1.1 Atomic structure

The Structure of the Atom
Mass Spectrometry
Electronic Structure
Ionisation Energies

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THE STRUCTURE OF THE ATOM

a) Protons, neutrons and electrons

Atoms are made up of three fundamental particles: protons, neutrons and electrons.

Protons and neutrons are found in the nucleus and are collectively called nucleons. Electrons orbit the nucleus in a similar way to that in which planets orbit a sun. In between the electrons and nucleus there is nothing (empty space).
The nucleus is very small; if an atom were the size of a football pitch, the nucleus would be the size of a drawing pin.

The basic properties of these three particles can be summarized in the following table:

|Particle |Charge |Mass |
|Proton |+1 unit |Approx 1 unit |
|Neutron |No charge |Approx 1 unit |
|Electron |-1 unit |Approx 1/1840 units (very small) |

1 unit of charge is 1.602 x 10-19 coulombs. A proton is given a charge of +1 and an electron a charge of -1. All charges are measured in these units.

1 unit of mass is 1.661 x 10-27 kg. This is also not a convenient number, so we use “atomic mass units”.
Since the mass of protons and neutrons varies slightly depending on the nucleus, then in order to define an “atomic mass unit” we need to choose one nucleus as a standard. For this purpose 126C , or “carbon-12”, was chosen because its mass per nucleon
(1.661 x 10 –27 kg) is around average, which means all the other nuclei