Cambridge International A-Level Chemistry

## Particles in the atom An atom is mostly empty space. Nearly all of its mass sits in a very small, dense **nucleus** made of protons and neutrons, and the electrons occupy shells in the space around it. The nucleus carries all of the positive charge and almost all of the mass, while the electrons carry all of the negative charge and almost none of the mass. | Particle | Relative charge | Relative mass | |---|---|---| | Proton | +1 | 1 | | Neutron | 0 | 1 | | Electron | $-1$ | $\frac{1}{1836}$ (negligible) | The **proton number** (atomic number, $Z$) is the number of protons in the nucleus, and it decides which element an atom is. The **nucleon number** (mass number, $A$) is the total number of protons and neutrons. In an atom the number of electrons equals the number of protons, so the atom is neutral; an ion has gained or lost electrons. For a species written as $\ce{^{A}_{Z}X^{n+}}$ or $\ce{^{A}_{Z}X^{n-}}$: - protons $= Z$ - neutrons $= A - Z$ - electrons $= Z - n$ for a cation of charge $n+$, and $Z + n$ for an anion of charge $n-$ For example, $\ce{^{27}_{13}Al^{3+}}$ has 13 protons, 14 neutrons and 10 electrons, while $\ce{^{32}_{16}S^{2-}}$ has 16 protons, 16 neutrons and 18 electrons. ### Beams in an electric field Suppose a beam of each particle, all travelling at one speed, is sent between two charged plates. The three beams behave differently. Protons are attracted towards the negative plate and electrons towards the positive plate. Neutrons have no charge and…