Everything around us is made of tiny particles called atoms. An atom is mostly empty space, but at its centre is a very small, dense nucleus. The nucleus contains two kinds of particles: protons, which carry a positive charge, and neutrons, which have no charge. Moving around the nucleus are the negatively charged electrons.
Charges and mass
A proton and a neutron have almost the same mass, so the nucleus holds nearly all of the atom's mass. An electron is far lighter, about 1836 times lighter than a proton. In a neutral atom the number of protons equals the number of electrons, so the positive and negative charges balance and the atom has no overall charge. Atoms are extremely small, far too tiny to see even with an ordinary microscope, yet the nucleus is thousands of times smaller still and sits right at the centre.
Key idea
Proton number (Z) = number of protons. Nucleon number (A) = number of protons + number of neutrons. So number of neutrons = A − Z.
Isotopes
Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. Because they have the same proton number they are the same element and share the same chemical properties. For example, carbon-12 and carbon-14 are isotopes of carbon: both have 6 protons, but carbon-12 has 6 neutrons while carbon-14 has 8. Many elements exist naturally as a mixture of two or more isotopes.
Remember
- Nucleus = protons + neutrons.
- Electrons orbit the nucleus.
- Isotopes: same protons, different neutrons.