**Oxidation Number of Nitrogen in NCl3**

The oxidation number, also known as the oxidation state, of an element is a measure of the number of electrons that an atom gains or loses when it forms a chemical compound. To determine the oxidation number of nitrogen in NCl3, we need to consider the following rules:

1. **Oxidation Number of Chlorine**: Chlorine is a halogen and usually has an oxidation number of -1 in its compounds. However, there are exceptions to this rule, such as in compounds with oxygen or fluorine. In NCl3, chlorine is in its usual oxidation state of -1.

2. **Total Charge of the Compound**: The sum of the oxidation numbers of all atoms in a neutral compound is zero. In the case of NCl3, since the compound is neutral, the sum of the oxidation numbers must be zero.

3. **Oxidation Number of Nitrogen**: Let's assume the oxidation number of nitrogen in NCl3 is x. Since there are three chlorine atoms with an oxidation number of -1 each, the sum of the oxidation numbers of chlorine is 3*(-1) = -3. According to the total charge of the compound rule, the sum of the oxidation numbers should be zero.

Therefore, x + (-3) = 0

Solving this equation, we find that x = +3.

Hence, the oxidation number of nitrogen in NCl3 is +3.

**Explanation in Detail:**

Nitrogen forms a covalent bond with three chlorine atoms in NCl3. In a covalent bond, the shared electrons are not transferred completely from one atom to another, as in ionic bonds. Instead, they are shared between the participating atoms.

In NCl3, nitrogen shares its three valence electrons with three chlorine atoms to form three covalent bonds. Each chlorine atom contributes one electron to the bond, and nitrogen contributes three electrons. By sharing these electrons, nitrogen fulfills its octet rule, which states that atoms tend to gain, lose, or share electrons to achieve a stable electron configuration with eight valence electrons.

To determine the oxidation number of nitrogen in NCl3, we assign an oxidation number to each atom. Since chlorine is a more electronegative element than nitrogen, chlorine is assigned an oxidation number of -1, as it tends to gain one electron to achieve a stable electron configuration. The total oxidation number contributed by the three chlorine atoms is -3.

According to the total charge of the compound rule, the sum of the oxidation numbers must be zero for a neutral compound. Therefore, the oxidation number of nitrogen, represented by x, can be calculated by subtracting the sum of the oxidation numbers of chlorine from zero: x + (-3) = 0.

Solving this equation, we find that x = +3. Hence, the oxidation number of nitrogen in NCl3 is +3.

Oxidation State of N is + 3