Class 9 Science Note

Valency and Molecular Formula

Element is the simplest form of substance that cannot be broken down by any chemical means but can be changed to another form of elements. E.g.  Sodium, chlorine

When two or more than two elements combined chemically, molecular compound is formed. Water is compound formed by hydrogen and oxygen.

There are 109 elements altogether, among them 92 elements occurs naturally.

The combining capacity of an element with other element is known as valency


The smallest unit of an element is called an atom.


Structure of an atom



All elements are composed of an atom consists of proton, neutron and electron. Proton and neuron lies inside the nucleus whereas an electron revolves around the nucleus in elliptical orbit. They are named as K, L, M, and N and so on. If there are two electrons in outermost shell then it is called duplet and if there are eight electrons then it’s called octet. During the chemical reaction all atom tries to remain in duplet or in octet state.



The force of attraction between the combining atoms that arises due to the complete transfer or mutual sharing of electrons is called chemical bond.


Electrovalent bond: The bond formed by the complete transfer of electrons between two atoms is known as electrovalent bond. They do not conduct electricity in solid state but conduct electricity in aqueous or molten state Example: NaCl, CaCl2 etc.


Covalent Bond: The bond formed by the mutual sharing of the electrons between combining of the same or different elements is called covalent bond. They do not conduct electricity in solid, molten or aqueous solution state. Example: H2O, HCl etc.



Molecular formula

Molecular formula is the symbolic representation of a molecule.


The information that can be obtained from the molecular formulaare:
i. It tells us about the elements constituting the molecule.

ii. It tells the number of atoms of each element present in one molecule of a compound.

iii. It also gives the number of molecules.

iv. It helps to find the molecular weight of the substance.


Example:  molecular formula of boron oxide is B2O3 which means it contains two atom of the boron and three atom of the oxygen and the valency associated with them is 2 and 3 for oxygen and boron respectively.

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