Class 9 Molecules, Ions and chemical formula of compounds

Topics to be covered

`=>` What is a molecule?
`=>` What is an ion?
`=>` Writing chemical formula
`=>` Formula of simple compounds

๐–๐ก๐š๐ญ ๐ข๐ฌ ๐š ๐Œ๐จ๐ฅ๐ž๐œ๐ฎ๐ฅ๐ž?

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

`color{green}("๐Œ๐Ž๐‹๐„๐‚๐”๐‹๐„๐’ ๐Ž๐… ๐„๐‹๐„๐Œ๐„๐๐“๐’")`

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

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

`color{green}(โ€ข)` 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}(โ€ข)` The number of atoms constituting a molecule is known as its `color{red}("๐š๐ญ๐จ๐ฆ๐ข๐œ๐ข๐ญ๐ฒ")`.

`color{green}("๐Œ๐Ž๐‹๐„๐‚๐”๐‹๐„๐’ ๐Ž๐… ๐‚๐Ž๐Œ๐๐Ž๐”๐๐ƒ๐’")`

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

๐–๐‡๐€๐“ ๐ˆ๐’ ๐€๐ ๐ˆ๐Ž๐?

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

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

`color{green}(โ€ข)` A negatively charged ion is called an โ€˜anionโ€™ and the positively charged ion, a โ€˜cationโ€™. 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^โ€“ ))`.

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

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

๐–๐ซ๐ข๐ญ๐ข๐ง๐  ๐‚๐ก๐ž๐ฆ๐ข๐œ๐š๐ฅ ๐…๐จ๐ซ๐ฆ๐ฎ๐ฅ๐š

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

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

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

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

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

๐…๐Ž๐‘๐Œ๐”๐‹๐€ ๐Ž๐… ๐’๐ˆ๐Œ๐๐‹๐„ ๐‚๐Ž๐Œ๐๐Ž๐”๐๐ƒ๐’

The simplest compounds, which are made up of two different elements are called binary compounds.

๐Ÿ. ๐…๐จ๐ซ๐ฆ๐ฎ๐ฅ๐š ๐จ๐Ÿ ๐ก๐ฒ๐๐ซ๐จ๐ ๐ž๐ง ๐œ๐ก๐ฅ๐จ๐ซ๐ข๐๐ž

Formula of the compound would be `color{red}(HCl).`

๐Ÿ. ๐…๐จ๐ซ๐ฆ๐ฎ๐ฅ๐š ๐จ๐Ÿ ๐ก๐ฒ๐๐ซ๐จ๐ ๐ž๐ง ๐ฌ๐ฎ๐ฅ๐ฉ๐ก๐ข๐๐ž

๐Ÿ‘. ๐…๐จ๐ซ๐ฆ๐ฎ๐ฅ๐š ๐จ๐Ÿ ๐œ๐š๐ซ๐›๐จ๐ง ๐ญ๐ž๐ญ๐ซ๐š๐œ๐ก๐ฅ๐จ๐ซ๐ข๐๐ž

The formulae of ionic compounds are simply the whole number ratio of the positive to negative ions in the structure.
For magnesium chloride, we write the symbol of cation `color{red}((Mg^(2+)))` first followed by the symbol of anion `color{red}((Cl^-))`. Then their charges are criss-crossed to get the formula.

๐Ÿ’. ๐…๐จ๐ซ๐ฆ๐ฎ๐ฅ๐š ๐จ๐Ÿ ๐ฆ๐š๐ ๐ง๐ž๐ฌ๐ข๐ฎ๐ฆ ๐œ๐ก๐ฅ๐จ๐ซ๐ข๐๐ž.

Thus, in magnesium chloride, there are two chloride ions `color{red}((Cl^-))` for each magnesium ion `color{red}((Mg^(2+)))`. The positive and negative charges must balance each other and the overall structure must be neutral. Note that in the formula, the charges on the ions are not indicated.