Singapore-Cambridge A-Level Chemistry (H2)

## Protons, neutrons and electrons An atom has a very small, dense, positively charged **nucleus** containing protons and neutrons, surrounded by electrons in the much larger space outside it. Almost all of the mass is in the nucleus and almost all of the volume is empty space occupied by the electron cloud. Alpha particle scattering showed this: nearly all particles passed straight through a thin gold foil, a few were deflected through large angles, and a very few rebounded, which means the positive charge and the mass are concentrated in a tiny region. | Particle | Relative charge | Relative mass | |---|---|---| | Proton | $+1$ | 1 | | Neutron | 0 | 1 | | Electron | $-1$ | $\dfrac{1}{1840}$ (about 0.0005) | In an electric field between two charged plates the three particles behave differently. Protons are deflected towards the negative plate, electrons towards the positive plate and neutrons are not deflected, because they carry no charge. The electron is deflected through a much larger angle than the proton, because its charge is the same size but its mass is about 1840 times smaller, so the same force gives it a far larger acceleration. A beam of ions with a higher charge, or a lower mass, is deflected more. ## Proton number, nucleon number and isotopes The **proton number** $Z$ is the number of protons in the nucleus, and it fixes the element. The **nucleon number** $A$ is the total number of protons and neutrons. An atom is written $\ce{^{A}_{Z}X}$, so…