The familiar image of the atom
Atoms are generally represented as miniature solar systems. This classical image with orbits is far removed from the reality of the atom, but provides a decent analogy for a system where practically all the matter is concentrated in a tiny volume at the centre. The rest of the atom consists of empty space where electrons, particles several thousands of times lighter than the nucleus, travel around it. The nucleus is a hundred thousand times smaller than the atom and the electrons, rather than going in precisely-defined orbits, share up the empty space between them.
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