An atom is made of three sub-atomic particles: protons, neutrons and electrons.Protons and neutrons are found in the nucleus at the centre of the atom. The nucleus is tiny compared with the whole atom but contains almost all of its mass. The electrons are found outside the nucleus in energy levels called quantum shells, so most of the atom is empty space.The actual masses and charges of these particles are very small, so chemists compare them using relative values:
An electron has only about 1/1840 of the mass of a proton, so electrons add almost nothing to the mass of an atom.Protons and electrons have equal but opposite charges. An atom contains equal numbers of protons and electrons, so it has no overall charge. The negative electrons are held in the atom by their electrostatic attraction to the positive nucleus.If an atom loses electrons it becomes a positive ion; if it gains electrons it becomes a negative ion.Worked example: a sodium atom has 11 protons and 11 electrons. When it loses one electron it has 11 protons (+11) and 10 electrons (–10), so its overall charge is +1 and it becomes the ion Na+.
| particle | relative mass | relative charge |
| proton | 1 | +1 |
| neutron | 1 | 0 |
| electron | 1/1840 | –1 |
An electron has only about 1/1840 of the mass of a proton, so electrons add almost nothing to the mass of an atom.Protons and electrons have equal but opposite charges. An atom contains equal numbers of protons and electrons, so it has no overall charge. The negative electrons are held in the atom by their electrostatic attraction to the positive nucleus.If an atom loses electrons it becomes a positive ion; if it gains electrons it becomes a negative ion.Worked example: a sodium atom has 11 protons and 11 electrons. When it loses one electron it has 11 protons (+11) and 10 electrons (–10), so its overall charge is +1 and it becomes the ion Na+.