Test: Classification Early Attempts - UPSC MCQ

In 1817 a German chemist Johann Wolfgang Döbereiner arranged the elements with similar properties into groups.

Some groups were identified having three elements each. So he called these groups ‘triads’. Only three triads could be identified from the elements discovered at that time.

The noble gases are missing from Newlands' octaves because these elements were not known at that time. 

Dobereiner's tried to arrange the element with similar properties into group. He identified some groups having three elements each. So he called these groups 'triads'.

Atomic mass of Y = (Atomic mass of X + Atomic mass of Z)/2

81 = (37 + Z) /2

162 -37 = Z

Z = 125

Mendeleev's Periodic Law states that the properties of elements are the periodic function of their relative atomic masses. Mendeleev arranged all 63 elements; which were discovered till his time; in the order of their increasing relative atomic masses in a tabular form. It is known as Mendeleev's Periodic Table.

The major limitations of Newlands' law of octaves were : 
It was applicable to only lighter elements having atomic masses upto 40 u, i.e., upto calcium. After calcium, the first and the eighth element did not have similar properties. For example chromium (Cr) and yttrium (Y) are the first and the eighth element placed in the same column but they have entirely different properties.  

Doberneiner's Law of Triads
When elements with similar properties are taken three at a time and arranged in the ascending order of their atomic weights, the atomic weight of the middle element is the average of the atomic weights of the first and third elements.

Dobereiner’s “triads” (1829): groups of three similar elements: Examples:

• Li, Na, K
• Ca, Sr, Ba
• Cl, Br, I

Mendeleev's Periodic Law states that the properties of elements are the periodic function of their relative atomic masses. Mendeleev arranged all 63 elements; which were discovered till his time; in the order of their increasing relative atomic masses in a tabular form. It is known as Mendeleev's Periodic Table.

Merits of Mendeleev's periodic table:

1) It helped in correcting the atomic masses of some of the elements e.g. beryllium was corrected from 13.5 to 9. Similarly, indium, gold, platinum etc.

2) He predicted the properties of some undiscovered elements and left gaps for them e.g. Eka-boron, eka-aluminum and eka-silicon names were given for scandium, gallium, and germanium (not discovered at the time of Mendeleev).

3) He framed the periodic table with vertical and horizontal columns and gave shape to it.

4) Isotopes are the atoms of the same element having different atomic masses. Therefore, according to Mendeleev’s classification, these should be placed at different places depending upon their atomic masses. For example, hydrogen isotopes with atomic masses 1,2 and 3 should be placed at three places. However, isotopes have not been given separate places in the periodic table because of their similar properties.

Hence, option A is correct.

Mendeleev started his work in the 1800’s when 63 elements were known to chemists. So Mendeleev first noted down the chemical properties of all the 63 elements on cards.

Hennig Brand was an alchemist looking for the philosopher's stone when he discovered phosphorus. Robert Boyle discovered it independently in 1860. Lavoisier separated metals and non-metals. Jons Jakob Berzelius used letters to signify elements.

Mendeleev missed receiving the Nobel Prize by one vote. Some atomic numbers of elements on the table were refined and corrected. Elements such as sulfur and hydrogen were difficult to place within the structure of the table but Mendeleev was able to construct the table around anomalous elements.

McMillan, Kennedy, and Wahl shared the discovery of plutonium with Seaborg. Element 106, seaborgium bears his name.

Mendeleev was very popular with the public because he was very good at teaching. He would ride third class on trains and talk to the other passengers about their farms and work and teach them relevant scientific information.

Dobereiner became a professor at the University of Jena in 1810. Between 1829 and 1858 a number of scientists continued work on these relationships and the "Law of Triads".