"Inside an insulator, the electrons are tightly held by nuclei." What is the force that hold the electrons? I found out from the Internet that insulators have strong covalent bonds inside and so electrons are tightly bounded to the nucleus. But in another source, it is stated that electrostatic attraction between the positively charged nucleus and the negative electrons keeps electrons inside an atom. So, should it be strong covalent bond or electrostatic attraction that hold the electrons? Thanks in advance.