Atoms and Molecules Olympiad (Class 8 Science)

NSO challenging questions on atomic structure, molecular composition, and chemical formulas. Olympiad-level logical reasoning on matter and substances.

Atoms and Molecules Olympiad (Class 8 Science)

NSO challenging questions on atomic structure, molecular composition, and chemical formulas. Olympiad-level logical reasoning on matter and substances.

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TL;DR: NSO challenging questions on atomic structure, molecular composition, and chemical formulas. Olympiad-level logical reasoning on matter and substances…

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NSO challenging questions on atomic structure, molecular composition, and chemical formulas. Olympiad-level logical reasoning on matter and substances.

Atoms and Molecules Olympiad MCQs with Answers & Explanations

Q1. An atom has 6 protons. Its atomic number is 6. How many electrons does it have in a neutral state?

  1. Cannot determine
  2. 6 ✓ (correct)
  3. 3
  4. 12

Explanation: In a neutral atom, the number of electrons equals the number of protons. Atomic number = protons = 6, so electrons = 6.

Q2. What is the correct formula for the compound formed between calcium and chlorine?

  1. Ca2Cl2
  2. Ca2Cl
  3. CaCl2 ✓ (correct)
  4. CaCl

Explanation: Calcium (Ca) is in Group 2 with valency +2. Chlorine (Cl) is in Group 17 with valency -1. Formula: CaCl2.

Q3. If the atomic mass of Oxygen is 16 and that of Carbon is 12, what is the molecular mass of CO2?

  1. 24
  2. 32
  3. 28
  4. 44 ✓ (correct)

Explanation: Molecular mass of CO2 = 12 + 2*16 = 12 + 32 = 44.

Q4. An element has atomic number 7. To which group does it belong?

  1. Group 15 ✓ (correct)
  2. Group 16
  3. Group 17
  4. Group 18

Explanation: Atomic number 7 is Nitrogen. Its electronic configuration is 2,5, placing it in Group 15 (5 valence electrons).

Q5. Which of these represents an ionic compound?

  1. CH4
  2. NaCl ✓ (correct)
  3. H2
  4. CO2

Explanation: NaCl is formed between Na (metal) and Cl (nonmetal) through ionic bonding with electron transfer. Others are covalent or elemental.

Q6. A compound contains 2 hydrogen atoms and 1 sulfur atom. What is its formula?

  1. H2S2
  2. HS
  3. H2S ✓ (correct)
  4. S2H2

Explanation: The formula directly lists the atoms: 2 hydrogen and 1 sulfur = H2S.

Q7. What is the valency of Nitrogen when it forms NH3?

  1. 4
  2. 1
  3. 5
  4. 3 ✓ (correct)

Explanation: In NH3 (ammonia), Nitrogen forms 3 covalent bonds with 3 hydrogen atoms, showing valency of 3.

Q8. An atom becomes a positive ion by losing electrons. How many electrons did it lose if it went from 18 electrons to 16?

  1. 2 ✓ (correct)
  2. 1
  3. 3
  4. 4

Explanation: 18 - 16 = 2 electrons lost, forming a 2+ ion.

Frequently Asked Questions

What is the difference between atomic number and mass number?

Atomic number is the number of protons in an atom's nucleus, which defines the element. Mass number is the total of protons and neutrons. For example, Carbon-12 has atomic number 6 (6 protons) and mass number 12 (6 protons + 6 neutrons). Atomic number determines chemical properties, while mass number distinguishes between isotopes of the same element.

How do we predict the formula of a compound from valencies?

Find the valencies of both elements. Write them as subscripts of the opposite element, then simplify to lowest terms. For example, Magnesium (valency +2) and Chlorine (valency -1): cross the valencies to get MgCl2. Aluminum (valency +3) and Oxygen (valency -2) gives Al2O3 after simplifying.

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