CBSE Class 9 Chemistry Structure of the Atom MCQs Set 10

Answer: (c) Air is a poor conductor of electricity under normal conditions

Explanation:
1. Gases at normal pressure are poor conductors because they have very few free charged particles.
2. Current passes only at very low pressure and high voltage.

Teacher's Note:
1. Discharge tubes work at low pressure.
2. Normal air is an insulator.

Answer: (a) J.J. Thomson

Explanation:
1. J.J. Thomson studied cathode rays and discovered the electron.
2. Goldstein discovered anode rays.

Teacher's Note:
1. Electron discovered by Thomson, nucleus by Rutherford.
2. Neutron by Chadwick.

Answer: (c) cathode rays consist of matter particles and possess kinetic energy

Explanation:
1. Only moving particles with mass and kinetic energy can push the paddle wheel.
2. So cathode rays consist of material particles.

Teacher's Note:
1. Cathode rays are streams of electrons.
2. Rotation shows momentum.

Answer: (b) depends upon the nature of gas

Explanation:
1. Anode rays are positive ions formed from the gas in the tube.
2. Different gases give ions of different mass and charge, so e/m depends on the gas.

Teacher's Note:
1. For cathode rays e/m is constant.
2. For anode rays it varies with the gas.

Answer: (c) \( \ce{1.6 x 10^-19} \) coulombs

Explanation:
1. The charge of an electron is \( \ce{1.6 x 10^-19} \) C (negative), measured by Millikan.

Teacher's Note:
1. This is the smallest unit of charge.
2. Remember the value.

Answer: (b) A proton

Explanation:
1. A proton has charge +1 and mass about 1 u.
2. A neutron has no charge, an electron has almost no mass and a helium nucleus has mass 4 u and charge +2.

Teacher's Note:
1. Proton and neutron have nearly the same mass.
2. The electron is about 1/1836 of it.

Answer: (a) nucleus

Explanation:
1. Most alpha particles passed through, but a few were deflected strongly.
2. This showed a small, dense, positive centre, the nucleus.

Teacher's Note:
1. Nucleus was discovered by Rutherford.
2. Electron by Thomson and neutron by Chadwick.

Answer: (d) alpha particles (helium nuclei) which impinged on a thin metal foil and got scattered

Explanation:
1. Rutherford bombarded a thin gold foil with alpha particles and studied how they were scattered.

Teacher's Note:
1. Alpha particles are helium nuclei.
2. The foil was very thin, only about a few atoms thick.

Answer: (c) Nucleus in the atom

Explanation:
1. The scattering pattern could be explained only if the positive charge and mass were concentrated in a tiny nucleus.

Teacher's Note:
1. The nucleus is about 10,000 times smaller than the atom.
2. Most of the atom is empty space.

Answer: (a) (i) and (iii)

Explanation:
1. Rutherford's model has a positive nucleus with electrons revolving around it like planets round the Sun, so (i) and (iii) are correct.
2. An alpha particle is about four times as heavy as hydrogen and the model disagreed with Thomson's.

Teacher's Note:
1. Rutherford replaced Thomson's model.
2. The solar system comparison is the standard one.

Answer: (b) (ii) and (iv)

Explanation:
1. Most alpha particles went straight through, so most of the atom is empty.
2. A few were deflected strongly, so the mass and positive charge are concentrated in a small nucleus.
3. The experiment did not show anything about electron charge or neutrons.

Teacher's Note:
1. Observation to inference: straight path means empty space, rebound means dense nucleus.
2. The neutron was found later.

Answer: (d) The position and velocity of the electrons in the orbit cannot be determined simultaneously

Explanation:
1. Statement (d) is the uncertainty principle of Heisenberg, which is part of the quantum model.
2. Bohr's model assumes electrons have definite orbits and speeds.

Teacher's Note:
1. Bohr gave fixed circular orbits.
2. Heisenberg's principle came later.

Answer: (c) Protium

Explanation:
1. Protium is the ordinary hydrogen atom with one proton and no neutron.
2. Helium has 2 neutrons, lithium has 4 and tritium has 2.

Teacher's Note:
1. Hydrogen isotopes: protium 0, deuterium 1, tritium 2 neutrons.
2. Only protium has none.

Answer: (d) \( \ce{10^-27} \) kg

Explanation:
1. Mass of a neutron = \( \ce{1.675 x 10^-27} \) kg, which is of the order of \( \ce{10^-27} \) kg.

Teacher's Note:
1. Proton mass is almost the same.
2. Electron mass is about \( \ce{9.1 x 10^-31} \) kg.

Answer: (b) Chadwick

Explanation:
1. James Chadwick discovered the neutron in 1932.

Teacher's Note:
1. Neutron is neutral and has mass about 1 u.
2. It explains the extra mass of nuclei.

Answer: (c) Ar

Explanation:
1. Calcium (Z = 20) has configuration 2, 8, 8, 2.
2. \( \ce{Ca^2+} \) loses two electrons to give 2, 8, 8, which is the argon configuration.

Teacher's Note:
1. Cations lose electrons from the outer shell.
2. Isoelectronic with argon.

Answer: (d) All of these

Explanation:
1. He has configuration 2, Be has 2, 2 and Mg has 2, 8, 2.
2. Each has two electrons in its outer shell.

Teacher's Note:
1. Helium has only one shell, the duplet.
2. Group 2 elements also have two valence electrons.

Answer: (b) The total number of neutrons and protons is always equal in a neutral atom

Explanation:
1. Neutrons and protons are not always equal, for example sodium has 11 protons and 12 neutrons.
2. Electrons and protons are equal in a neutral atom.

Teacher's Note:
1. Neutral atom means protons equal electrons.
2. Neutron number varies between isotopes.

Answer: (a) -2

Explanation:
1. Sulphur has 16 protons.
2. With 18 electrons, charge = 16 - 18 = -2, the sulphide ion \( \ce{S^2-} \).

Teacher's Note:
1. Charge = protons minus electrons.
2. Sulphur gains two electrons to complete its octet.

Answer: (c) 1

Explanation:
1. 3 protons means atomic number 3 (lithium) with configuration 2, 1.
2. It loses one electron, so its valency is 1.

Teacher's Note:
1. Valency depends on electrons, not neutrons.
2. Lithium is an alkali metal.

Answer: (c) either metals or non-metals

Explanation:
1. Sodium (metal) and chlorine (non-metal) both have valency 1.
2. So elements of valency 1 can be metals or non-metals.

Teacher's Note:
1. Hydrogen is a non-metal of valency 1.
2. Alkali metals also have valency 1.

Answer: (a) 12

Explanation:
1. Sodium has atomic number 11 and mass number 23.
2. Neutrons = 23 - 11 = 12.

Teacher's Note:
1. Neutrons = mass number - atomic number.
2. The isotope here is sodium-23.