* Electrons are the key: Electric charge is carried by electrons. When an object gains a negative charge, it gains extra electrons.
* Mass of electrons: Electrons have a very small mass (9.11 x 10^-31 kg).
* The effect on mass: While electrons do contribute to the total mass of an object, the increase in mass due to gaining a few extra electrons is incredibly tiny. It's practically negligible.
In essence: For all practical purposes, an object's mass remains virtually unchanged when it gains a negative charge. The mass increase is so small that it's impossible to detect with everyday instruments.
