Edexcel A-Level Chemistry

## Sub-atomic particles An atom has a tiny, dense nucleus made of protons and neutrons, surrounded by electrons in shells. Almost all the mass is in the nucleus, because a proton and a neutron are each about 1836 times heavier than an electron. | Particle | Relative mass | Relative charge | |---|---|---| | Proton | 1 | +1 | | Neutron | 1 | 0 | | Electron | $\tfrac{1}{1836}$ (negligible) | −1 | The **atomic (proton) number**, $Z$, is the number of protons in the nucleus, and it decides which element the atom is. The **mass number**, $A$, is the total number of protons and neutrons. An atom is written with $A$ as a superscript and $Z$ as a subscript before the symbol, so $\ce{^{27}_{13}Al}$ has 13 protons and $27 - 13 = 14$ neutrons. In a neutral atom the number of electrons equals the number of protons, and an ion has gained or lost electrons. | Species | Protons | Neutrons | Electrons | |---|---|---|---| | $\ce{^{27}_{13}Al^{3+}}$ | 13 | 14 | 10 | | $\ce{^{35}_{17}Cl^{-}}$ | 17 | 18 | 18 | | $\ce{^{56}_{26}Fe^{2+}}$ | 26 | 30 | 24 | | $\ce{^{32}_{16}S^{2-}}$ | 16 | 16 | 18 | For a molecule or a polyatomic ion, add up the numbers over all the atoms and then take account of the overall charge. The ammonium ion has $7 + 4 = 11$ protons, and because it carries a charge of +1 it has $11 - 1 = 10$ electrons. ## Isotopes **Isotopes** are atoms of the same element with the same number of protons but different numbers of neutrons, so they have the same atomic number and different…