Chemical formula: The symbolic representation of all the elements(atoms) in the compound is called chemical formula. OR The way of representing of a molecule or a compound using the symbols of its elements.
How to write a Chemical Formula:
Compounds are represented by chemical formulae as elements are represented by symbols. Chemical formulae of compounds are written keeping the following steps in consideration.
- Symbols of two elements are written side by side, in the order of positive ion first and negative ion later.
- The valency of each ion is written on the right top corner of its symbol, e.g. Na+, Ca2+, CI– and O2–.
3. This valency of each ion is brought to the lower right corner of other ion by ‘cross-exchange’ method.
4. If the valencies are same, they are offset (ignored) and are not written in the chemical formula. But if they are different, they are indicated as such at the same position, e.g. in case of sodium chloride both the valencies are offset and formula is written as NaCl, whereas, calcium chloride is represented by formula CaCl2.
5. If an ion is a combination of two or more atoms which is called radical, bearing a net charge on it, e.g. SO42–(sulphate) and PO43–(phosphate), then the net charge represents the valency of the radical. The chemical formula of such compounds is written as explained in (3) and (4); writing the negative radical within the parenthesis. For example, chemical formula of aluminium sulphate is written as Al2(SO4)3 and that of calcium phosphate as Ca3(PO4)2
Significance of chemical formula:
* It tells the name of the elements as present in the compound.
* It represents the name of the substance specially compound. For example, water.
* It also represent one mole of the molecules in the balanced chemical equation.
* It is in fact one molecule or formula unit of the compound.
* It indicates the mass of the compound in amus or grams.
Types of chemical formulas: There are two types of chmical formulas.
1) Emperical formula. 2) Molecular formula.
Empirical formula: The formula that shows simplest whole number ratio of elements present in a compound is called empirical formula.
It is the simplist type of formula.
Determination of emperical formula; The empirical formula of a compound is determined by knowing the percentage composition of a compound.
The covalent compound silica (sand) has simplest ratio of 1:2 of silicon and oxygen respectively. Therefore, its empirical formula is SiO2 Similarly, glucose has 2 simplest ratio 1:2:1 of carbon, hydrogen and oxygen, respectively. Hence, its empirical formula is CH2O.
Formula unit: The simplest whole number ratio of ions, as present in the ionic compound is called formula unit. OR The simplest unit taken as a representative of an ionic compound is called formula unit. Ionic compounds have only empirical formulae. For example, formula unit of common salt consists of one Na+ and one CI– ion and its empirical formula is NaCl. Similarly, formula unit of potassium bromide is KBr, which is also its empirical formula.
Molecular Formula: formulae that show actual number of atoms of each element present in a molecule of that compound is called molecular formula.
Molecules are formed by the combination of atoms. These molecules are represented by molecular formula.
Derivation of molecular formula: molecular formula is derived from empirical formula by the following relationship:
Molecular formula = (Empirical formula)n
Where n is 1,2,3 and so on.
For example, molecular formula of benzene is C6H6 which is derived from the
empirical formula CH where the value of n is 6.
The molecular formula of a compound may be same or a multiple of the empirical formula. A few compounds having different empirical and molecular formulae are shown in Table.
Some Compounds with their Empirical and Molecular Formulae.
|Compound||Emperical Formula||Molecular Formula|
Some compounds may have same empirical and molecular formula e.g. water (H2O) hydrochloric acid (HCl), etc.