# Atoms and Molecules

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## 07 Writing Chemical Formulae of Common Compounds

WRITING CHEMICAL FORMULAE OF COMMON COMPOUNDS

• Various chemical substances are used in our daily life, some in our house such as the staples present in the medicine cabinet.
• Such chemicals are bound up with atoms in a fixed proportion and also these are arranged in a certain way which can be determined by the chemical formula.
• Chemistry mostly comprises of reactions along with formulas.
• The chemical formulaindicates clearly the number and constituent chemical elements of the periodic table.
• The arrangement of the chemical substances is also given in the periodic table.

Importance of Chemistry Formulae:

• In Chemistry, chemical formulae are like shorthand.
• They are used to represent the elements in a compound.
• The Periodic Tablehas elements along with the symbols that represent different individual elements like is hydrogen, O for oxygen, Ca for calcium, Na for Sodium and so on.
• Compounds are the substances which have more than two atoms bonded together, and also more than one or more elements.
• The nature of these chemical compounds and their reactions is determined by chemistry formulas.
• Chemistry formulas also give an idea of how a substance has formed and the proportion of elements involved in that particular reaction.

Types of Chemistry Formulae or Chemical Formulae:

In chemistry, there are various chemical formulae, some of which are:

• Molecular Formula- The molecular formula determines the number of elements present in a compound. The elements are written in the form of their symbols as in the periodic table where the number of atoms is written as a subscript.

For example- the molecular formula for glucose is written as .

${\mathrm{C}}_{6}{\mathrm{H}}_{12}{\mathrm{O}}_{6}$

• Empirical Formula- This formula indicates the ratio of the elements present in a compound and is usually obtained based on the analysis of experimental data. Empirical Formulas are usually said to be derived from the molecular formulas.

The empirical formula of glucose is written as

${\mathrm{CH}}_{2}\mathrm{O}$

.

• Structural Formula- It provides an idea of how the atoms in the molecule or the compound are arranged along with their bond formations.

Writing a Chemical Formula:

In order to write a chemical formula, the symbol of the elements present in the compound must be known along with formula of the radicals and the valency of the elements in that compound. In addition the following points should be noted while writing a chemical formula:

• Compounds with two and more elements are known. But most of the compounds are binary compounds i.e. they have two elements.
• For a compound containing metal and a non-metal, the metal is named first followed by the non-metal. For example:  NaCl which consists of
${\mathrm{Na}}^{+}$

(metal ion) and

${\mathrm{Cl}}^{–}$

(non-metal ion)

• Cation is an atom with a positive charge whereas anion is an atom with the negative charge.
• Anions having -1 negative charge usually have a suffix as –ide. For example:
${\mathrm{F}}^{–}$

– Flouride while those having oxy anions (oxygen + another element) usually have a suffix as –ate. For example –

${\mathrm{SO}}_{4}^{2–}$

(Sulphate). For a polyatomic anion having

${\mathrm{H}}^{–}$

ion, or hydrogen is used as a suffix. For example

${\mathrm{HCO}}_{3}^{–}$

Bicarbonate or hydrogen carbonate

• Some polyatomic anions can be named as:
 ${\mathrm{NH}}_{2}^{–}$ Amide ${\mathrm{PO}}_{4}^{3–}$ Phosphate ${\mathrm{CN}}^{–}$ Cyanide

Writing of formula of molecular compounds:

Formula of Hydrogen Sulphide

The formula for aluminium sulphate