Which of the following element is not a gas?

Mercury

The correct answer is option 3. Mercury.

Let us discuss each element:

1. Hydrogen:

Hydrogen is a gas at room temperature. It is the lightest and most abundant element in the universe. Its chemical symbol is \(H\), and it exists as diatomic molecules \((H_2)\) in its gaseous state.

2. Oxygen:

Oxygen is a gas at room temperature. It is essential for life and supports combustion. The chemical symbol for oxygen is \(O_2\), and it is diatomic in its gaseous form.

3. Mercury:

Mercury is a metal and is in a liquid state at room temperature. It is the only metal that is in liquid form under standard conditions. The chemical symbol for mercury is \(Hg\).

4. Nitrogen:

Nitrogen is a gas at room temperature. It makes up a significant portion of the Earth's atmosphere, and its chemical symbol is \(N_2\), indicating that it exists as diatomic molecules in its gaseous state.

So, among the given options, Mercury is an element that is not a gas; it is a liquid at room temperature.