Class 9 Science


You have read that noble gases have fully filled outermost shell. Due to this, they are stable and they do not react with other elements. Other elements also tend to attain stable configuration by completing the octet in their outermost orbit.

This is important to note that, the number of electrons in the outermost orbit of an element is closer to octet. An element can lose or gain electron in order to complete the octet. This tendency of losing or gaining electrons imparts valency to an element.

Let us take example of hydrogen. Hydrogen can readily lose or gain an electron. So, its valency is one. Now, let us take example of Hydrochloric Acid (HCl). One atom of chlorine combines with one atom of hydrogen to form hydrochloric acid. In this case, hydrogen loses one electron and thus gets +1 charge. On the other hand, chlorine gains an electron and thus gets – 1 charge. So, valency of hydrogen and chlorine are one.

Valency can be defined as combining capacity of an atom.


Hydrogen molecule: Hydrogen has only one electron in its outermost orbit, thus it requires one more electrons to complete its outermost orbit. Therefore, in order to complete outermost orbit, hydrogen shares one electron with another hydrogen atom and form H2 (hydrogen molecule).

In the case of LiCl (Lithium chloride); Lithium has three electrons in its outermost orbit and chlorine has seven electrons in its outermost orbit. Thus in order to make outermost orbit completely filled lithium loses one electrons and chlorine gains one electron. After losing one electron, lithium has two electrons in its outermost orbit and after gaining one electron, chlorine has eight electrons in its outermost orbit. And they form LiCl (Lithium chloride)


Elements having same atomic number but different atomic masses are known as Isotopes.


Carbon-12, Carbon-13, Carbon-14 are thee isotopes of carbon atom. Here 12, 13 and 14 are the atomic masses of isotopes of carbon respectively. Since, atomic number is the unique property of an atom, thus the atomic number of carbon is 6 even in the case of three types of carbon (isotopes)

126Carbon, 136Carbon, 146Carbon,

Hydrogen-1, Deuterium–2, Tritium-3 are three isotopes of hydrogen.

The isotopes of hydrogen are written as:

11Hydrogen, 21Deuterium, 31Tritium

Uses of Isotopes:

Carbon–14 (C-14) is used in carbon dating. This technology is utilised for finding the age of materials found in archaeological excavation. This helps in determining the periods of various events in history. Even fossil's age can be determined by using this technology.


Atoms having same atomic mass and different atomic numbers are known as Isobars.


4018Ar (Argon) and 4020Ca (Calcium)

Both the elements have same atomic mass equal to 40 but different atomic numbers, i.e. argon has atomic number equal to 18 and calcium has atomic number equal to 20.