Yes, electrons of an atom do contribute to the atom's mass. However, their contribution is very small compared to the protons and neutrons in the nucleus.

Here's why:

* Mass of particles:

* Protons: 1 atomic mass unit (amu)

* Neutrons: 1 amu

* Electrons: 0.00054858 amu (approximately 1/1836 of an amu)

* Number of particles: Atoms typically have an equal number of protons and electrons. The number of neutrons can vary, leading to isotopes.

Therefore, while electrons do contribute to the atom's mass, their contribution is negligible compared to the protons and neutrons. This is why atomic mass is often approximated as the sum of protons and neutrons, ignoring the electrons.

However, in some cases, the mass of electrons becomes significant:

* High-precision measurements: In very precise measurements of atomic mass, the contribution of electrons needs to be considered.

* High-energy physics: In high-energy physics, where particles are accelerated to near the speed of light, the relativistic mass increase due to the electrons' velocity can become significant.

Overall, while electrons contribute to an atom's mass, their contribution is usually small and often ignored for practical purposes.