Classification of Elements and Periodicity in Properties — Chemistry STD 11 Science — Question

1. Position of hydrogen: Hydrogen is placed in group I-A of the periodic table. However, it resembles the elements of both group I A (alkali metals) and group VII A (halogens). Therefore, the position of hydrogen in the periodic table is not clear.

2. Anomalous pairs of elements: In some cases, elements of higher atomic masses are placed before those having lower atomic masses.

Examples:

• $\text{Ar}\ \ \ \ \ \ \ \ \text{ and }\ \ \ \ \ \ \text{K}\\39.95\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 39.1$

• $\text{CO}\ \ \ \ \ \ \text{ and}\ \ \ \ \ \text{Ni}\ \ \ \ \ \ \ \ \ \ \text{Te}\ \ \ \ \ \text{and}\ \ \ \ \text{I}\\58.93\ \ \ \ \ \ \ \ \ \ \ \ \ 58.71\ \ \ \ \ \ 127.6\ \ \ \ \ \ \ \ \ \ 126.9$

3. Metals and non-metals: No attempt has been made to place metals and non-metals separately in it.

4. Lanthanoides and Actinodes: Fifteen elements are placed in one position.

Group (III) B

• 6^th period: La + 14 lathanoide elements. (₅₈Ce to ₇₁Lu).
• 7^th period: Ac + 14 actinoide elements. (₉₀Th to ₁₀₃Lr)

Thus, lanthanoids and actinoides have not been provided separate and proper places in the Mendeleev's periodic table.

5. Position of isotopes: Isotopes of elements are placed in the same position in the table though according to their atomic masses they should have been placed in different positions.

6. Similar elements separated in the table: Certain elements such as copper and mercury or gold and platinum which possess similar chemical properties are placed in different groups.

7. Dissimilar elements placed together in the same group: Alkali metals such as Li, Na, K etc. (group IA) are grouped together with coinage metals such as Cu, Ag and Au (group IB) though their properties are quite different.

Number of elements in the periods. The presence of only 2 elements in the first period, 8 in the second and third etc. cannot be explained.