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CodeC10-CHEM-5-CW
Periodic Classification of Elements — Chapter Assignment
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- This is a full-length test covering the whole chapter — every topic is included.
- 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 (1 mark each)
6 × 1 = 6 marks
1.
Who gave the Law of Octaves?
- A.Dobereiner
- B.Newlands
- C.Mendeleev
- D.Moseley
2.
The modern periodic table is arranged in order of increasing:
- A.atomic mass
- B.atomic number
- C.valency
- D.density
3.
The number of periods in the modern periodic table is:
- A.7
- B.8
- C.18
- D.9
4.
Across a period, atomic size:
- A.increases
- B.decreases
- C.stays the same
- D.doubles
5.
The valency of an element with configuration 2, 8, 2 is:
- A.8
- B.2
- C.6
- D.0
6.
Down a group, metallic character:
- A.decreases
- B.increases
- C.stays the same
- D.becomes zero
Section B (2 marks each)
4 × 2 = 8 marks
7.
State Dobereiner's law of triads with one example.
8.
Give two limitations of Mendeleev's periodic table.
9.
An element has atomic number 13. Find its period and group.
10.
Why does atomic size increase down a group?
Section C (3 marks each)
2 × 3 = 6 marks
11.
Explain how Moseley's work changed the basis of the periodic table and state the modern periodic law.
12.
An element X has configuration 2, 8, 1 and Y has 2, 8, 7. Compare their metallic character and predict the formula of their compound.
Section D (5 marks each)
2 × 5 = 10 marks
13.
Explain the trends in valency, atomic size and metallic character across a period and down a group, giving the reason for each.
14.
Trace the history of element classification from Dobereiner to the modern periodic table, giving the basis of each scheme and how each problem was overcome.
Answer Key
Section A (1 mark each)
- (B) Newlands
- (B) atomic number
- (A) 7
- (B) decreases
- (B) 2
- (B) increases
Section B (2 marks each)
- In a triad of three similar elements arranged by atomic mass, the mass of the middle element is the average of the other two; e.g. Li(7), Na(23), K(39).
- Hydrogen had no fixed position and isotopes (same element, different masses) had to share one place; also some pairs had reversed mass order.
- Configuration 2, 8, 3; three shells means Period 3 and three valence electrons means Group 13 (aluminium).
- A new shell is added at each step, so the outermost electrons lie farther from the nucleus and the atom becomes larger.
Section C (3 marks each)
- Moseley's X-ray studies showed that atomic number (number of protons), not atomic mass, is the fundamental property of an element. Rearranging by atomic number removed anomalies such as Ar before K. The modern periodic law states that the properties of elements are a periodic function of their atomic numbers.
- X has 1 valence electron, loses it easily and is strongly metallic; Y has 7 valence electrons, gains 1 and is non-metallic. X loses 1 electron to Y, forming the ionic compound XY (like NaCl).
Section D (5 marks each)
- Valency: across a period it rises 1 to 4 then falls to 0 (Na=1 ... Si=4 ... Ar=0), but down a group it stays the same because valence electrons are unchanged. Atomic size: decreases across a period (nuclear charge increases while shells stay fixed, pulling electrons in) and increases down a group (a new shell is added each time). Metallic character: decreases across a period (smaller size and stronger pull make electrons harder to lose) and increases down a group (larger size loosens outer electrons). Each trend is a balance between rising nuclear charge and added shells; across a period charge dominates, down a group the new shells dominate.
- Dobereiner grouped elements into triads by atomic mass (middle mass = average) but found only a few triads. Newlands arranged elements by increasing atomic mass and saw every eighth element repeat (Law of Octaves), but it failed beyond calcium. Mendeleev arranged 63 elements by atomic mass and similar properties, leaving gaps and predicting eka-elements (later Sc, Ga, Ge) and fitting noble gases, but could not place hydrogen or isotopes and had reversed mass pairs. Moseley then showed atomic number is fundamental, giving the modern periodic law (properties are a periodic function of atomic number); arranging by atomic number fixed the reversed pairs and let isotopes share one position.
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