Here it means that when we compare the mass of protons and electrons .Electrons weigh much less than protons and therefore when the mass of an atom is calculated then just the mass of protons and neutrons is taken into consideration.

Mass of an Electron in Comparison to Other Particles

Introduction:
The given question asks us to compare the mass of an electron to that of other particles, namely the proton, beta particle, neutron, and alpha particle. We need to determine which particle has a mass approximately 1/2000 times that of an electron.

Explanation:
To answer this question, let's look at each particle individually and compare their masses to that of an electron.

Proton:
- A proton is a subatomic particle found in the nucleus of an atom.
- It carries a positive charge equal in magnitude to that of an electron.
- The mass of a proton is approximately 1.673 x 10^-27 kilograms.
- Comparing this mass to that of an electron, we can see that the mass of a proton is significantly larger than that of an electron. Therefore, the mass of an electron is not approximately 1/2000 times that of a proton.

Beta Particle:
- A beta particle is either an electron or a positron emitted from the nucleus during radioactive decay.
- Both electrons and positrons are considered beta particles.
- Since we are comparing the mass of an electron to other particles, we will consider the mass of an electron as a beta particle.
- As per the question, we need to find a particle whose mass is approximately 1/2000 times that of an electron.
- If we compare the mass of an electron to itself, we can conclude that the mass of an electron is not approximately 1/2000 times its own mass.
- Therefore, the mass of an electron is not approximately 1/2000 times that of a beta particle.

Neutron:
- A neutron is a subatomic particle found in the nucleus of an atom.
- It carries no charge and is electrically neutral.
- The mass of a neutron is approximately 1.675 x 10^-27 kilograms.
- Comparing this mass to that of an electron, we can see that the mass of a neutron is similar to that of a proton and is significantly larger than that of an electron. Therefore, the mass of an electron is not approximately 1/2000 times that of a neutron.

Alpha Particle:
- An alpha particle is a helium nucleus consisting of two protons and two neutrons.
- It carries a charge of +2.
- The mass of an alpha particle is approximately 6.646 x 10^-27 kilograms.
- Comparing this mass to that of an electron, we can see that the mass of an alpha particle is significantly larger than that of an electron. Therefore, the mass of an electron is not approximately 1/2000 times that of an alpha particle.

Conclusion:
After comparing the masses of an electron to those of a proton, beta particle, neutron, and alpha particle, we can conclude that the mass of an electron is approximately 1/2000 times that of a proton. Therefore, the correct answer is option 'A'.

Proton