Mendeleev's periodic table had a gap for an element with properties similar to aluminum, later discovered and named: MCQ with Answer and Explanation

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Mendeleev's periodic table had a gap for an element with properties similar to aluminum, later discovered and named:

A. Germanium

B. Boron

C. Scandium

D. Gallium

Answer: Option D

Solution (By JKSSB Mock Tests)

Mendeleev named the missing element eka-aluminium. When gallium was discovered (1875), its properties (density, oxide formula) closely matched Mendeleev's predictions, validating his table. Germanium was eka-silicon; scandium was eka-boron. These predictions were a triumph of the periodic law. Gallium has a low melting point (~30°C) and is used in semiconductors. Mendeleev corrected atomic masses based on group properties.

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Question #1

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Ammonia gas is manufactured by the Haber process. The catalyst employed is:

A. Ni

B. Pt

C. V₂O₅

D. Fe

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Correct Answer: Option D

Explanation:
Haber process: N₂ + 3H₂ ⇌ 2NH₃, conditions: 400-450°C, 200 atm, finely divided iron with promoters (K₂O, Al₂O₃). Pt is used in Ostwald process for HNO₃; V₂O₅ in Contact process for H₂SO₄.

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Question #2

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The element with atomic number 17 belongs to which group and period?

A. Group 17, Period 2

B. Group 17, Period 3

C. Group 7, Period 2

D. Group 7, Period 3

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Correct Answer: Option B

Explanation:
The element with atomic number 17 is Chlorine. Its electronic configuration is 2, 8, 7. It has 3 shells, so it is in Period 3. It has 7 valence electrons, so it belongs to Group 17 (10 + 7 = 17 in modern IUPAC notation) along with fluorine, bromine, and iodine.

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Question #3

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Rusting of iron requires:

A. Both oxygen and water

B. Only oxygen

C. Only water

D. Carbon dioxide and water

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Correct Answer: Option A

Explanation:
Rusting is an electrochemical process requiring both oxygen and water (moisture). The overall reaction: 4Fe + 3O₂ + 2xH₂O → 2Fe₂O₃·xH₂O. In the absence of either, rusting does not occur. Salt accelerates rusting by increasing conductivity. Prevention methods include painting, greasing, galvanization, and cathodic protection. Rust is hydrated iron(III) oxide, a reddish-brown flaky substance, not protective.

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