Which of the following pairs of 'number – composition' is correct?

I. Atomic number – number of protons

II. Mass number – Sum of number of neutrons and protons

Question

Question

This question was previously asked in

SSC CHSL Tier-I Exam 2022 Official Paper (Held On: 09 March, 2023 Shift 4)

Answer 1

The correct answer is Both I and II.

Key Points

I. The atomic number of an element is indeed equal to the number of protons in an atom of that element.

Hence correct.

II. The mass number of an atom is defined as the total number of protons and neutrons (collectively known as nucleons) in an atom.

Hence correct.

Additional Information

Element	Symbol	Atomic Number (Number of Protons)	Mass Number (Number of Protons + Neutrons)
Carbon	C	6	12

In this example, Carbon has an atomic number of 6, which means it has 6 protons in the nucleus of each of its atoms.
- This atomic number defines the element
- In other words, any atom with 6 protons is a Carbon atom.
- The mass number of Carbon is 12, indicating that the total number of protons and neutrons in the nucleus is 12.
  - Since we know there are 6 protons (from the atomic number), this means there must also be 6 neutrons (because 12 total nucleons - 6 protons = 6 neutrons).
The atomic number and mass number are crucial in defining an atom's identity and properties.
- The atomic number determines the element and its place in the periodic table, while the mass number helps to identify isotopes (varieties of the same element with different numbers of neutrons).