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CBSE Class 9 Atoms and Molecules — practice questions

Explore 50 free Atoms and Molecules questions in CBSE Class 9 Science. Open each question or exercise to read its content, reference answers and available explanation.

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Atoms and Molecules questions and exercises with answers

Showing questions 1–30 of 50.

According to the Bohr model, the ground-state energy of a hydrogen atom is:Read the answer and worked solution →In the reaction H₂ + O₂ → H₂O, when the products are weighed, the mass equals:Read the answer and worked solution →When an atom LOSES one or more electrons, it becomes:Read the answer and worked solution →As per your syllabus, who proposed the LAW OF CONSERVATION OF MASS in 1789 — and is called the Father of Modern Chemistry?Read the answer and worked solution →As per your syllabus, who is credited with the LAW OF CONSTANT PROPORTIONS (or Definite Proportions)?Read the answer and worked solution →As per your syllabus, 'Bridging Science and Society' (§9.4.1), which Indian scientist is often called the 'Father of the Indian Nuclear Programme'?Read the answer and worked solution →As per your syllabus, naming rules for covalent compounds (§9.4.1 C), how is CO named?Read the answer and worked solution →As per your syllabus, how is SF₆ named?Read the answer and worked solution →As per your syllabus, what is the COMMON NAME for the covalent compound NH₃ (which would otherwise be called nitrogen trihydride)?Read the answer and worked solution →As per your syllabus, the formula NH₄⁺ represents which kind of ion?Read the answer and worked solution →As per your syllabus, what is the VALENCY of magnesium ion (Mg²⁺)?Read the answer and worked solution →As per your syllabus, what is the CHEMICAL FORMULA for calcium chloride — using the cross-over rule?Read the answer and worked solution →CLASS9 · Chemistry · Atom or MoleculeRead the answer and worked solution →Sort: element, compound, or mixture?Read the answer and worked solution →Pop the elements — skip the compoundsRead the answer and worked solution →Atomic structure — true or false?Read the answer and worked solution →An element has two isotopes: one of mass 9 amu (50% abundance) and another of mass 10 amu (50% abundance). What is its average atomic mass?Read the answer and worked solution →How many entities are present in exactly one mole of any substance (the value of Avogadro's number)?Read the answer and worked solution →Who proposed the Law of Conservation of Mass?Read the answer and worked solution →The total mass after a chemical reaction is:Read the answer and worked solution →The Law of Conservation of Mass was proposed in:Read the answer and worked solution →Pure water always contains hydrogen and oxygen in the mass ratio:Read the answer and worked solution →The Law of Constant Proportions is also called the law of:Read the answer and worked solution →Who proposed the atomic theory?Read the answer and worked solution →According to Dalton, atoms are:Read the answer and worked solution →A hydrogen molecule consists of how many atoms?Read the answer and worked solution →A hydrogen chloride molecule is formed from:Read the answer and worked solution →Two oxygen atoms in a molecule are joined by a:Read the answer and worked solution →Hydrogen and chlorine in HCl are joined by a:Read the answer and worked solution →A compound formed by sharing electrons is called:Read the answer and worked solution →

Worked practice questions

According to the Bohr model, the ground-state energy of a hydrogen atom is:

  1. $-13.6$ eV
  2. $-6.8$ eV
  3. $-27.2$ eV
  4. $-3.4$ eV
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Correct answer: A. $-13.6$ eV

Bohr's energy formula: $E_n = -\dfrac{13.6}{n^2}$ eV for hydrogen. For the ground state $n = 1$: $E_1 = -13.6$ eV. - Option A ($-3.4$ eV) is $E_2$. - Option D ($-27.2$ eV) is the magnitude of *twice* the ground-state energy (a common confusion with the virial-theorem relation $E = -\frac{1}{2}|U|$).

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In the reaction H₂ + O₂ → H₂O, when the products are weighed, the mass equals:

  1. Zero
  2. Less than the reactants
  3. Same as the reactants
  4. More than the reactants
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Correct answer: C. Same as the reactants

By the law of conservation of mass, the products weigh exactly the same as the reactants (provided no gas escapes the closed system).

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When an atom LOSES one or more electrons, it becomes:

  1. An anion (negatively charged ion)
  2. A neutron
  3. A cation (positively charged ion)
  4. A molecule
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Correct answer: C. A cation (positively charged ion)

Losing electrons means losing negative charge. With more protons than electrons, the atom becomes net positive — a cation. Conversely, gaining electrons makes it an anion (negative). **Example:** Na → Na⁺ + e⁻ (sodium cation formed by losing 1 electron).

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