Question

Explain symbols and chemical formulae. Differentiate between empirical formula and molecular formula with examples.

Answer 1

Symbols: In 1813, Berzelius gave a system to represent the element by symbols. A symbol is the shortest form of representing an element. A symbol is a letter or a group of letters of English alphabet which represent an atom of an element.

1. Generally, the first letter of the English name of the element is used as its symbol, e.g.

For example:

Name of the element	Symbol
Hydrogen	H
Oxygen	O
Nitrogen	N
Carbon	C
Phosphorous	P
Sulphur	S

2. If there is more than one element which has the first letter in the name common, one small prominent letter in pronunciation is used in addition to the first capital letter.

For example:

Name of the element	Symbol
Carbon	C
Calcium	Ca
Chlorine	Cl
Chromium	Cr
Cadmium	Cd

3. In some cases, the symbols have been derived from the Latin names of the elements.

For example:

English name	Latin name	Symbol
Potassium	Kalium	K
Iron	Ferrum	Fe
Copper	Cuprum	Cu
Silver	Argentum	Ag
Mercury	Hydrargyrum	Hg
Lead	Plumbum	Pb
Gold	Aurum	Au
Tin	Stannum	Sn

The significance of symbol: 

1. A symbol is the shortest form of the name of an element. 

2. It denotes an atom of the element.

For example, O means one atom of oxygen 2C means two atoms of carbon.

3. It also represents the atomic weight of that element. For example, 2H, 12C and 32S mean that atomic weights of hydrogen, carbon and sulphur are 1, 12, and 32 units, respectively.

4. Formula: Formula is a short name of any element or compound. A group of symbols of elements which represents one molecule of any element or compound is called formula. For example, the formula of chlorine is Cl₂. The formula of common salt is NaCl.

The significance of the formula:

1. Formula represents one molecule of the substance. For example, NaCl represents one molecule of sodium chloride.

2. The formula gives the name of all constituent elements present in the molecule. For example, constituent elements of Na₂SO₄ are sodium (Na), one atom of sulphur (S) and oxygen (O).

3. The formula gives information about the number of atoms of elements present in one molecule e.g., Na₂SO₄ tells that there are two atoms of sodium (Na), one atom of sulphur (s) and four atoms of Oxygen (O).

4. If atomic weights are known, the molecular weight of the substance can be found.

For example, atomic weights of sodium, sulphur and oxygen are 23, 32 and 16 respectively, so the molecular weight of sodium sulphate (Na₂SO₄) is 

23 x 2 + 32 + 16 x 4 

= 46 + 32 + 64 = 142. 

5. The number written at the beginning of a formula denotes the number of molecules.

For example, in the formula (CuSO₄ . 5H₂O) of copper sulphate molecule, there are 5 molecules of water (HaO).

Difference between empirical formula and molecular formula:

Empirical formula represents only the simple ratio of the number of atoms of elements in its molecule.

For example, the empirical formula of acetic acid is CH₂O. It means in the molecule of acetic acid the ratio of atoms of carbon, hydrogen and oxygen is 1: 2: 1. While molecular formula represents the exact number of various constituent elements in a molecule of that compound. For example, the molecular formula of acetic acid is C₂H₄O₂ which shows that a molecule of acetic acid contains 2 atoms of carbon 4 atoms of hydrogen and 2 atoms of oxygen.