An atom contains equal numbers of protons and electrons. Since protons and electrons have equal and opposite charges, this means that atoms are have no overall electrical charge.
For example, the atomic number of sodium is 11. Every sodium atom has 11 protons and 11 electrons. It has 11 positive charges and 11 negative charges.
Atoms of different elements usually have different mass numbers, but they can be the same. For example, the mass number of argon atoms and calcium atoms can both be 40.
The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.
To calculate the numbers of subatomic particles in an atom, use its atomic number and mass number:
The atomic number of a sodium atom is 11 and its mass number is 23. Calculate the number of protons, neutrons and electrons it contains.
Number of protons = 11
Number of electrons = 11
Number of neutrons (mass number - atomic number) = 23 - 11 = 12