete answer:
-The oxidation state is defined as the total number of electrons that can be lost or gained by any atom to form bonds or to form any complex. If due to a reaction the oxidation state increases then it is known as oxidation and if the oxidation state decreases then it is known as reduction.
We can also say that oxidation state is the degree of oxidation or reduction of any atom in a chemical compound. It can be positive, negative or zero depending on the type of bonds formed by it and whether it has lost or gained electrons.
-IUPAC defines oxidation state as: “Oxidation state of the atom is the charge of this atom after ionic approximation of its heteronuclear bonds.”

This ionic approximation is done on the basis of following criteria:
 1) Bond’s polarity: from electronegativity difference, dipole moment and from quantum-chemical calculations of charges.
 2) Electron assignment according to the atom’s contribution to bonding MO.
-An element in free state has an oxidation state zero and also when we say a neutral compound, then it means that it has oxidation state zero (neutral means no charge).
For example: neutral molecules such as ${O_2}$, ${P_4}$, ${O_3}$, ${S_8}$, $KMn{O_4}$, etc. for these the sum of oxidation numbers of all atoms is zero. Let us see for$KMn{O_4}$:
O.No. for K = (+1), for Mn = (+7) and for O = (-2). Now let us find oxidation number of entire molecule (let it be x for now):
                                                          x = (+1) + (+7) + (-2)4
                                                          x = 8 + (-8) = 0
The oxidation state came out to be zero and we know that this compound is also zero.
-For any ion the sum of oxidation states of all the atoms present in it would be equal to its charge.
For example: In $C{N^ - }$ ion the sum of oxidation numbers of both the atoms is (-1) and in $N{H_4}^ + $ it is (+1).
Hence we can say that the statement given in the question is true and the correct option will be: (A) True.

Note: Oxidation state is not constant for all elements which means that it is not necessary that every element will show only one oxidation state in every compound. There are many elements which show variable valency due to electrons of the outermost shell. The d-block elements or the transition metals have the ability to show variable valency states.