With reference to Atomic Number and Mass Number, consider the following statements: 1. Atomic number is defined as the total number of neutrons present in the nucleus. 2. Mass number is the sum of the total number of protons and neutrons in an atom. 3. Elements are defined by the number of protons they possess. Which of the statements given above is/are correct?

The correct option is (c). Explanation The fundamental properties of an atom are determined by the composition of its nucleus, specifically the number of protons and neutrons. These subatomic particles define the atomic number and mass number, which are crucial for identifying elements and their isotopes. Statement-wise Analysis: - Statement 1 is Incorrect: The Atomic Number (denoted by 'Z') is defined as the total number of protons present in the nucleus of an atom. It is not determined by the number of neutrons. For a neutral atom, the number of protons is equal to the number of electrons. - Statement 2 is Correct: The Mass Number (denoted by 'A') is the sum of the total number of protons and neutrons present in the nucleus of an atom. Protons and neutrons are collectively referred to as nucleons. Therefore, Mass Number (A) = Number of Protons (Z) + Number of Neutrons (n). - Statement 3 is Correct: Elements are defined strictly by the number of protons they possess (Atomic Number). While the number of neutrons can vary within the same element (creating isotopes) and the number of electrons can change (creating ions), the number of protons remains constant for a specific element. Key Takeaway: The identity of an element is determined by its Atomic Number (Z), which equals the number of protons. The Mass Number (A) represents the total mass of the nucleus, calculated as the sum of protons and neutrons.

NCERT Class 9 Science — Chapter 4: Structure of the atom

mcqeasy

With reference to Atomic Number and Mass Number, consider the following statements:

Atomic number is defined as the total number of neutrons present in the nucleus.
Mass number is the sum of the total number of protons and neutrons in an atom.
Elements are defined by the number of protons they possess.

Which of the statements given above is/are correct?

Official previous-year question

Verified 1 Aug 2026.

Options

A1 and 2 only
B2 only
C2 and 3 only
D1, 2 and 3

Did you get this right?

Solution

The correct option is (c).

Explanation

The fundamental properties of an atom are determined by the composition of its nucleus, specifically the number of protons and neutrons. These subatomic particles define the atomic number and mass number, which are crucial for identifying elements and their isotopes.

Statement-wise Analysis:

Statement 1 is Incorrect: The Atomic Number (denoted by 'Z') is defined as the total number of protons present in the nucleus of an atom. It is not determined by the number of neutrons. For a neutral atom, the number of protons is equal to the number of electrons.
Statement 2 is Correct: The Mass Number (denoted by 'A') is the sum of the total number of protons and neutrons present in the nucleus of an atom. Protons and neutrons are collectively referred to as nucleons. Therefore, Mass Number (A) = Number of Protons (Z) + Number of Neutrons (n).
Statement 3 is Correct: Elements are defined strictly by the number of protons they possess (Atomic Number). While the number of neutrons can vary within the same element (creating isotopes) and the number of electrons can change (creating ions), the number of protons remains constant for a specific element.

Key Takeaway:

The identity of an element is determined by its Atomic Number (Z), which equals the number of protons. The Mass Number (A) represents the total mass of the nucleus, calculated as the sum of protons and neutrons.

Your note

More from Chapter 4: Structure of the atom

Which sub-atomic particle was identified by J. Chadwick in 1932?

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With reference to Neils Bohr’s model of the atom, consider the following statements: 1. Only certain special orbits known as discrete orbits of electrons are allowed inside the atom. 2. Electrons radiate energy continuously while revolving in discrete orbits. 3. These discrete orbits are called energy levels. Which of the statements given above is/are correct?

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Consider the following statements regarding the discovery and properties of neutrons: 1. They were discovered by J.J. Thomson in 1897. 2. They are present in the nucleus of all atoms including hydrogen. 3. A neutron has no charge and a mass nearly equal to that of a proton. Which of the statements given above is/are correct?

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With reference to 'Valency', consider the following statements: 1. It is the combining capacity of the atoms of elements. 2. Atoms react to achieve a duplet in the outermost shell in all cases. 3. If the outermost shell has 8 electrons, the valency is 8. Which of the statements given above is/are correct?

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With reference to the definition of an element, consider the following statements: 1. Elements are defined by the number of protons they possess. 2. Atoms of a given element can have different numbers of protons. 3. The atomic number is characteristic of a particular element.

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The correct option is (c).

Explanation

Statement-wise Analysis:

Statement 1 is Incorrect: The Atomic Number (denoted by 'Z') is defined as the total number of protons present in the nucleus of an atom. It is not determined by the number of neutrons. For a neutral atom, the number of protons is equal to the number of electrons.
Statement 2 is Correct: The Mass Number (denoted by 'A') is the sum of the total number of protons and neutrons present in the nucleus of an atom. Protons and neutrons are collectively referred to as nucleons. Therefore, Mass Number (A) = Number of Protons (Z) + Number of Neutrons (n).
Statement 3 is Correct: Elements are defined strictly by the number of protons they possess (Atomic Number). While the number of neutrons can vary within the same element (creating isotopes) and the number of electrons can change (creating ions), the number of protons remains constant for a specific element.

Key Takeaway: