Step-by-step explanation:
It is known that nucleus of an atom contains protons and neutrons. Protons have a positive charge whereas neutrons have no charge.
Therefore, electrons closer to the nucleus are strongly held due to the force of attraction between opposite charges of protons and electrons.
So, as the electrons go far away from the nucleus then force of attraction between the nucleus and electrons decreases.
As a result, it becomes easier to remove valence electrons of an atom. Thus, they can be removed by friction.