Vidaara.orgClass 10 · Chemistry
CodeVID-C10-05-T1-01
Assignment — Early Attempts at Classification
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
Dobereiner arranged elements in groups of:
- A.two
- B.three
- C.eight
- D.ten
2.
The Law of Octaves was given by:
- A.Dobereiner
- B.Newlands
- C.Mendeleev
- D.Moseley
3.
Mendeleev arranged elements in increasing order of:
- A.atomic number
- B.atomic mass
- C.valency
- D.melting point
4.
Eka-aluminium predicted by Mendeleev was later discovered as:
- A.scandium
- B.gallium
- C.germanium
- D.argon
5.
The Law of Octaves was found to work well only up to:
- A.calcium
- B.iron
- C.zinc
- D.bromine
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
State Dobereiner's observation about the atomic masses in a triad, using one example.
7.
Why was Newlands' Law of Octaves rejected?
8.
Give any two limitations of Mendeleev's periodic table.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
State Mendeleev's periodic law and explain how the discovery of the noble gases supported his table.
10.
How did Mendeleev's prediction of eka-silicon demonstrate the power of his classification?
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Trace the development of element classification from Dobereiner to Mendeleev, mentioning the basis, one achievement and one limitation of each scheme.
Answer Key
Section A — Multiple Choice Questions
- (B) three
- (B) Newlands
- (B) atomic mass
- (B) gallium
- (A) calcium
Section B — Short Answer (2 marks)
- The atomic mass of the middle element is the average of the other two; e.g. in Li(7), Na(23), K(39), (7+39)/2 = 23 = mass of Na.
- It worked only up to calcium, assumed no new elements would be found, and forced unlike elements into the same slot, so it broke down for heavier elements.
- Hydrogen had no fixed position, and isotopes of an element had different masses but shared one position; also a few mass orders were reversed.
Section C — Short Answer (3 marks)
- Mendeleev's law: properties of elements are a periodic function of their atomic masses. When He, Ne and Ar were discovered, they were placed in a new group (zero group) without disturbing the existing arrangement, supporting the table's strength.
- Mendeleev left a gap below silicon and predicted the properties of eka-silicon. When germanium was discovered later, its atomic mass, density and oxide formula matched the prediction closely, proving his periodic arrangement was sound.
Section D — Long Answer (5 marks)
- Dobereiner (triads): grouped three similar elements; achievement was spotting the average-mass pattern (Li, Na, K); limitation was that only three or four triads could be made. Newlands (octaves): arranged by increasing atomic mass and found every eighth element similar; achievement was the first periodic idea; limitation was failure beyond calcium and forcing unlike elements together. Mendeleev (periodic table): arranged 63 elements by atomic mass and similar chemical properties; achievement was leaving gaps and predicting eka-elements (Sc, Ga, Ge) and fitting noble gases; limitation was no fixed place for hydrogen, isotopes sharing a position and a few reversed mass orders.
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