But some of the outer electrons are held very loosely. They can move from one atom to another. It is positively charged. An atom that gains electrons has more negative than positive particles. It has a negative charge.
Some materials hold their electrons very tightly.
Electrons do not move through them very well. These things are called insulators. Plastic, cloth, glass and dry air are good insulators. Other materials have some loosely held electrons, which move through them very easily. These are called conductors.