Atoms and Molecules (Class 9 Science)

Atoms are the smallest units of elements that retain chemical properties, while molecules are combinations of atoms. Understanding atomic structure and che

Atoms and Molecules (Class 9 Science)

Atoms are the smallest units of elements that retain chemical properties, while molecules are combinations of atoms. Understanding atomic structure and che

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TL;DR: Atoms are the smallest units of elements that retain chemical properties, while molecules are combinations of atoms. Understanding atomic structure an…

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Atoms are the smallest units of elements that retain chemical properties, while molecules are combinations of atoms. Understanding atomic structure and che

Atoms and Molecules MCQs with Answers & Explanations

Q1. Who proposed the Dalton's atomic theory?

  1. John Dalton ✓ (correct)
  2. Bohr
  3. Thomson
  4. Rutherford

Explanation: John Dalton proposed the atomic theory in 1808, stating that matter consists of tiny indivisible particles called atoms.

Q2. The mass number of an atom is equal to:

  1. Number of electrons + neutrons
  2. Number of protons + neutrons ✓ (correct)
  3. Number of protons
  4. Number of electrons

Explanation: The mass number is the sum of protons and neutrons, which constitute most of the atom's mass.

Q3. Which of the following is the correct electronic configuration of Oxygen (O)?

  1. 2, 4
  2. 2, 8
  3. 2, 6 ✓ (correct)
  4. 8, 2

Explanation: Oxygen has atomic number 8, so its electronic configuration is 2, 6 (2 electrons in first shell, 6 in second).

Q4. The molecular mass of CO2 is approximately:

  1. 32
  2. 28
  3. 64
  4. 44 ✓ (correct)

Explanation: Molecular mass of CO2 = 12 + (16 x 2) = 12 + 32 = 44 g/mol.

Q5. An ion with a positive charge is called:

  1. Cation ✓ (correct)
  2. Anion
  3. Atom
  4. Radical

Explanation: A cation is an ion with a positive charge, formed when an atom loses electrons.

Q6. The atomic mass unit is defined as:

  1. Mass of proton
  2. 1/12 of carbon-12 mass ✓ (correct)
  3. 1/16 of oxygen mass
  4. Mass of electron

Explanation: One atomic mass unit (u) is defined as 1/12 of the mass of a carbon-12 atom.

Q7. The formula for common salt is:

  1. Na2Cl2
  2. NaCl2
  3. NaCl ✓ (correct)
  4. Na2Cl

Explanation: The formula for sodium chloride (common salt) is NaCl, with one sodium and one chloride ion.

Q8. Which of the following is a diatomic molecule?

  1. O3
  2. P4
  3. S8
  4. H2 ✓ (correct)

Explanation: H2 (hydrogen) is a diatomic molecule consisting of two hydrogen atoms bonded together.

Q9. The valency of oxygen in most compounds is:

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

Explanation: Oxygen has a valency of 2 in most compounds because it needs 2 more electrons to complete its octet.

Q10. The molar mass of H2SO4 is approximately:

  1. 102
  2. 98 ✓ (correct)
  3. 100
  4. 96

Explanation: Molar mass of H2SO4 = (2 x 1) + 32 + (4 x 16) = 2 + 32 + 64 = 98 g/mol.

Frequently Asked Questions

What is the difference between atoms and molecules?

An atom is the smallest unit of an element, while a molecule is a combination of atoms bonded together.

How is the atomic mass of an element determined?

The atomic mass is the weighted average of the masses of all naturally occurring isotopes of that element.

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