`color{green}(•)` A molecule is in general a group of two or more atoms that are chemically bonded together, that is, tightly held together by attractive forces.

`color{green}(•)` A molecule can be defined as the smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.

`color{green}(•)` Atoms of the same element or of different elements can join together to form molecules.

`color{green}("𝐌𝐎𝐋𝐄𝐂𝐔𝐋𝐄𝐒 𝐎𝐅 𝐄𝐋𝐄𝐌𝐄𝐍𝐓𝐒")`

`color{green}(•)` The molecules of an element are constituted by the same type of atoms.

`color{green}(•)` Molecules of many elements, such as argon (Ar), helium (He) etc. are made up of only one atom of that element.

`color{green}(•)` But the case is different with most of the nonmetals. For example, a molecule of oxygen consists of two atoms of oxygen and hence it is known as a diatomic molecule, `color{red}(O_2)`. If 3 atoms of oxygen unite into a molecule, instead of the usual 2, we get ozone.

`color{green}(•)` The number of atoms constituting a molecule is known as its `color{red}("𝐚𝐭𝐨𝐦𝐢𝐜𝐢𝐭𝐲")`.

`color{green}("𝐌𝐎𝐋𝐄𝐂𝐔𝐋𝐄𝐒 𝐎𝐅 𝐂𝐎𝐌𝐏𝐎𝐔𝐍𝐃𝐒")`

Atoms of different elements join together in definite proportions to form molecules of compounds.

`color{green}(•)` Compounds that are composed of metals and nonmetals contain charged species known as ions.

`color{green}(•)` An ion is a charged particle and can be negatively or positively charged.

`color{green}(•)` A negatively charged ion is called an ‘anion’ and the positively charged ion, a ‘cation’. For example, sodium chloride `color{red}((NaCl))`. Its constituent particles are positively charged sodium ions `color{red}((Na^+))` and negatively charged chloride ions `color{red}((Cl^– ))`.

`color{green}(•)` Ions may consist of a single charged atom or a group of atoms that have a net charge on them.

`color{green}(•)` A group of atoms carrying a charge is known as a polyatomic ion.

`color{green}(•)` The chemical formula of a compound is a symbolic representation of its composition.

`color{green}(•)` The combining power (or capacity) of an element is known as its valency.

`color{green}(•)` Valency can be used to find out how the atoms of an element will combine with the atom(s) of another element to form a chemical compound.


`color{green}(•)` The rules that you have to follow while writing a chemical formula are as follows:

`color{green}(•)` the valencies or charges on the ion must balance.

`color{green}(•)` when a compound consists of a metal and a non-metal, the name or symbol of the metal is written first. For example: calcium oxide `color{red}((CaO))`, sodium chloride `color{red}((NaCl))`, iron sulphide `color{red}((FeS))`, copper oxide `color{red}((CuO))` etc., where oxygen, chlorine, sulphur are non-metals and are written on the right, whereas calcium, sodium, iron and copper are metals, and are written on the left.

`color{green}(•)` in compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.