Periodic Classification Elements Class 10 Notes PDF

Periodic Classification Elements Chapter 5 CBSE Class 10 Notes PDF Free Download

Periodic Classification Elements Class 10 Notes

In the year 1817, Johann Wolfgang Döbereiner, a German chemist, tried to arrange the elements with similar properties into groups.

He identified some groups having three elements each. So he called these groups ‘triads’.

Döbereiner showed that when the three elements in a triad were written in the order of increasing atomic masses; the atomic mass of the middle element was roughly the average of the atomic masses of the other two elements.

For example, take the triad consisting of lithium (Li), sodium (Na) and potassium (K) with the respective atomic masses 6.9, 23.0 and 39.0. What is the average of the atomic masses of Li and K? How does this compare with the atomic mass of Na? Given below (Table 5.1) are some groups of three elements.

These elements are arranged downwards in order of increasing atomic masses. Can you find out which of these groups form Döbereiner triads?

The attempts of Döbereiner encouraged other chemists to correlate the properties of elements with their atomic masses. In 1866, John Newlands, an English scientist, arranged the then known elements in the order of increasing atomic masses.

He started with the element having the lowest atomic mass (hydrogen) and ended at thorium which was the 56th
element. He found that every eighth element had properties similar to that of the first. He compared this to the octaves found in music. Therefore, he called it the ‘Law of Octaves’.

It is known as ‘Newlands’ Law of Octaves’. In Newlands’ Octaves, the properties of lithium and sodium were found to be the same. Sodium is the eighth element after lithium. Similarly, beryllium and magnesium resemble each other.

Achievements of Mendeleev’s Periodic Table

1. A systematic study of elements: Elements with similar properties were grouped together, which made the study of
their chemical and physical properties easier.
2. Correction of atomic masses: Placement of elements in Mendeleev’s periodic table helped in correcting the atomic
masses of certain elements. For example, the atomic mass of beryllium was corrected from 13.5 to 9. Similarly, atomic masses of indium, gold, platinum, etc., were also corrected.
3. Prediction of properties of yet to be discovered elements: Eka-boron, eka-aluminum, and eka-silicon were the names
given to yet to be discovered elements. The properties of these elements could be predicted accurately from the elements that belonged to the same group. These elements, when discovered were named scandium, gallium, and germanium, respectively.
4. Placement of noble gases: When discovered, they were placed easily in a new group called zero groups of Mendeleev’s table, without disturbing the existing order.

Limitations of Mendeleev’s Periodic Table

1. Position of hydrogen: Hydrogen resembles both, the alkali metals (IA) and the halogens (VIIA) in properties, so,
Mendeleev could not justify its position.
2. Position of isotopes: Atomic weight of isotopes differ, but,they were not placed in different positions in Mendeleev’s
periodic table.
3. Anomalous pairs of elements: Cobalt (Co) has higher atomic weights but was placed before Nickel (Ni) in the periodic table.
4. Placement of like elements in different groups: Platinum (Pt) and Gold (Au) has similar properties but were placed in
different groups.
5. Cause of periodicity: He could not explain the cause of periodicity among the elements.

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