Periodic classification of Elements

Periodic classification of Elements

Father of periodic table — Mendeleev.
The arrangement of the known elements in certain groups in such a way so that the elements with similar properties are grouped together is known as classification of elements.

Genesis of periodic classification :

1. Lavoisier's classification

Lavoisier's classification was based on the basis of the physical properties of the elements, such as hardness, malleability and lustre. He classified the elements into two groups, i.e. as metals and non metals. The first elements, he classified, was sodium and lead.

2. Dobereinier's Triads

In 1829, Dobereiner, a German chemist arranged certain elements with similar properties in groups of three in such a way that the atomic mass of the middle element was nearly the same as the average atomic masses of the first and third elements.

Triad	Lithium	Sodium	Potassium
Atomic mass	7	13	39

atomic mass of sodium = 39+7/2 = 23
But only few elements could be covered under triads

3. Newland's law of octaves

In 1866, John Newlands, An English Chemist proposed the law of octaves by stating that, When elements are arranged in order to increasing atomic masses, every eighth element has properties similar to the first, just like musical notes.

But this generalization was also rejected because it could not be extended to the elements with atomic mass more than 40.

Example : Starting from lithium (Li), the eighth element is sodium (Na), and its properties are similar to those of the lithium. Similarly, beryllium (Be), magnesium (Mg), and calcium (Ca) show similar properties. Fluorine (F) and chlorine (Cl) are also similar chemically.

4. Lother's-Mayer's atomic volume curve

In 1869 Lother mayer plotted a graph between atomic volume of the elements and their atomic mass and he pointed that the elements with similar properties occupy similar position in the curve.

5. Mendeleev's periodic law

The physical and chemical properties of the elements are the periodic function of their atomic masses.

Mendeleev's arranged the elements known at that time in increasing order of atomic masses and this arrangement was periodic table.

But in modern periodic table, it states that ‘physical and chemical properties of the elements are periodic functions of their atomic numbers’.

In periodic table :
      Horizontal line is called periods.
      Vertical line is called group.

In Mendeleev's periodic table :
      Period— 7
      Group— 9 (I, II, III, IV, V, VI, VII, VIII, Zero)

6. Modem Periodic law

Modem periodic law was given by Moseley. According to Moseiev : "The physical and chemical properties of the elements are the periodic function of their atomic numbers.

In modern periodic table :
      Period — 7
      Group — 18

Modern periodic table are classified as :
      (i) s-block
      (ii) p-block
      (iii) d-block
      (iv) f-block