What does hund's rule state?

an orbital can contain two electrons only if the electrons have opposite spins.

an orbital can contain two electrons only if all other orbitals at that sublevel are empty.

an orbital can contain two electrons only if the electrons have different energy levels.

an orbital can contain two electrons only if all other orbitals in that sublevel contain at least one electron.

 An orbital can contain two electrons only if all other orbitals in that sublevel contain at least one electron.

Explanation: When electrons are filled in orbitals of equal energy, one electron enters each orbital until all the orbitals contain one electron with the same spin direction.  then the pairing of electrons take place.

The first electron in an orbital could have up spin or down spin. All of the electrons in singly occupied orbitals have the same spin so as to maximize total spin.

Two electrons must have opposite spin is given by Pauli's exclusion principle, which states that no two electrons can have same set of quantum numbers.

answer: - 3.) the oxidation state of nitrogen in no changes from +2 to 0, and the oxidation state of carbon in co changes from +2 to +4 as the reaction proceeds.

solution: - as per the rules, oxidation number of oxygen in its compounds is -2. the sum of oxidation numbers of all the elements of a molecule or compound is zero.

also, the oxidation number in elemental form is zero.

in no, the oxidation number of o is -2 so the oxidation number of n is +2 so that the sum is zero.

similarly, the oxidation number of c in co is +2 and o is -2.

on product side the oxidation number of n is 0 as it is in the elemental form() .

oxidation number of c in carbon dioxide is +4 as there are two oxygen in it and we know that each oxygen is -2.

so, oxidation number of n is changing from +2 to 0 and the oxidation number of c is changing from +2 to +4.

since nitrogen is reduced and hence no is acting as an oxidizing agent. carbon is oxidized and so co is acting as a reducing agent.

so, the right choice is number 3.)the oxidation state of nitrogen in no changes from +2 to 0, and the oxidation state of carbon in co changes from +2 to +4 as the reaction proceeds.