All known elements could not be classified into groups of triads on the basis of their properties.

Limitations of Newlands’ law of octaves:

→ It was not applicable throughout the arrangements. It was applicable up to calcium only. The properties of the elements listed after calcium showed no resemblance to the properties of the elements above them.

→ Those elements that were discovered after Newlands’ octaves did not follow the law of octaves.

→ The position of cobalt and nickel in the group of the elements (F, Cl) of different properties could not be explained.

→ Placing of iron far away from cobalt and nickel, which have similar properties as iron, could also not be explained.

[**NCERT In Text**]

Use Mendeleev’s Periodic Table to predict the formulae for the oxides of the following elements:

K, C, Al, Si, Ba.

K is in group 1. Therefore, the oxide will be K_{2}O.

C is in group 4. Therefore, the oxide will be CO_{2}.

Al is in group 3. Therefore, the oxide will be Al_{2}O_{3}.

Si is in group 4. Therefore, the oxide will be SiO_{2}.

Ba is in group 2. Therefore, the oxide will be BaO.

[**NCERT In Text**]

Besides gallium, which other elements have since been discovered that were left by Mendeleev in his Periodic Table? (any two)

Scandium and germanium.