Atomic Structure
An atom consists of a central nucleus (containing protons and neutrons), around which are electrons.
Protons are positively charged; electrons are negatively charged; neutrons are neutral (have no electrical charge).
An atom has no overall electrical charge. It has the same number of protons as electrons.
Atoms can lose or gain electrons. This changes them into ions (charged particles).
Atoms of the same element always have the same number of protons. However, they can have different numbers of neutrons, making different isotopes.
The atomic number of an atom is the total number of protons in its nucleus.
The mass number of an atom is the total number of protons and neutrons in its nucleus. (Due to having such a small mass that it is negligible, electrons are not included in the mass.)