A charged atom is called an ion. Atoms are neutral because they have the same numbers of positive and negative charges. By adding or taking away negative charges, atoms can become positive or negative.
Protons and neutrons exist in the nucleus of an atom. Protons have a positive charge, whereas a neutron has no charge. Electrons are negatively charged particles that zoom around the nucleus.
Some elements are electropositive, while others are more electronegative. Atoms like to have a full outer electron shell, so they either try to gain or lose electrons to fill their outer shells. Electropositive elements such as lithium, whose atoms have few electrons in their outer level, will donate them to other atoms. They then become positively charged because they have more protons than electrons. Elements such as oxygen are electronegative and want to take electrons from other atoms, thus gaining a negative charge.