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Atomic structure Typeit
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An atom is the smallest part of an that can exist. Atoms contain tiny negative particles called , whose discovery led to the development of the plum pudding model. This model was later replaced by the nuclear model, which suggests that most of the mass of an atom is concentrated at its centre, the . This mass consists of neutral particles called , and positively charged particles called , both of which have a relative mass of 1.
The atomic number, i.e. the number of in an atom, is unique for each element. The is the sum of the number of protons and neutrons in an atom. The number of neutrons in an element is not always the same, so there can be several of the same element. Their natural abundance is taken into account when calculating the of an element.