Most of the mass of an atom is not from the electrons. Electrons are extremely light particles, and the vast majority of an atom's mass comes from its nucleus. The nucleus contains protons and neutrons, which are much heavier than electrons.

- Protons have a positive charge and a mass of about 1 atomic mass unit (amu).

- Neutrons have no charge and a mass of about 1 amu.

- Electrons have a negative charge and a mass of about 0.0005 amu.

In comparison, the mass of an atom is typically measured in daltons (Da) or atomic mass units (amu), where 1 Da or 1 amu is approximately equal to the mass of a single proton or neutron.

Here's an example with carbon as a reference to illustrate the mass difference:

- An atom of carbon has six protons, six neutrons, and six electrons.

- The six protons contribute 6 amu to the atom's mass.

- The six neutrons contribute another 6 amu.

- The six electrons contribute a negligible 0.003 amu to the total mass.

Therefore, the majority of carbon's mass (12 amu) comes from the protons and neutrons in its nucleus, with the electrons contributing a very tiny fraction.

In summary, while electrons play a crucial role in the chemical behavior and properties of atoms, they contribute very little to their overall mass.