Periodic Trends — Set 3
Chemistry · आवर्ती प्रवृत्तियां · Questions 21–30 of 50
Which of the following sets of elements represents Dobereiner's Triads?
Correct Answer: D. Li, Na, K
• **Li-Na-K triad** = Lithium, sodium, and potassium form a classic Dobereiner's triad. • **Atomic weight pattern** — Sodium's atomic weight is approximately the average of lithium and potassium. • 💡 Wrong-option analysis: A: CNO doesn't form triad; B: FCl O doesn't follow pattern; C: HHe Li doesn't form triad.
Which element has the electronic configuration [Ar] 4s¹?
Correct Answer: C. Potassium
• **Potassium config** = Potassium with atomic number 19 has configuration [Ar] 4s¹. • **Period 4 start** — It's the first element of period 4 with one electron in the 4s orbital. • 💡 Wrong-option analysis: A: Magnesium is 3s²; B: Sodium is 3s¹; D: Calcium is 4s².
As we move across a period, the non-metallic character of elements?
Correct Answer: A. Increases
• **Non-metal increase** = Non-metallic character increases from left to right across a period. • **Metal to nonmetal** — Transition from metallic elements on the left to nonmetallic on the right. • 💡 Wrong-option analysis: B: Character definitely changes; C: Monotonic increase, not first decrease; D: Opposite trend.
Which period in the modern periodic table is the shortest?
Correct Answer: C. Period 1
• **Period 1 shortest** = Period 1 is the shortest, containing only two elements: hydrogen and helium. • **Single shell** — Elements only have electrons in the first shell (1s and 1s²). • 💡 Wrong-option analysis: A: Period 2 has 8 elements; B: Period 7 is longest with 32; D: Period 4 has 18.
Which of the following is not an alkaline earth metal?
Correct Answer: B. Sodium
• **Sodium exception** = Sodium is an alkali metal (Group 1), not an alkaline earth metal (Group 2). • **Different group** — Alkaline earth metals are in Group 2 with +2 valency, while sodium is Group 1 with +1. • 💡 Wrong-option analysis: A: Barium is Group 2; C: Calcium is Group 2; D: Magnesium is Group 2.
The shielding effect is responsible for the trend in which property?
Correct Answer: B. Atomic radius
• **Shielding cause** = The shielding effect causes atomic radius to increase down a group. • **Inner electrons** — Inner electrons shield outer electrons from full nuclear charge. • 💡 Wrong-option analysis: A: Metallic character depends on multiple factors; C: Electronegativity has different causes; D: Melting point varies irregularly.
Which of the following correctly shows increasing order of atomic radius?
Correct Answer: A. Na > Al > Cl
• **Left-to-right decrease** = Atomic radius decreases across period 3: Na > Al > Cl. • **Nuclear charge** — Increasing protons in same shell pull electrons closer. • 💡 Wrong-option analysis: B: Reverse of correct order; C: Incorrect arrangement; D: Incorrect arrangement.
What is the oxidation state of oxygen in most of its compounds?
Correct Answer: B. -2
• **Oxygen -2 state** = Oxygen has an oxidation state of -2 in almost all compounds (except peroxides). • **Electron affinity** — Oxygen readily gains two electrons to complete its octet. • 💡 Wrong-option analysis: A: -1 is in peroxides only; C: Oxygen rarely oxidizes; D: Zero in elemental form.
Which metal is the best conductor of electricity?
Correct Answer: C. Silver
• **Silver conductivity** = Silver is the best electrical conductor among all metals. • **Free electrons** — Its electron configuration allows maximum free electron mobility. • 💡 Wrong-option analysis: A: Copper is second; B: Iron is lower; D: Gold is similar but less than silver.
Which element shows allotropy most prominently?
Correct Answer: A. Carbon
• **Carbon allotropy** = Carbon displays allotropy most prominently with diamond, graphite, and fullerenes. • **Different structures** — Each form has distinct properties despite same element composition. • 💡 Wrong-option analysis: B: Sulfur shows some allotropy; C: Phosphorus shows some allotropy; D: Carbon shows most forms.