In an atom the number of electrons equals the number of protons in the nucleus. Protons are positive and electrons are negative, so the charges cancel and atoms have no overall electrical charge. All atoms of a particular element have the same number of protons. This number is the atomic number. The total number of protons and neutrons in an atom is its mass number.
An atom is written with its mass number at the top and its atomic number at the bottom, as in 146C. This carbon atom has 6 protons and a mass number of 14. The number of neutrons is the mass number minus the atomic number, so 14 − 6 = 8 neutrons.
Atoms of the same element can have different numbers of neutrons. These atoms are called isotopes of that element. For example 126C has 6 neutrons and 146C has 8. Isotopes have the same atomic number but different mass numbers, and both are neutral atoms.
An atom turns into a positive ion if it loses one or more outer electrons.