The element with atomic number 20 has the electronic configuration: MCQ with Answer and Explanation

Question
The element with atomic number 20 has the electronic configuration:
A. 2,8,8,2
B. 2,8,18,2
C. 2,8,10
D. 2,8,8,4
Answer: Option A
Solution (By JKExamLibrary)
Atomic number 20 is calcium (Ca). Electron shells fill according to 2n²: K=2, L=8, M can hold 18 but fourth shell (N) starts after 8 electrons in M due to energy level considerations (Aufbau principle). So Ca configuration is 2,8,8,2. It has 2 valence electrons, belongs to Group 2, Period 4. Its nearest noble gas is Ar (2,8,8).
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Question #1
The role of a catalyst in a chemical reaction is to:
A. Increase the equilibrium constant
B. Decrease the enthalpy change
C. Increase the temperature of the reaction
D. Provide an alternative pathway with lower activation energy
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Correct Answer: Option D

Explanation:
A catalyst lowers the activation energy (Ea) by providing an alternative reaction mechanism, thereby increasing the rate of both forward and reverse reactions equally. It does not affect the equilibrium constant, enthalpy change (ΔH), or the position of equilibrium. It remains chemically unchanged after reaction. Positive catalysts speed up, negative catalysts slow down. Enzymes are biological catalysts.
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Question #2
Which salt is used in the manufacture of soap?
A. Sodium carbonate
B. Sodium bicarbonate
C. Sodium chloride
D. Sodium hydroxide
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Correct Answer: Option D

Explanation:
Soap is made by saponification using NaOH (for hard soap) or KOH (for soft soap). NaCl is used to precipitate soap; Na₂CO₃ is washing soda.
This question belongs to: Science chemistry
Question #3
Which gas diffuses slowest?
A. CH₄
B. H₂
C. CO₂
D. NH₃
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Correct Answer: Option C

Explanation:
Rate ∝ 1/√M. Molar masses: H₂=2, CH₄=16, NH₃=17, CO₂=44. CO₂ has highest mass, diffuses slowest.
This question belongs to: Science chemistry
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