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Atoms and Molecules

Relative Atomic Mass and Relative Molecular Mass


We know that the outermost shell of an atom can hold a maximum of eight electrons. The elements, whose atoms have a completely filled outermost shell, have very little chemical activity. Such elements are said to have zero combining capacity or valency. For e.g., helium, neon, argon. (The elements of the 18th group in the periodic table). These elements have either completely filled outermost shells or have 8 electrons in their outermost shell. Hence, their valency is zero. They are called inert or noble gases.  

The combining capacity of atoms of the elements is their tendency to react with other atoms of the same or different molecules to attain a filled outermost shell. The outermost shell, which has eight electrons, is said to possess an octet and every atom tends to achieve an octet in its outermost shell. This is done by gaining, losing, or sharing its electrons. The number of electrons gained, lost, or shared by an atom to complete its octet is called the combining capacity or valency of that atom.

Both hydrogen and sodium contain one electron each in their outermost shells. Thus, both can lose one electron. Hence, their valency is one.

It is not always true that the number of electrons present in the outermost shell of an atom represents its valency. For example, in fluorine, there are seven electrons in the outermost shell, but the valency of fluorine is one. This is because it is energetically suitable for fluorine atom to accept one electron, rather than donating seven electrons. Hence, its valency is obtained by subtracting seven electrons from the octet.

Concept of valency

We know that the combining power or the combining capacity of an atom or an element is called its valency. The number of atoms of other elements with which one atom of …

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