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 __________.