Do the electrons of an atom contribute to mass of an atom?

Protons have a positive charge and weigh 1 AMU

Neutrons have a neutral charge and weigh 1 AMU

Electrons have a negative charge and 1184 electrons weigh 1AMU so they are thought to add nothing to the mass of an atom.

You may want to check out Bohr's model or the duet/octet rule.