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

### 𝐖𝐡𝐚𝐭 𝐢𝐬 𝐚 𝐌𝐨𝐥𝐞𝐜𝐮𝐥𝐞?

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.

### 𝐖𝐇𝐀𝐓 𝐈𝐒 𝐀𝐍 𝐈𝐎𝐍?

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.

### 𝐖𝐫𝐢𝐭𝐢𝐧𝐠 𝐂𝐡𝐞𝐦𝐢𝐜𝐚𝐥 𝐅𝐨𝐫𝐦𝐮𝐥𝐚

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.

### 𝐅𝐎𝐑𝐌𝐔𝐋𝐀 𝐎𝐅 𝐒𝐈𝐌𝐏𝐋𝐄 𝐂𝐎𝐌𝐏𝐎𝐔𝐍𝐃𝐒

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

𝟏. 𝐅𝐨𝐫𝐦𝐮𝐥𝐚 𝐨𝐟 𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧 𝐜𝐡𝐥𝐨𝐫𝐢𝐝𝐞

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

𝟐. 𝐅𝐨𝐫𝐦𝐮𝐥𝐚 𝐨𝐟 𝐡𝐲𝐝𝐫𝐨𝐠𝐞𝐧 𝐬𝐮𝐥𝐩𝐡𝐢𝐝𝐞

𝟑. 𝐅𝐨𝐫𝐦𝐮𝐥𝐚 𝐨𝐟 𝐜𝐚𝐫𝐛𝐨𝐧 𝐭𝐞𝐭𝐫𝐚𝐜𝐡𝐥𝐨𝐫𝐢𝐝𝐞

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.

𝟒. 𝐅𝐨𝐫𝐦𝐮𝐥𝐚 𝐨𝐟 𝐦𝐚𝐠𝐧𝐞𝐬𝐢𝐮𝐦 𝐜𝐡𝐥𝐨𝐫𝐢𝐝𝐞.

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.