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.
Hydrogen, `H_2` is very less abundant in the atmosphere due to

Options

A. inflammable nature of `H_(2)`
B. weak earth's gravity which is not able to hold light `H_(2)` molecules
C. diatomic nature of hydrogen
D. very rapid reaction between hydrogen and atmosphere oxygen

Answer

To answer the question regarding why hydrogen is very less abundant in the Earth's atmosphere, we can break down the reasoning into clear steps:

### Step-by-Step Solution:

1. **Understanding Hydrogen's Abundance in the Universe**:
- Hydrogen accounts for approximately 75% of the mass of the universe and is the most abundant element in stars, including our Sun.

2. **Hydrogen's Presence on Earth**:
- On Earth, hydrogen is rarely found in its uncombined state (as H₂ gas) due to various reasons, primarily related to the planet's gravity.

3. **Effect of Earth's Gravity**:
- The Earth's gravity is relatively weak compared to the mass of hydrogen molecules. Hydrogen (H₂) is a very light molecule, and thus, it is difficult for Earth's gravity to retain it in the atmosphere.

4. **Loss of Hydrogen to Space**:
- Because of this weak gravitational hold, nearly all of the hydrogen that was originally present in the Earth's atmosphere has escaped into space over time.

5. **Presence in Compounds**:
- While hydrogen is not abundant in its elemental form, it is found in various compounds such as water (H₂O), hydrocarbons (petroleum), proteins, and carbohydrates.

6. **Isotopes of Hydrogen**:
- Hydrogen has three isotopes: protium (¹H), deuterium (²H), and tritium (³H). The differences in their masses lead to different physical properties, but their chemical properties remain similar.

7. **Chemical Properties and Reactions**:
- Although hydrogen is highly reactive and can form explosive mixtures with oxygen (as indicated by the exothermic reaction), this does not significantly contribute to its scarcity in the atmosphere.

8. **Conclusion**:
- The primary reason for the low abundance of hydrogen in the Earth's atmosphere is the weak gravitational force of the Earth, which cannot hold onto the light hydrogen molecules, leading to their loss to space.

### Final Answer:
The reason hydrogen is very less abundant in the atmosphere is due to **weak Earth's gravity**, which is not able to hold light H₂ molecules.