Consider the following statements regarding the mass of an atom: 1. The mass of an electron is considered negligible. 2. The mass of an atom is practically due to electrons and protons alone. 3. The mass of a proton is taken as one unit.

Correct Option The correct option is (c). Explanation An atom consists of three fundamental subatomic particles: electrons, protons, and neutrons. The distribution of mass within an atom is determined by the properties of these particles, specifically their location and individual masses. Statement-wise Analysis - Statement 1 is Correct: The mass of an electron is approximately \(9.1 \times 10^{-31}\) kg, which is about \(\frac{1}{1840}\) times the mass of a hydrogen atom. Due to this extremely low value, the mass of an electron is considered negligible when calculating the total atomic mass. - Statement 2 is Incorrect: The entire mass of an atom is practically concentrated in the nucleus, which contains protons and neutrons. Since the mass of electrons is negligible, the mass of an atom is determined by the sum of the masses of protons and neutrons, not electrons and protons. - Statement 3 is Correct: The mass of a proton is approximately \(1.672 \times 10^{-27}\) kg. On the atomic mass scale, this is taken as approximately 1 atomic mass unit (u). This standard unit allows for convenient comparison of atomic masses. Key Takeaway: The mass of an atom is derived almost entirely from the nucleus (protons and neutrons), while the volume of an atom is defined by the electron cloud. The electron's mass is negligible in atomic mass calculations.

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

mcqeasy

Consider the following statements regarding the mass of an atom:

The mass of an electron is considered negligible.
The mass of an atom is practically due to electrons and protons alone.
The mass of a proton is taken as one unit.

Official previous-year question

Verified 1 Aug 2026.

Options

A1 and 2 only
B2 only
C1 and 3 only
D1, 2 and 3

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Solution

Correct Option

The correct option is (c).

Explanation

An atom consists of three fundamental subatomic particles: electrons, protons, and neutrons. The distribution of mass within an atom is determined by the properties of these particles, specifically their location and individual masses.

Statement-wise Analysis

Statement 1 is Correct: The mass of an electron is approximately \(9.1 \times 10^{-31}\) kg, which is about \(\frac{1}{1840}\) times the mass of a hydrogen atom. Due to this extremely low value, the mass of an electron is considered negligible when calculating the total atomic mass.
Statement 2 is Incorrect: The entire mass of an atom is practically concentrated in the nucleus, which contains protons and neutrons. Since the mass of electrons is negligible, the mass of an atom is determined by the sum of the masses of protons and neutrons, not electrons and protons.
Statement 3 is Correct: The mass of a proton is approximately \(1.672 \times 10^{-27}\) kg. On the atomic mass scale, this is taken as approximately 1 atomic mass unit (u). This standard unit allows for convenient comparison of atomic masses.

Key Takeaway: The mass of an atom is derived almost entirely from the nucleus (protons and neutrons), while the volume of an atom is defined by the electron cloud. The electron's mass is negligible in atomic mass calculations.

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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Correct Option

The correct option is (c).

Explanation

Statement-wise Analysis

Statement 1 is Correct: The mass of an electron is approximately \(9.1 \times 10^{-31}\) kg, which is about \(\frac{1}{1840}\) times the mass of a hydrogen atom. Due to this extremely low value, the mass of an electron is considered negligible when calculating the total atomic mass.
Statement 2 is Incorrect: The entire mass of an atom is practically concentrated in the nucleus, which contains protons and neutrons. Since the mass of electrons is negligible, the mass of an atom is determined by the sum of the masses of protons and neutrons, not electrons and protons.
Statement 3 is Correct: The mass of a proton is approximately \(1.672 \times 10^{-27}\) kg. On the atomic mass scale, this is taken as approximately 1 atomic mass unit (u). This standard unit allows for convenient comparison of atomic masses.