| Element | Symbol | Atomic Number (Z) | Mass (nucleon) Number (A) |
| hydrogen | H | 1 | 1 |
| carbon | C | 6 | 12 |
| nitrogen | N | 7 | 14 |
| oxygen | O | 8 | 16 |
| chlorine | Cl | 17 | 35 |
| particle name | mass /atomic mass units |
| proton (p+) | 1.007 |
| neutron (n0) | 1.009 |
| electron (e–) | 5.5 × 10–4 |
The atomic number equals the number of protons in the nucleus of the atom.
The table shows a few atoms with their atomic number.
Because atoms have no overall electrical charge, the numbers of protons and electrons are equal.
Atomic number equals
number of protons equals
number of electrons
Because atoms have no overall electrical charge, the numbers of protons and electrons are equal.
Atomic number equals
number of protons equals
number of electrons
The number of neutrons in an atom can be deduced if the mass (nucleon) number (A) of the atom is also known.
Because protons and neutrons are in the nucleus, they are referred to as nucleons.
The mass (nucleon) number (A) equals the sum of the numbers of protons and neutrons in the nucleus.
Why aren't electrons included in the mass number?
It is important to remember that this is a number NOT a mass.
The data at the right shows that the mass number is close to the relative mass of the nucleus in atomic mass units but not exactly equal to it.
The data at the right shows that the mass number is close to the relative mass of the nucleus in atomic mass units but not exactly equal to it.
Why aren't electrons included in the mass number?
As the data shows, the mass of the electron is more than 1000 times smaller than the mass of the neutron and the proton.
Combining these ideas to deduce the number of neutrons in the nucleus
| If you know this | and any of these, | you can calculate |
mass (nucleon) number and  | atomic number | number of neutrons |
| number of protons | ||
| number of electrons |