Among `NH_(3), HNO_(3), N_(2)` and `Mg_(3)N_(2)` , the number of molecules having nitrogen in negative oxidation state is :

Explanation

To determine the number of molecules among \( NH_3 \), \( HNO_3 \), \( N_2 \), and \( Mg_3N_2 \) that have nitrogen in a negative oxidation state, we will calculate the oxidation state of nitrogen in each compound step by step.

### Step 1: Calculate the oxidation state of nitrogen in \( NH_3 \)

1. **Write the equation for oxidation state**:
\[ x + 3(+1) = 0 \]
where \( x \) is the oxidation state of nitrogen and the oxidation state of hydrogen is +1.

2. **Solve for \( x \)**:
\[ x + 3 = 0 \implies x = -3 \]

**Conclusion**: The oxidation state of nitrogen in \( NH_3 \) is -3.

### Step 2: Calculate the oxidation state of nitrogen in \( HNO_3 \)

1. **Write the equation for oxidation state**:
\[ +1 + x + 3(-2) = 0 \]
where the oxidation state of hydrogen is +1 and that of oxygen is -2.

2. **Solve for \( x \)**:
\[ 1 + x - 6 = 0 \implies x - 5 = 0 \implies x = +5 \]

**Conclusion**: The oxidation state of nitrogen in \( HNO_3 \) is +5.

### Step 3: Calculate the oxidation state of nitrogen in \( N_2 \)

1. **Write the equation for oxidation state**:
\[ 2x = 0 \]
since \( N_2 \) is a diatomic molecule of nitrogen.

2. **Solve for \( x \)**:
\[ x = 0 \]

**Conclusion**: The oxidation state of nitrogen in \( N_2 \) is 0.

### Step 4: Calculate the oxidation state of nitrogen in \( Mg_3N_2 \)

1. **Write the equation for oxidation state**:
\[ 3(+2) + 2x = 0 \]
where the oxidation state of magnesium is +2.

2. **Solve for \( x \)**:
\[ 6 + 2x = 0 \implies 2x = -6 \implies x = -3 \]

**Conclusion**: The oxidation state of nitrogen in \( Mg_3N_2 \) is -3.

### Final Count of Molecules with Negative Oxidation State

- From the calculations:
- \( NH_3 \): -3
- \( HNO_3 \): +5
- \( N_2 \): 0
- \( Mg_3N_2 \): -3

**Molecules with nitrogen in negative oxidation state**: \( NH_3 \) and \( Mg_3N_2 \).

**Total count**: 2 molecules.

### Final Answer
The number of molecules having nitrogen in a negative oxidation state is **2**.

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