Hydrogen accounts for approximately `75%` of the mass of the universe. Hydrogen serves as the nuclear fuel of our Sun and other stars, and these are mainly composed of hydrogen. On the earth, though hydrogen is rarely found in the uncombined state. Since the earth's gravity is too weak to hold such light molecules, nearly all the `H_2` originally present in the earth's atmosphere has been lost to space. In the earth's crust and oceans, hydrogen is found in water, petroleum, proteins, carbohydrates and other compounds and it is the ninth most abundant element on a mass basis. Hydrogen has three isotopes : hydrogen or protium `()`, deuterium or heavy hydrogen (D or ), tritium `(T or )`. The physical properties of the three isotopes are different due to the difference in their masses, i.e. isotope effect. The chemical properties of the three isotopes are similar as they have the same electronic configuration. Reaction between hydrogen and oxygen is highly exothermic, and gas mixtures that contain as little as `4%` by volume hydrogen in oxygen (or in air) are highly flammable and potentially explosive.
`2H_(2(g))+O_(2(g)), DeltaH^(ɵ)=-485 kJmol^(-1)`
As hydrogen is environmentally clean it is an enormously attractive fuel. 'Hydrogen economy' is an emerging field in which it is thought that our energy needs can be met by gaseous, liquid and solid hydrogen. As hydrogen is no a naturally occuring substance such as coal, oil or natural gas, energy must be exploaded to produce hydrogen before it can be used.
Which of the following is the lightest gas?

Options

A. hydrogen
B. oxygen
C. nitrogen
D. helium

Answer

To determine which of the following gases is the lightest, we will analyze the options provided: hydrogen, oxygen, nitrogen, and helium.

### Step-by-Step Solution:

1. **Identify the Gases**:
- The gases in question are:
- Hydrogen (H₂)
- Oxygen (O₂)
- Nitrogen (N₂)
- Helium (He)

2. **Determine the Molar Mass of Each Gas**:
- **Hydrogen (H₂)**: The molar mass of hydrogen is approximately 2 g/mol (1 g/mol for each hydrogen atom, and there are two atoms in H₂).
- **Oxygen (O₂)**: The molar mass of oxygen is approximately 32 g/mol (16 g/mol for each oxygen atom, and there are two atoms in O₂).
- **Nitrogen (N₂)**: The molar mass of nitrogen is approximately 28 g/mol (14 g/mol for each nitrogen atom, and there are two atoms in N₂).
- **Helium (He)**: The molar mass of helium is approximately 4 g/mol.

3. **Compare the Molar Masses**:
- Hydrogen (H₂): 2 g/mol
- Oxygen (O₂): 32 g/mol
- Nitrogen (N₂): 28 g/mol
- Helium (He): 4 g/mol

4. **Identify the Lightest Gas**:
- Among the calculated molar masses, hydrogen (H₂) has the lowest molar mass of 2 g/mol.

5. **Conclusion**:
- Therefore, the lightest gas among the options provided is **hydrogen**.

### Final Answer:
**Hydrogen (H₂)** is the lightest gas.

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