AQA A-Level Chemistry

## Particles in the atom An atom has a small, dense nucleus made of protons and neutrons, with electrons in the space around it. The model has changed over time: Dalton pictured solid indivisible spheres, Thomson found electrons and described a positive sphere with electrons embedded in it, Rutherford's scattering experiment showed that the positive charge and nearly all the mass sit in a tiny nucleus, and Bohr and later quantum models placed the electrons in shells and orbitals of fixed energy. | Particle | Relative charge | Relative mass | |---|---|---| | Proton | +1 | 1 | | Neutron | 0 | 1 | | Electron | -1 | $\frac{1}{1840}$ (about 0.0005) | The **atomic (proton) number**, $Z$, is the number of protons in the nucleus and fixes which element it is. The **mass number**, $A$, is the number of protons plus neutrons. An atom is neutral, so it has as many electrons as protons; an ion has gained or lost electrons. ### Counting particles For an ion of element X with mass number $A$, atomic number $Z$ and charge $n$: - protons $= Z$ - neutrons $= A - Z$ - electrons $= Z - n$ for a positive ion of charge $n+$, and $Z + n$ for a negative ion of charge $n-$ For $\ce{^{27}Al^{3+}}$ (atomic number 13) there are 13 protons, 14 neutrons and $13 - 3 = 10$ electrons. For $\ce{^{32}S^{2-}}$ (atomic number 16) there are 16 protons, 16 neutrons and 18 electrons. ## Isotopes **Isotopes** are atoms of the same element with the same number of protons but different numbers of neutrons, so they…