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An element is a simple substance which cannot be further divided into any simpler substance by ordinary chemical means. Copper, hydrogen, iron are some examples of elements. There are about 109 elements known to man. Uranium is the heaviest element and hydrogen is the lightest element.

One element differs from another because each has its own special kind of atoms. The figure given below represents atoms of an element A and those of an element B. An element cannot be broken down into simpler substance because its atoms are indivisible. When two elements combine, atoms are neither created nor destroyed. Thus, the mass of the product is the same as that of the reactants. If atoms of element A combine with atoms of element B, the proportion of mass of the elements in the resulting compound (molecule) will be constant. Thus, if one atom of A combines with two atoms of B, the resulting compound (molecule) will always have A and B in 1:2 ratio (by mass) as shown in the figure. In a mixture, however, the atoms do not combine and they may be present in any proportion (Fig.)

Atoms of
element A and element B



Molecule of AB2   Mixture of elements A and B


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