An atom has the same number of electrons as protons, so the positive and negative charges cancel and the atom has no overall 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 symbol, its mass number above and its atomic number below, both on the left. For carbon-12 this is 12 above 6, so the atom has 6 protons and 12 − 6 = 6 neutrons. The number of neutrons is always the mass number minus the atomic number.
Atoms of the same element can have different numbers of neutrons. These atoms are called isotopes. Carbon-14 has 6 protons, like carbon-12, but it has 8 neutrons. The isotopes have the same atomic number but different mass numbers, so they have different masses.
An atom turns into a positive ion if it loses one or more outer electrons. It then has more protons than electrons, so it has an overall positive charge.