Substances can be categorized into three classes: elements, compounds and mixtures. An element is the purest form of a substance, since all the atoms of an element are identical. This means that they have the same number of protons, neutrons and electrons. While this may be true for elements that do not have isotopes (atoms that have the same atomic number but different numbers of neutrons), elements with isotopes may have atoms with different mass numbers. In general, one isotope dominates a given element and hence we can assume that all the atoms are identical to each other. Since atoms are identical to each other, they have exactly the same properties, thus making an element a pure substance and also incapable of further sub-division.
Some examples of elements are iron, copper, oxygen, sodium, chlorine, lead, uranium, etc.
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