The mass of an electron is negligible compared to protons and neutrons.
Explanation
An atom consists of three fundamental subatomic particles: protons, neutrons, and electrons. The mass number (A) is defined as the sum of the number of protons and neutrons (collectively called nucleons) present in the nucleus. It provides an integer approximation of the atom's total mass.
Detailed Analysis:
- Electrons have no mass. is Incorrect: Electrons are matter and possess a definite rest mass (approximately $9.1 \times 10^{-31}$ kg). They are not massless particles.
- Electrons are not present in the nucleus. is Incorrect: While electrons are present in the extranuclear region, their location does not determine whether their mass is counted. If electrons possessed significant mass, they would contribute to the atomic mass regardless of their position relative to the nucleus.
- The mass of an electron is negligible compared to protons and neutrons. is Correct: The mass of a proton or a neutron is approximately $1.67 \times 10^{-27}$ kg. In contrast, the mass of an electron is roughly $\frac{1}{1837}$ times the mass of a proton. Due to this vast difference, the contribution of electrons to the total mass is statistically insignificant. Consequently, the entire mass of the atom is practically assumed to be concentrated in the nucleus, and electron mass is neglected in mass number calculations.
- Electrons are constantly moving and cannot be weighed. is Incorrect: The constant motion of electrons does not prevent the determination of their mass. The mass of an electron is a well-established physical constant.
Key Takeaway:
The mass number is derived solely from protons and neutrons because the mass of an electron is negligible relative to nucleons, making no significant contribution to the atom's total mass.