CBSE Class 9 Chemistry Structure of the Atom MCQs Set 15

Answer: (a) 22

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

Teacher's Note:
1. Add the atomic numbers of all atoms.
2. 44 is the molar mass.

Answer: (a) 11, 11, 12

Explanation:
1. Protons = atomic number = 11 and electrons = 11 in a neutral atom.
2. Neutrons = 23 - 11 = 12.

Teacher's Note:
1. Order matters: electrons, protons, neutrons.
2. Mass number is protons plus neutrons.

Answer: (a) \( {}^{31}_{15}\mathrm{X} \)

Explanation:
1. 15 electrons in a neutral atom means atomic number 15.
2. Mass number = 15 + 16 = 31, so the symbol is \( {}^{31}_{15}\mathrm{X} \).

Teacher's Note:
1. Mass number goes on top, atomic number at the bottom.
2. This element is phosphorus.

Answer: (b) 10

Explanation:
1. Protons = 27 - 14 = 13 (aluminium).
2. The ion has charge +3, so electrons = 13 - 3 = 10.

Teacher's Note:
1. Positive charge means electrons were lost.
2. Atom had 13 electrons.

Answer: (c) same chemical properties but different physical properties

Explanation:
1. Isotopes have the same number of electrons and so the same chemical properties.
2. Their masses differ, so physical properties such as density differ.

Teacher's Note:
1. Chemical behaviour depends on electrons.
2. Physical properties depend on mass.

Answer: (d) Hydrogen and deuterium

Explanation:
1. Hydrogen and deuterium are atoms of the same element with different mass numbers (1 and 2).
2. Oxygen and ozone are allotropes, and the other pairs are compounds.

Teacher's Note:
1. Deuterium is heavy hydrogen.
2. Isotopes need the same atomic number.

Answer: (a) the same nuclear charge but different mass number

Explanation:
1. Isotopes have the same number of protons, so the same nuclear charge.
2. They have different numbers of neutrons, so different mass numbers.

Teacher's Note:
1. Nuclear charge = number of protons.
2. Different neutron counts change the mass number.

Answer: (a) the nucleus is unstable

Explanation:
1. Radioactivity arises from an unstable nucleus, usually because of a wrong neutron to proton ratio.
2. The number of electrons has no role.

Teacher's Note:
1. Unstable nuclei emit alpha, beta or gamma radiation.
2. They change into more stable nuclei.

Answer: (a) 90 percent, 10 percent

Explanation:
1. Let the fraction of A-16 be x.
2. 16x + 18(1 - x) = 16.2, so 18 - 2x = 16.2 and x = 0.9.
3. So 90 percent A-16 and 10 percent A-18.

Teacher's Note:
1. Average mass = sum of mass times abundance.
2. Check: 0.9 x 16 + 0.1 x 18 = 16.2.

Answer: (d) Cathode rays

Explanation:
1. X-rays, gamma rays and infrared are electromagnetic waves.
2. Cathode rays are streams of electrons, which are particles.

Teacher's Note:
1. Electromagnetic radiation consists of photons.
2. Cathode rays have mass.

Answer: (c) Gamma rays

Explanation:
1. Gamma rays are the most penetrating.
2. Alpha rays are stopped by paper and beta rays by a few mm of aluminium.

Teacher's Note:
1. Penetration: gamma greater than beta greater than alpha.
2. Thick lead or concrete is needed to stop gamma rays.

Answer: (d) 0.75

Explanation:
1. 10 years is two half-lives, so 1/4 of the nuclei remain.
2. The fraction that disintegrates is 1 - 0.25 = 0.75.

Teacher's Note:
1. After n half-lives the fraction left is (1/2) to the power n.
2. Probability of decay is the fraction decayed.

Answer: (b) 12 hours

Explanation:
1. 1/64 = (1/2) to the power 6, so 6 half-lives are needed.
2. Time = 6 x 2 = 12 hours.

Teacher's Note:
1. Count the number of halvings.
2. Each halving takes one half-life.

Answer: (b) Co (60)

Explanation:
1. Cobalt-60 emits gamma rays that are used in radiotherapy to destroy cancer cells.
2. Iodine-131 is used for the thyroid.

Teacher's Note:
1. Cobalt-60 is the common radiotherapy source.
2. Isotopes are also used as tracers.

Answer: (c) 0.693 N/n sec

Explanation:
1. The activity is n = lambda N, so the decay constant lambda = n/N.
2. Half-life = 0.693/lambda = 0.693 N/n seconds.

Teacher's Note:
1. Half-life and decay constant are inversely related.
2. t half = 0.693 / lambda.

Answer: (b) Kinetic energy of products

Explanation:
1. Most of the energy released in a nuclear explosion appears as the kinetic energy of the fission products, which then becomes heat and blast.

Teacher's Note:
1. Fission fragments fly apart at very high speed.
2. Gamma rays carry only a smaller part.

Answer: (a) A and C are isotopes

Explanation:
1. Alpha emission lowers Z by 2 and mass by 4.
2. Emission of two beta particles raises Z by 2 with no change in mass.
3. So C has the same atomic number as A but mass lower by 4, so A and C are isotopes.

Teacher's Note:
1. Alpha then two betas returns the atomic number to its original value.
2. Same Z means isotopes.

Answer: (a) 90, 234

Explanation:
1. Mass number: 238 = A + 4, so A = 234.
2. Atomic number: 92 = Z + 2, so Z = 90.

Teacher's Note:
1. Alpha decay lowers the mass number by 4 and atomic number by 2.
2. The product is thorium-234.

Answer: (b) \( {}^{234}_{90}\mathrm{Th} \)

Explanation:
1. Alpha emission reduces the mass number by 4 and the atomic number by 2, giving mass 234 and Z = 90.
2. The element with Z = 90 is thorium.

Teacher's Note:
1. Alpha particle is a helium nucleus.
2. The daughter has a different atomic number, so it is a different element.

Answer: (c) \( {}^{216}_{83}\mathrm{X} \)

Explanation:
1. Three alpha: mass 228 - 12 = 216 and Z = 88 - 6 = 82.
2. One beta raises Z by 1 to 83 with the mass unchanged.
3. The product is \( {}^{216}_{83}\mathrm{X} \).

Teacher's Note:
1. Each alpha: -4 mass, -2 charge.
2. Each beta: +1 charge, no mass change.

Answer: (c) fusion of protons during synthesis of heavier elements

Explanation:
1. In the Sun hydrogen nuclei fuse to form helium and release energy.
2. This fusion is the source of solar energy.

Teacher's Note:
1. Fusion needs very high temperature.
2. Fission is used in nuclear reactors.