JEE Chemistry Structure Of Atom MCQs Set 03

Chemistry Objective Questions and Answers: Structure Of Atom

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Multiple Choice Questions

Question 1: The discovery of the neutron came very late because
(a) Neutrons are present in the nucleus
(b) Neutrons are highly unstable particles
(c) Neutrons are chargeless
(d) Neutrons do not move
Show Answer & Explanation

Answer: (c) Neutrons are chargeless

Explanation:
1. Charged particles are deflected by electric and magnetic fields and are easy to detect.
2. The neutron carries no charge, so it passes through matter without such effects and was identified only in 1932 by Chadwick.

Teacher's Note:
1. Neutral particles are detected through their collisions with nuclei, not through deflection.

Question 2: The fundamental particles present in the nucleus of an atom are
(a) Alpha particles and electrons
(b) Neutrons and protons
(c) Neutrons and electrons
(d) Electrons, neutrons and protons
Show Answer & Explanation

Answer: (b) Neutrons and protons

Explanation:
1. The nucleus is made of protons and neutrons, together called nucleons.
2. Electrons occupy the space outside the nucleus.

Teacher's Note:
1. Read the data carefully and write the relevant formula before substituting.
2. Check units and the order of magnitude of the answer.

Question 3: The number of neutrons in 12C is
(a) 6
(b) 7
(c) 8
(d) 9
Show Answer & Explanation

Answer: (a) 6

Explanation:
1. Neutrons = mass number − atomic number = 12 − 6 = 6.

Teacher's Note:
1. Convert every quantity to moles or equivalents first.
2. Then apply the stoichiometric ratio or the definition asked for.

Question 4: The nucleus of a helium atom (4He) contains
(a) Four protons
(b) Four neutrons
(c) Two neutrons and two protons
(d) Four protons and two electrons
Show Answer & Explanation

Answer: (c) Two neutrons and two protons

Explanation:
1. Helium has atomic number 2, so there are 2 protons.
2. Mass number 4 gives 4 − 2 = 2 neutrons.

Teacher's Note:
1. Eliminate options that disagree with the basic definition.
2. Confirm the remaining option by a quick calculation.

Question 5: Which statement about the proton is correct?
(a) A proton is the nucleus of deuterium
(b) A proton is an ionised hydrogen molecule
(c) A proton is an ionised hydrogen atom
(d) A proton is an α-particle
Show Answer & Explanation

Answer: (c) A proton is an ionised hydrogen atom

Explanation:
1. Ordinary hydrogen (protium) has one proton and one electron.
2. Removing the electron leaves a bare proton, H+.

Teacher's Note:
1. Deuterium's nucleus is a deuteron (one proton and one neutron).

Question 6: Cathode rays are made up of
(a) Positively charged particles
(b) Negatively charged particles
(c) Neutral particles
(d) None of these
Show Answer & Explanation

Answer: (b) Negatively charged particles

Explanation:
1. Cathode rays are deflected towards the positive plate of an electric field.
2. They are streams of electrons, which are negatively charged.

Teacher's Note:
1. Read the data carefully and write the relevant formula before substituting.
2. Check units and the order of magnitude of the answer.

Question 7: Anode rays were discovered by
(a) Goldstein
(b) J. Stoney
(c) Rutherford
(d) J.J. Thomson
Show Answer & Explanation

Answer: (a) Goldstein

Explanation:
1. Goldstein used a perforated cathode in a discharge tube and observed rays moving towards the cathode.
2. These positively charged rays are called anode (canal) rays.

Teacher's Note:
1. Convert every quantity to moles or equivalents first.
2. Then apply the stoichiometric ratio or the definition asked for.

Question 8: The radius of an atom is of the order of
(a) 10−10 cm
(b) 10−13 cm
(c) 10−15 cm
(d) 10−8 cm
Show Answer & Explanation

Answer: (d) 10−8 cm

Explanation:
1. Atomic radii are about 1 Å = 10−10 m.
2. In centimetres this is 10−8 cm; the nucleus is about 10−13 cm.

Teacher's Note:
1. Remember: atom about 10−8 cm, nucleus about 10−13 cm.

Question 9: The neutron is a fundamental particle carrying
(a) A charge of +1 unit and a mass of 1 unit
(b) No charge and a mass of 1 unit
(c) No charge and no mass
(d) A charge of −1 and a mass of 1 unit
Show Answer & Explanation

Answer: (b) No charge and a mass of 1 unit

Explanation:
1. The neutron is electrically neutral.
2. Its mass (1.675 × 10−27 kg) is almost the same as that of the proton, about 1 amu.

Teacher's Note:
1. Eliminate options that disagree with the basic definition.
2. Confirm the remaining option by a quick calculation.

Question 10: The size of the nucleus is measured in
(a) amu
(b) Angstrom
(c) Fermi
(d) cm
Show Answer & Explanation

Answer: (c) Fermi

Explanation:
1. Nuclear radii are about 10−15 m, which is one fermi (femtometre).

Teacher's Note:
1. Read the data carefully and write the relevant formula before substituting.
2. Check units and the order of magnitude of the answer.

Question 11: Which one of the following pairs is not correctly matched?
(a) Rutherford – Proton
(b) J.J. Thomson – Electron
(c) J.H. Chadwick – Neutron
(d) Bohr – Isotope
Show Answer & Explanation

Answer: (d) Bohr – Isotope

Explanation:
1. Rutherford identified the proton, Thomson the electron and Chadwick the neutron.
2. Isotopes were discovered by Soddy, not Bohr, who proposed the model of the hydrogen atom.

Teacher's Note:
1. Convert every quantity to moles or equivalents first.
2. Then apply the stoichiometric ratio or the definition asked for.

Question 12: The minimum charge that can exist on any particle is
(a) 1.6 × 10−19 coulomb
(b) 1.6 × 10−10 coulomb
(c) 4.8 × 10−10 coulomb
(d) Zero
Show Answer & Explanation

Answer: (a) 1.6 × 10−19 coulomb

Explanation:
1. Charge is quantised in multiples of the electronic charge.
2. The smallest free charge is e = 1.6 × 10−19 C.

Teacher's Note:
1. Eliminate options that disagree with the basic definition.
2. Confirm the remaining option by a quick calculation.

Question 13: The nature of anode rays depends upon
(a) The nature of the electrode
(b) The nature of the residual gas
(c) The nature of the discharge tube
(d) All of the above
Show Answer & Explanation

Answer: (b) The nature of the residual gas

Explanation:
1. Anode rays are positive ions formed from the gas filling the tube.
2. Their charge-to-mass ratio therefore depends on the residual gas, not on the electrodes or the tube.

Teacher's Note:
1. Read the data carefully and write the relevant formula before substituting.
2. Check units and the order of magnitude of the answer.

Question 14: The mass of one mole of protons and of one mole of electrons are respectively
(a) 6.023 × 1023 g each
(b) 1.008 g and 0.55 mg
(c) 9.1 × 10−28 kg
(d) 2 g each
Show Answer & Explanation

Answer: (b) 1.008 g and 0.55 mg

Explanation:
1. Mass of a proton = 1.673 × 10−24 g, so one mole = 1.007 g ≈ 1.008 g.
2. Mass of an electron = 9.1 × 10−28 g, so one mole = 5.5 × 10−4 g = 0.55 mg.

Teacher's Note:
1. Convert every quantity to moles or equivalents first.
2. Then apply the stoichiometric ratio or the definition asked for.

Question 15: The ratio of the specific charge of a proton to that of an α-particle is
(a) 2 : 1
(b) 1 : 2
(c) 1 : 4
(d) 1 : 1
Show Answer & Explanation

Answer: (a) 2 : 1

Explanation:
1. Specific charge = charge/mass. For a proton it is e/m_p.
2. An α-particle has charge 2e and mass 4m_p, so its specific charge is e/2m_p.
3. Ratio = 2 : 1.

Teacher's Note:
1. Eliminate options that disagree with the basic definition.
2. Confirm the remaining option by a quick calculation.

Question 16: The proton and the neutron are collectively called
(a) Deutrons
(b) Positrons
(c) Mesons
(d) Nucleons
Show Answer & Explanation

Answer: (d) Nucleons

Explanation:
1. Particles that make up the nucleus are called nucleons.

Teacher's Note:
1. Read the data carefully and write the relevant formula before substituting.
2. Check units and the order of magnitude of the answer.

Question 17: Which of the following has the same mass as an electron?
(a) Photon
(b) Neutron
(c) Positron
(d) Proton
Show Answer & Explanation

Answer: (c) Positron

Explanation:
1. The positron is the antiparticle of the electron, with the same mass but a positive charge.

Teacher's Note:
1. Convert every quantity to moles or equivalents first.
2. Then apply the stoichiometric ratio or the definition asked for.

Question 18: The ratio of the mass of an electron to the mass of a proton is approximately
(a) 1 : 2
(b) 1 : 1
(c) 1 : 1837
(d) 1 : 3
Show Answer & Explanation

Answer: (c) 1 : 1837

Explanation:
1. Mass of a proton ≈ 1837 times the mass of an electron.

Teacher's Note:
1. Eliminate options that disagree with the basic definition.
2. Confirm the remaining option by a quick calculation.

Question 19: The nucleus of an element of atomic number 25 and atomic weight 55 contains
(a) 25 protons and 30 neutrons
(b) 25 neutrons and 30 protons
(c) 55 protons
(d) 55 neutrons
Show Answer & Explanation

Answer: (a) 25 protons and 30 neutrons

Explanation:
1. Protons = atomic number = 25.
2. Neutrons = 55 − 25 = 30.

Teacher's Note:
1. Read the data carefully and write the relevant formula before substituting.
2. Check units and the order of magnitude of the answer.

Question 20: The total number of neutrons in the dipositive zinc ion (Zn2+) of mass number 70 is
(a) 34
(b) 40
(c) 36
(d) 38
Show Answer & Explanation

Answer: (b) 40

Explanation:
1. Zinc has atomic number 30.
2. Neutrons = 70 − 30 = 40; removing electrons does not change the nucleus.

Teacher's Note:
1. Convert every quantity to moles or equivalents first.
2. Then apply the stoichiometric ratio or the definition asked for.

Question 21: Which of the following are isoelectronic with one another?
(a) Na+ and Ne
(b) K+ and O
(c) Ne and O
(d) Na+ and K+
Show Answer & Explanation

Answer: (a) Na+ and Ne

Explanation:
1. Na+ has 11 − 1 = 10 electrons and Ne has 10 electrons.
2. The other pairs have unequal numbers of electrons.

Teacher's Note:
1. Eliminate options that disagree with the basic definition.
2. Confirm the remaining option by a quick calculation.

Question 22: The number of electrons in one molecule of CO2 is
(a) 22
(b) 44
(c) 66
(d) 88
Show Answer & Explanation

Answer: (a) 22

Explanation:
1. Carbon has 6 electrons and each oxygen has 8.
2. 6 + 2 × 8 = 22.

Teacher's Note:
1. Read the data carefully and write the relevant formula before substituting.
2. Check units and the order of magnitude of the answer.

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