•    Valence Electrons – Electrons existing in the outermost orbit of an atom are called Valence Electrons.

•    The atoms which have completely filled the outermost shell are not very active chemically.
         •    The valency of an atom or the combining capacity of an atom is given by the number of elements present in the outermost shell.

        •    For Example, Helium contains two electrons in its outermost shell which means its valency is two. In other words, it can share two electrons to form a chemical bond with another element.

           •    What happens when the outermost shell contains a number of electrons that are close to its maximum capacity?

Valency in such cases is generated by subtracting the number of electrons present in the outermost orbit from octet (8). For example, oxygen contains 6 electrons in its outermost shell. Its valency is calculated as: 8 – 6 = 2. This means oxygen needs two electrons to form a bond with another element.

Atomic Number of an Element

Atomic Number (Z) = Number of protons in an atom
Mass Number of an Element
Mass Number = Number of protons + Number of neutrons









•    The atoms of an element can exist in several forms having similar atomic numbers but varying mass numbers.
•    Isotopes are pure substances.
•    Isotopes have a similar chemical nature.
•    Isotopes have distinct physical characteristics.








Where can we use Isotopes?

1. The fuel of Nuclear Reactor – Isotope of Uranium
2. Treatment of Cancer – Isotope of Cobalt
3. Treatment of Goitre – Isotope of Iodine


The atoms of several elements can have a similar mass number but distinct atomic masses. Such elements are called Isobars. 


•    Species having same number of neutrons but different number of protons are called Isotones.





•    Examples include boron-12 and carbon-13 nuclei both contain 7 neutrons, and so are isotones. 

Difference Between Isotopes, Isobars & Isotones