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 `(H)`, 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.
If an isotope of hydrogen has one neutron in its atom, its atomic number and atomic mass will respectively be

Options

A. `1,2`
B. `1,3`
C. `1,1`
D. `2,1`

Explanation

To solve the question regarding the atomic number and atomic mass of a hydrogen isotope that has one neutron, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Isotope**: The question specifies that we are dealing with a hydrogen isotope that has one neutron. The isotopes of hydrogen are:
- Protium (¹H) - 0 neutrons
- Deuterium (²H) - 1 neutron
- Tritium (³H) - 2 neutrons

Since we have one neutron, we are referring to **Deuterium (²H)**.

2. **Determine the Atomic Number**: The atomic number of an element is defined as the number of protons in its nucleus. For all isotopes of hydrogen, including deuterium, the number of protons is always 1. Therefore, the atomic number of deuterium is:
\[ \text{Atomic Number} = 1 \]

3. **Calculate the Atomic Mass**: The atomic mass of an isotope is the sum of the number of protons and neutrons in its nucleus. For deuterium:
- Number of protons = 1
- Number of neutrons = 1

Therefore, the atomic mass is calculated as follows:
\[ \text{Atomic Mass} = \text{Number of Protons} + \text{Number of Neutrons} = 1 + 1 = 2 \]

4. **Final Answer**: The atomic number and atomic mass of the hydrogen isotope with one neutron (deuterium) are:
- Atomic Number = 1
- Atomic Mass = 2

### Summary:
- Atomic Number: 1
- Atomic Mass: 2