Periodic Classification of Elements

Dobereiner's triads According to Dobereiner's triads - 'when three elements in a triad are arranged in order of increasing atomic masses, the atomic mass of the middle element is roughly the average of the atoms of the other two elements.' E.g. Li (6.9), Na (23), K (39). Avg at mass of Na = (6.9+39)/2 = 22.95.

Limitations of Dobereiner's triads . Dobereiner could identify only three triads from the elements known at that time. So, all the elements could not be arranged in triads.

Newlands' Law of octaves According to the Newlands' Law of octaves - when the elements are arranged according to increasing atomic mass, the properties of every eighth element are similar to that of first.

E.g. elements with atomic mass up to 40 are

H F Cl

Li Na K

Be Mg Ca

B Al

C Si

N P

O S

Limitations of Newlands’ Law of octaves

1. This law was applicable only to lighter elements (up to atomic mass 40).
2. Newlands assumed that only 56 elements existed in nature. But, later on many new elements were discovered whose properties did not fit into law of octaves.
3. To fit the elements in his table, he adjusted two elements in the same slot. He also put some unlike elements under same note.

Mendeleev’s Periodic Law According to Mendeleev’s Periodic Law – ‘The physical and chemical properties of elements are the periodic function of their atomic masses.’

Achievement of Mendeleev’s Periodic Table.            

1. Mendeleev placed an element with slightly larger atomic mass before an element with slightly lower atomic mass so that the elements with same properties fell in the same group. E.g. Co (58.93) was placed before Ni (58.71).
2. Mendeleev left some gaps for the elements to be discovered.
3. Correction of doubtful atomic mass.

Limitations of Mendeleev’s Periodic Table

1. Correct position of hydrogen could not be assigned.
2. Anomalous position of isotopes.
3. Uncertainty in discovery of new elements.
4. Wrong order of atomic masses.

Modern Periodic Law. According to Modern Periodic Law - ‘The physical and chemical properties of elements are the periodic function of their atomic numbers.’

Explanation of limitations of Mendeleev’s Periodic Table.

1. Position of isotopes. Since all isotopes of an element have same atomic number, they can be placed in same group. E.g. C-12 and C-13 both have atomic number 6.
2. Anomalous position of some elements. Co (58.93) and Ni (58.71)Cobalt with higher atomic mass is placed before nickel because the atomic numbers of cobalt and nickel are 27 and 28.
3. Uncertainty in prediction of new elements. Atomic masses do not increase in regular manner i.e. they may be in decimals. But, atomic numbers increase in regular manner i.e. increase by 1 in going from one element to the next.

Groups. The vertical columns in the Modern Periodic Table are called groups. There are 18 groups in the Modern Periodic Table.

Periods. The horizontal rows in the Modern Periodic Table are called periods. There are 7 periods in the Modern Periodic Table.

Period no	1	2	3	4	5	6	7
No of elements	2	8	8	18	18	32	incomplete
Name of period	Very short	Short	Short	Long	Long	Very long	incomplete