Chapter 8 — Periodic Classification of Elements: Multiple Choice Questions
Instructions: Each question carries 1 mark. Choose the best answer.
1. The modern periodic law states that properties of elements are periodic functions of their ______.
Answer
(a) Atomic mass (b) Atomic number (c) Atomic radius (d) Number of neutrons
✅ Answer: (b) Atomic number
ExplanationMoseley's Modern Periodic Law (1913): Properties of elements are periodic functions of their atomic number (number of protons). This corrected Mendeleev who based it on atomic mass (which placed some elements in wrong positions).
2. The number of periods in the modern periodic table is ______.
Answer
(a) 7 (b) 8 (c) 18 (d) 4
✅ Answer: (a) 7
ExplanationThe modern periodic table has 7 periods (horizontal rows, numbered 1–7) and 18 groups (vertical columns). The period number indicates the number of electron shells in elements of that period.
3. Elements in the same group have ______.
Answer
(a) same atomic mass (b) same number of neutrons (c) same number of valence electrons (d) same number of electron shells
✅ Answer: (c) same number of valence electrons
ExplanationElements in the same group (vertical column) have the same number of valence electrons → similar chemical properties. Elements in the same period (horizontal row) have the same number of electron shells.
4. Mendeleev's periodic law was based on ______.
Answer
(a) Atomic number (b) Atomic mass (c) Valence electrons (d) Electronic configuration
✅ Answer: (b) Atomic mass
ExplanationDmitri Mendeleev (1869) arranged elements in order of increasing atomic mass and observed that properties repeated periodically. He called this the periodic law. Henry Moseley (1913) later corrected this to atomic number.
5. Moving from left to right across a period, atomic radius ______.
Answer
(a) increases (b) decreases (c) remains the same (d) first increases then decreases
✅ Answer: (b) decreases
ExplanationAcross a period (left to right), nuclear charge (protons) increases while the number of shells stays the same. Higher nuclear charge pulls electrons closer → atomic radius decreases. Moving down a group, a new shell is added each time → radius increases.
6. The most electronegative element is ______.
Answer
(a) Caesium (b) Francium (c) Fluorine (d) Oxygen
✅ Answer: (c) Fluorine
ExplanationFluorine (F) is the most electronegative element (electronegativity = 4.0 on Pauling scale). Electronegativity increases across a period and decreases down a group. Fluorine is in period 2, group 17 (top-right of periodic table, excluding noble gases).
7. Noble gases are placed in ______.
Answer
(a) Group 1 (b) Group 2 (c) Group 17 (d) Group 18
✅ Answer: (d) Group 18
ExplanationNoble gases (He, Ne, Ar, Kr, Xe, Rn) are in Group 18 (also called Group 0 in older notation). They have completely filled valence shells (8 electrons, except He with 2), making them chemically inert (very unreactive).
8. Alkali metals are placed in ______.
Answer
(a) Group 1 (b) Group 2 (c) Group 17 (d) Group 18
✅ Answer: (a) Group 1
ExplanationAlkali metals (Li, Na, K, Rb, Cs, Fr) are in Group 1. They have 1 valence electron, are very reactive (especially with water), and form +1 ions. They are soft metals stored in oil (to prevent reaction with air and moisture).
9. The period number of an element indicates ______.
Answer
(a) number of valence electrons (b) number of protons (c) number of electron shells (d) atomic mass
✅ Answer: (c) number of electron shells
ExplanationThe period number equals the number of electron shells (energy levels) in atoms of that period. Period 1 elements (H, He) have 1 shell; Period 2 elements (Li to Ne) have 2 shells; Period 3 (Na to Ar) have 3 shells, etc.
10. Which scientist proposed the first periodic table?
Answer
(a) Henry Moseley (b) John Newlands (c) Dmitri Mendeleev (d) Lothar Meyer
✅ Answer: (c) Dmitri Mendeleev
ExplanationDmitri Mendeleev (1869) proposed the first systematic periodic table, arranging 63 known elements by atomic mass. He was remarkable for:
- Leaving gaps for undiscovered elements (gallium, scandium, germanium)
- Predicting properties of missing elements
- His predictions were later confirmed, validating his table
(John Newlands proposed the Law of Octaves in 1866; Moseley improved Mendeleev's table in 1913)
Quick Revision — Periodic Table Trends
| Property | Across Period (→) | Down Group (↓) |
|---|
| Atomic radius | Decreases | Increases |
| Ionisation energy | Increases | Decreases |
| Electronegativity | Increases | Decreases |
| Metallic character | Decreases | Increases |
| Non-metallic character | Increases | Decreases |
Key Groups to Remember
| Group | Name | Examples | Valence electrons |
|---|
| 1 | Alkali metals | Li, Na, K | 1 |
| 2 | Alkaline earth metals | Be, Mg, Ca | 2 |
| 17 | Halogens | F, Cl, Br, I | 7 |
| 18 | Noble gases | He, Ne, Ar | 8 (He=2) |
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