Periodic Classification of Elements NCERT Solution, study material, CBSE Notes

NCERT Solution: Periodic Classification of Elements

Did Dobereiner’s triads also exist in the columns of Newlands’ Octaves? Compare and find out.

Only one triad of Dobereiner's triads exists in the columns of Newlands' octaves. The triad formed by the elements Li, Na, and K of Dobereiner's triads also occurred in the columns of Newlands' octaves.

Dobereiner's triads

Li Na K

Ca Sr Br

Cl Br I

Newlands' octaves

H	Li	Be	B	C	N	O
F	Na	Mg	Al	Si	P	S
Cl	K	Ca	Cr	Ti	Mn	Fe
Co and Ni	Cu	Zn	Y	In	As	Se
Br	Rb	Sr	Ce and La	Zr	-	-

What were the limitations of Dobereiner’s classification?

Limitation of Dobereiner’s classification:
All known elements could not be classified into groups of triads on the basis of their properties.

What were the limitations of Newlands’ Law of Octaves?

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.

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 K₂O.
C is in group 4. Therefore, the oxide will be CO₂.
Al is in group 3. Therefore, the oxide will be Al₂O₃.
Si is in group 4. Therefore, the oxide will be SiO₂.
Ba is in group 2. Therefore, the oxide will be BaO.

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

Scandium and germanium.

What were the criteria used by Mendeleev in creating his Periodic Table?

Mendeleev used atomic mass of the elements as the unique criteria of the elements. He proposed that the chemical properties of elements are the periodic function of their atomic masses. And thus, he arranged the elements in the increasing order of their atomic masses.

Why do you think the noble gases are placed in a separate group?

oble gases are inert elements. Their properties are different from the all other elements. Therefore, the noble gases are placed in a separate group.

How could the Modern Periodic Table remove various anomalies of Mendeleev’s Periodic Table?

Various anomalies of Mendeleev’s Periodic Table removed as follows in the Modern Periodic Table:

→ Elements are arranged in the increasing order of their atomic number in Modern Periodic Table, thus there was no need for keeping more than one element in one slot.
→ In Modern Periodic Table there was no problem of the place of isotopes, as isotopes have same atomic mass with different atomic numbers.

→ Elements having same valence electron are kept in same group.
→ Elements having same number of shells were put under the same period.
→ Position of hydrogen became clarified in as it is kept in the group with the elements of same valence electrons.