Atoms have a small central nucleus made up of protons and neutrons around which there are electrons.
The relative masses of protons, neutrons and electrons and their relative charges are as shown:
We know this due to the work done by Rutherford, Geiger and Marsden, in their particle scattering experiment. Before this, the best model of the atom was the so called ‘’ model, in which the had not been considered. The negative are evenly distributed through the cloud that makes up the body of the atom, like plums in the pudding.
Rutherford and his team tested this model by firing alpha particles at a thin piece of leaf, only a few atoms wide. They expected to find that the alpha particles passed straight through, and most did. Some, however, were and very few were towards the source. This led them to conclude that there must be a very concentration of charge, and a lot of within the atom. Hence the concept of the and the ‘solar system’ model of the atom was put forward. A discrepancy between the mass of the atom and its positive charge was found, and so to account for this the was proposed, having only mass and not charge.
In an atom, the number of electrons is to the number of protons in the nucleus. The atom as a whole has no electrical charge. All atoms of a particular element have the same number of . Atoms of different elements have different numbers of . The total number of protons and neutrons (nucleons) in an atom is called its mass (nucleon) number. Atoms of the same element which have different numbers of neutrons are called .
