EARLY CLASSIFICATION OF ELEMENTS:
1. Dӧbereiner’s Triads
- The elements were divided into a group of three such that their atomic masses were in increasing order and the atomic mass of the middle element was roughly the average of the atomic mass of the other two elements.
- For e.g.: The Li, Na and K triad. Atomic masses of Li, Na and K are 6.9, 23.0 and 39.0 respectively. Average of Li and K atomic masses= (6.9+39.0)/2= 22.95≈ 23.0
2. Newland’s Laws of octaves
- John Newland, an English scientist, arranged all elements in increasing order of their atomic masses and found that every eighth element had properties similar to that of the first element.
- It was however found that this law was only applicable to elements upto calcium. The elements after calcium did not follow this rule.
Mendeleev’s Periodic Table
Mendeleev formulated the Periodic Law which states that the properties of elements are the periodic function of their atomic masses. He arranged the elements in a table called the periodic table consisting of vertical columns called ‘groups’ and horizontal rows called ‘periods’.
- No fixed position could be given to hydrogen.
- Isotopes were discovered after Mendeleev’s Periodic table was formed. Accommodating isotopes in the table posed a problem
- It was not possible to predict how many elements could be discovered between two elements.
THE MODERN PERIODIC TABLE
Henry Moseley showed that that the atomic number of an element is a more fundamental property. The modern periodic law states that properties of elements are a periodic function of their atomic numbers.
Trends in the modern periodic table:
1. Valency: The valency of an element is determined by the number of valence electrons present in the outermost shell of its atom. Valency increases in a period on going from left to right. Valency remains constant on going down the group.
2. Atomic size: Atomic size refers to the radius of an atom i.e. the distance between the centre of the nucleus and the outermost shell of the isolated atom. The atomic size decreases in moving from left to right along a period but it increases as we go down the group.
3. Metallic and non-metallic character: Metallic character decreases across a period and increases down a group. Non-metallic character increases across a period and decreases down a group.