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Science practice

Chemistry - Atomic Structure Questions with Answers

Atomic structure questions cover the discovery of the electron, proton and neutron, the atomic models of Thomson, Rutherford and Bohr, atomic number, mass number, isotopes and electronic configuration, at NCERT class 9 level. RRB Group D, ALP, NTPC and SSC papers typically ask who discovered a particle, the number of neutrons in an atom, or the electronic configuration of a simple element. Tip: always compute neutrons as mass number minus atomic number before reading the options.

Formulas and key facts

  • Atomic number Z = number of protons = number of electrons in a neutral atom
  • Mass number A = number of protons + number of neutrons
  • Number of neutrons = A - Z
  • Electron: negative charge, discovered by J.J. Thomson; proton: positive charge; neutron: no charge, discovered by James Chadwick
  • Maximum electrons in a shell = 2n²: K shell 2, L shell 8, M shell 18
  • Outermost shell holds at most 8 electrons (octet rule); the first shell is full with 2
  • Valency = number of electrons in the outermost shell if 4 or fewer, otherwise 8 minus that number
  • Isotopes: same atomic number, different mass numbers (for example, chlorine-35 and chlorine-37)
  • Isobars: same mass number, different atomic numbers (for example, argon-40 and calcium-40)
  • Rutherford's gold foil experiment with alpha particles showed a small, dense, positive nucleus
  • Thomson's model: positive sphere with electrons embedded (plum pudding model)
  • Bohr's model: electrons move in fixed orbits (shells) without radiating energy

Worked examples

Q1. Chlorine has atomic number 17 and mass number 35. How many neutrons does an atom of it have?

  1. Neutrons = A - Z.
  2. Neutrons = 35 - 17 = 18.

Answer: 18 neutrons

Q2. Write the electronic configuration of calcium (atomic number 20) and state its valency.

  1. A neutral atom has 20 electrons.
  2. Fill shells in order: K = 2, L = 8, M = 8, N = 2 (K, L and M follow the 2, 8, 8 rule; remaining 2 go to N).
  3. Configuration is 2, 8, 8, 2.
  4. Two electrons in the outermost shell, so valency = 2 (it loses 2 electrons).

Answer: 2, 8, 8, 2; valency 2

Q3. An ion X²⁺ has 10 electrons and a mass number of 24. Find the atomic number and the number of neutrons of the element.

  1. The 2+ charge means the atom lost 2 electrons.
  2. Neutral atom electrons = 10 + 2 = 12, so Z = 12.
  3. Neutrons = A - Z = 24 - 12 = 12.
  4. The element is magnesium.

Answer: Atomic number 12; 12 neutrons

Shortcuts that save time

  • Always compute protons, then neutrons, then electrons in that order. For ions, electrons change but protons and neutrons do not. Mixing these up is the usual source of negative marks.
  • For the first 20 elements, use the 2, 8, 8, 2 filling pattern. The valency of elements with 1, 2 or 3 outer electrons equals that number; with 5, 6 or 7 outer electrons it is 8 minus that number.
  • Match the scientist to the idea with one phrase each: Thomson with electron, Rutherford with nucleus, Bohr with orbits, Chadwick with neutron. If two options both seem right, skip rather than guess.

Where this appears

RRB Group DGeneral Science section
RRB ALPGeneral Science section
RRB NTPCGeneral Awareness section
SSC CGLGeneral Awareness section

Chemistry - Atomic Structure

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The cation present in sodium chloride (NaCl) is -

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Chemistry - Atomic Structure questions with answers

10 sample questions from this topic with the correct option and the solution.

  1. Q1. The cathode ray experiment was done for the first time by-

    1. J.J. Thomson
    2. Ernest Rutherford
    3. John Dalton
    4. Goldstein

    Answer: (A) J.J. Thomson

    Solution
    J.J. Thomson performed cathode ray experiments in a discharge tube in 1897. He showed that the rays are negatively charged particles, which led to the discovery of the electron. Goldstein discovered anode (canal) rays, Rutherford discovered the nucleus, and Dalton proposed the atomic theory.

  2. Q2. The calculated formula unit mass of Na 2 SO 4 is:

    1. 146 u
    2. 148 u
    3. 142 u
    4. 140 u

    Answer: (C) 142 u

    Solution
    Formula unit mass is the sum of atomic masses of all atoms in the formula. For Na2SO4: 2 × 23 (Na) = 46, 1 × 32 (S) = 32, 4 × 16 (O) = 64. Total = 46 + 32 + 64 = 142 u.

  3. Q3. The atomic number of an element is determined by the number of:

    1. neutrons
    2. both protons and neutrons
    3. protons
    4. electrons

    Answer: (C) protons

    Solution
    The atomic number of an element equals the number of protons in the nucleus of its atom. It is the unique identity of an element, and in a neutral atom it also equals the number of electrons. Neutrons affect only the mass number, not the atomic number.

  4. Q4. The atomic number is defined as the total number of __________ present in the nucleus of an atom.

    1. only protons
    2. only neutrons
    3. neutrons and half of the protons
    4. protons and neutrons

    Answer: (A) only protons

    Solution
    The atomic number of an element is defined as the number of protons in the nucleus of its atom. It is represented by Z and identifies the element. Neutrons do not affect the atomic number; they only change the mass number. So the correct answer is only protons.

  5. Q5. The atomic mass of sulphur is 32 u, and sulphur exists as S ₈ molecules. What is the molecular mass of sulphur?

    1. 256 u
    2. 32 u
    3. 64 u
    4. 128 u

    Answer: (A) 256 u

    Solution
    Molecular mass is the atomic mass multiplied by the number of atoms in the molecule. Sulphur exists as S8, so molecular mass = 8 × 32 u = 256 u.

  6. Q6. The atomic mass of an atom is primarily determined by the number of:

    1. electrons
    2. nucleons
    3. protons
    4. neutrons

    Answer: (B) nucleons

    Solution
    Almost all the mass of an atom is concentrated in its nucleus, which contains protons and neutrons. These particles are together called nucleons. Electrons have negligible mass, so the atomic mass is determined primarily by the number of nucleons (the mass number).

  7. Q7. The formula unit mass of CaCl 2 is:

    1. 115u
    2. 114u
    3. 111u
    4. 110u

    Answer: (C) 111u

    Solution
    Formula unit mass of CaCl2 = atomic mass of Ca + 2 x atomic mass of Cl. Using Ca = 40 u and Cl = 35.5 u: 40 + 2 x 35.5 = 40 + 71 = 111 u. So the answer is 111 u.

  8. Q8. The fractional atomic mass of chlorine is due to which of the following?

    1. Experimental error
    2. Presence of ions
    3. Its compound nature
    4. Presence of isotopes

    Answer: (D) Presence of isotopes

    Solution
    Chlorine occurs naturally as two isotopes, Cl-35 (about 75%) and Cl-37 (about 25%). Its atomic mass is the weighted average of these isotopic masses, which comes to about 35.5 u. Hence the fractional atomic mass is due to the presence of isotopes.

  9. Q9. The electronic configuration of an element is 2, 8, 5, then find its number of protons?

    1. 8
    2. 15
    3. 2
    4. 5

    Answer: (B) 15

    Solution
    In a neutral atom the number of electrons equals the number of protons. The configuration 2, 8, 5 gives 2 + 8 + 5 = 15 electrons. Hence the element has 15 protons (it is phosphorus).

  10. Q10. The electron added with the highest principal quantum number, along with the electronic shell, is called __________.

    1. Effective Electron
    2. Band Electron
    3. Core Electron
    4. Valence Electron

    Answer: (D) Valence Electron

    Solution
    The outermost shell of an atom has the highest principal quantum number. The electrons present in this shell are called valence electrons. They take part in chemical bonding and determine the chemical properties of the element.