Question

The lightest gas is :
A. nitrogen
B. helium
C. oxygen
D. hydrogen

Answer 1

Hint: Consider the options mentioned in gaseous form. The lightness of the gas is judged by the mass of the gases. The lesser the mass of the gas, the lighter it will be. Check the mass of the gases, arrange accordingly to find the lightest gas.

Complete step by step answer:
Let us discuss the options one-by one and their physical properties:
A. nitrogen: The nitrogen talked about here is dinitrogen or nitrogen gas $\left( {{\text{N}}_{2}} \right)$. It is a colourless and odourless gas. It is a diatomic gas. Nitrogen is the lightest element of group 15 of the periodic table. The atomic mass of nitrogen gas $\left( {{\text{N}}_{2}} \right)$ is twice the mass of nitrogen element $\left( \text{N} \right)$. The atomic mass of nitrogen is 7 grams. The mass of $\left( {{\text{N}}_{2}} \right)$ will be $2\times 7$ or 14 grams. This gas forms 78% of earth’s atmosphere.

B. helium: Helium exists as helium gas $\left( \text{He} \right)$ only with 2 protons present in it. It is the first element of the noble gas group. It is tasteless, colourless, inert and odourless gas. It is a monatomic gas. Helium is the second lightest gas with lowest boiling point. The atomic mass of helium gas $\left( \text{He} \right)$ is 4 grams.

C. oxygen: The oxygen talked about here is dioxygen or oxygen gas $\left( {{\text{O}}_{2}} \right)$. It is a colourless and odourless gas. It is a diatomic gas. It is the third most abundant element by mass after helium and hydrogen. The atomic mass of oxygen gas $\left( {{\text{O}}_{2}} \right)$ is twice the mass of oxygen element $\left( \text{O} \right)$. The atomic mass of the oxygen element is 8 grams. The mass of $\left( {{\text{O}}_{2}} \right)$ will be $2\times 8$ or 16 grams. This gas forms 21% of earth’s atmosphere.

D. hydrogen: The hydrogen talked about here is dihydrogen or hydrogen gas $\left( {{\text{H}}_{2}} \right)$. It is a colourless and odourless gas. It is a diatomic gas. Hydrogen is the lightest element of the periodic table, thus, hydrogen gas is the lightest gas. The atomic mass of hydrogen gas $\left( {{\text{H}}_{2}} \right)$ is twice the mass of hydrogen element $\left( \text{H} \right)$. The atomic mass of the hydrogen element is 1 gram. The mass of $\left( {{\text{H}}_{2}} \right)$ will be $2\times 1$ or 2 grams.
The order of atomic masses of gases will be ${{\text{O}}_{2}}>{{\text{N}}_{2}}>\text{He}>{{\text{H}}_{2}}$. So, the minimum gas is hydrogen gas. It has the least mass so it will be the lightest gas.
So, the correct answer is “Option D”.

Note: In balloons, hydrogen and helium gases are used. Helium gas is lighter than air and it is known as noble gas. As it is a noble gas, it is non-reactive. In balloons, helium gas is filled not hydrogen gas. This is because hydrogen gas is also lighter than air and even lighter than helium. But, hydrogen gas is not used because it is highly explosive and flammable.