All 24 MCQs of 9th Class Chemistry Chapter 2, Atomic Structure, with answers and the reason each option is right or wrong: the 10 textbook exercise MCQs plus 14 more board-style MCQs. Punjab Board new syllabus.
Try each one first: tap an option to check it, then read the reasoning and why the others are wrong.
A note about the questions
The wording follows published solved exercises and every answer was re-checked by hand, so the wording or the order of options may differ slightly from your print.
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Set A: Textbook exercise 10 MCQs
The multiple-choice questions at the end of Chapter 2 in your book, in the book's option order.
Q2.1Atomic Models and Shells
How many electrons can be accommodated at the most in the third shell of the elements?
Answer(b) 18
Step by step
Use 2n².
n = 3: 2 × 9 = 18.
Why the others are wrong
(a) 8 is the maximum for the second shell (and the outermost shell in an octet).
(c) 10 is the capacity of a d sub-shell.
(d) 32 is the fourth shell.
Remember
Shells hold 2, 8, 18, 32.
Q2.2Subatomic Particles
What information was obtained from discharge tube experiments?
Answer(c) Electrons and protons were discovered.
Step by step
Cathode rays: electrons.
Canal (anode) rays: positive particles, protons.
Why the others are wrong
(a) the structure (nucleus and shells) came later from Rutherford and Bohr.
(b) neutrons were not found in discharge tubes.
(d) the nucleus was found in the gold-foil experiment.
Remember
Discharge tube: e⁻ and p⁺.
Q2.3Isotopes and Radioactivity
Why have isotopes not been shown in the periodic table?
Answer(c) All the isotopes have the same atomic number, so there is no need to give them separate places.
Step by step
Place in the table = atomic number.
Isotopes: same atomic number, so the same place.
Why the others are wrong
(a) space is not the reason.
(b) instability does not decide their place.
(d) isotopes have the same chemical behaviour as their element.
Remember
Iso-tope = "same place".
Q2.4Isotopes and Radioactivity
Which particle is present in different numbers in the isotopes?
Answer(b) neutron
Step by step
Same element: same protons.
Different mass number: different neutrons.
Why the others are wrong
(a) neutral isotopes have the same number of electrons.
(c) a different proton number would be a different element.
(d) the electrons are the same.
Remember
Isotopes differ only in neutrons.
Q2.5Isotopes and Radioactivity
In which isotope of oxygen are there equal numbers of protons, electrons and neutrons?
Answer(b) ¹⁶O
Step by step
Z = 8, so 8 protons and 8 electrons.
¹⁶O: 8 neutrons; ¹⁷O: 9; ¹⁸O: 10.
Why the others are wrong
(a) ¹⁷O has 9 neutrons.
(c) ¹⁸O has 10 neutrons.
(d) ¹⁶O works.
Remember
Equal numbers when A = 2Z.
Q2.6Relative Atomic Mass
What will be the relative atomic mass of nitrogen, given the abundances of its two isotopes ¹⁴N and ¹⁵N are 99.64 and 0.35 respectively?
Answer(b) 14.0021
Step by step
14 × 99.64 = 1394.96.
15 × 0.35 = 5.25.
Sum = 1400.21; ÷ 100 = 14.0021.
Why the others are wrong
(a) it misplaces the digits.
(c) far too much ¹⁵N for 0.35 %.
(d) also too high.
Remember
Almost all ¹⁴N, so the answer is only just above 14.
Q2.7Isotopes and Radioactivity
How is radiocarbon dating useful for archaeologists?
Answer(a) It helps determine the age of organic matter.
Step by step
A dead plant or animal stops taking in ¹⁴C.
The less ¹⁴C left, the older the sample.
Why the others are wrong
(b) composition is found by analysis.
(c) it says nothing about usefulness.
(d) it measures age, not whether matter is radioactive.
Remember
Carbon dating = age.
Q2.8Subatomic Particles
What keeps the particles present in the nucleus intact?
Answer(a) They are held together by the strong nuclear force.
Step by step
Electrostatic force pushes protons apart.
The strong nuclear force pulls all nucleons together.
Why the others are wrong
(b) the weak force is involved in radioactive decay, not in holding the nucleus.
(c) the electrostatic force between protons is repulsive.
(d) dipolar forces act between molecules.
Remember
Nucleus = strong nuclear force.
Q2.9Atomic Models and Shells
How do electrons keep themselves away from the oppositely charged nucleus?
Answer(b) By revolving around the nucleus
Step by step
A stationary electron would be pulled into the nucleus.
Moving in an orbit, the attraction only keeps it going round.
Why the others are wrong
(a) a stationary electron would fall into the nucleus.
(c) part of the modern explanation, but not the answer the book's model gives.
(d) there is no such magnetic field.
Remember
Book answer: they revolve.
Q2.10Relative Atomic Mass
Rubidium consists of two isotopes, ⁸⁵Rb and ⁸⁷Rb. The percent abundance of the light isotope is 72.2 %. What is the percent abundance of the heavier isotope? (Its atomic mass is 85.47.)
Answer(b) 27.8 %
Step by step
Total abundance = 100 %.
100 − 72.2 = 27.8 %.
Why the others are wrong
(a) 72.2 + 15 is not 100.
(c) 72.2 + 37 is more than 100.
(d) that is roughly the lighter isotope.
Remember
Two isotopes: subtract from 100.
Set B: More MCQs from this chapter 14 MCQs
Extra questions on the same ideas, of the kind set in board papers and class tests.
QB1Subatomic Particles
Cathode rays are deflected towards the positive plate. This shows that they are:
Answer(b) negatively charged
Step by step
Bends to + plate: negative charge.
Why the others are wrong
(a) positive particles would bend towards the negative plate.
(c) neutral rays would not bend.
(d) waves would not be deflected by an electric field.
Remember
Bends to + = negative.
QB2Subatomic Particles
Canal rays (anode rays) were discovered by:
Answer(b) E. Goldstein
Step by step
Thomson: electrons (1897).
Goldstein: canal rays (1886).
Chadwick: neutron (1932).
Why the others are wrong
(a) Thomson studied cathode rays.
(c) Chadwick discovered the neutron.
(d) Bohr proposed the shell model.
Remember
Goldstein: Goes through the holes.
QB3Subatomic Particles
A proton is about how many times heavier than an electron?
Answer(b) 1836
Step by step
1.673 × 10⁻²⁷ ÷ 9.109 × 10⁻³¹ ≈ 1836.
Why the others are wrong
(a) far too small.
(c) that is the electron compared with the proton.
(d) far too small.
Remember
Proton = 1836 electrons.
QB4Subatomic Particles
The particle with no charge and a mass almost equal to that of a proton is:
Answer(c) neutron
Step by step
Charge 0; mass ≈ proton.
Why the others are wrong
(a) the electron is charged and very light.
(b) a positron is positive and as light as an electron.
(d) an alpha particle is charged (+2) and four times heavier.
Remember
Neutron = neutral.
QB5Atomic Models and Shells
Rutherford's gold-foil experiment showed that:
Answer(b) an atom has a tiny, dense, positive nucleus with mostly empty space around it
Step by step
Most pass straight: mostly empty space.
A few bounce back: a small, dense, positive centre.
Why the others are wrong
(a) that was Bohr's idea.
(c) atoms were already known to contain particles.
(d) neutrons were found later.
Remember
Gold foil = nucleus.
QB6Atomic Models and Shells
According to Bohr, an electron in the orbit closest to the nucleus has:
Answer(b) minimum energy
Step by step
E₁ < E₂ < E₃ < E₄.
Why the others are wrong
(a) outer shells have more energy.
(c) it still has energy.
(d) each shell has its own fixed energy.
Remember
Closer = lower energy.
QB7Atomic Models and Shells
The second shell (n = 2) contains the sub-shells:
Answer(b) s and p
Step by step
n = 1: s. n = 2: s, p. n = 3: s, p, d. n = 4: s, p, d, f.
Why the others are wrong
(a) that is the first shell.
(c) that is the third shell.
(d) that is the fourth shell.
Remember
Number of sub-shells = n.
QB8Atomic Number and Mass Number
The atomic number of an element is the number of:
Answer(b) protons in its nucleus
Step by step
Z = protons.
A = protons + neutrons.
Why the others are wrong
(a) neutrons are A − Z.
(c) that is the mass number.
(d) shells are not counted in Z.
Remember
Z = protons.
QB9Atomic Number and Mass Number
How many neutrons are present in ¹³⁷₅₆Ba?
Answer(b) 81
Step by step
A = 137, Z = 56.
N = 137 − 56 = 81.
Why the others are wrong
(a) 56 is the number of protons.
(c) 137 is the mass number.
(d) it adds instead of subtracting.
Remember
Neutrons = top − bottom.
QB10Isotopes and Radioactivity
Which isotope of hydrogen is radioactive?
Answer(c) tritium ³H
Step by step
³H: too many neutrons for one proton, so unstable.
Why the others are wrong
(a) protium is stable.
(b) deuterium is stable.
(d) only tritium.
Remember
Tritium = the radioactive one.
QB11Isotopes and Radioactivity
Technetium-99m is used:
Answer(b) for diagnostic imaging of organs such as the brain and lungs
Step by step
Tc-99m: medical imaging.
C-14: dating.
Why the others are wrong
(a) that is carbon-14.
(c) uranium is the usual nuclear fuel.
(d) not a use of Tc-99m.
Remember
Tc-99m = scans.
QB12Isotopes and Radioactivity
Isotopes of an element have:
Answer(a) the same chemical properties but different physical properties
Step by step
Same electrons: same chemistry.
Different neutrons: different mass and physical properties.
Why the others are wrong
(b) it is the other way round.
(c) they differ in mass.
(d) they have the same atomic number.
Remember
Same chemistry, different mass.
QB13Subatomic Particles
When a sodium atom is hit by ionising radiation and loses an electron, it becomes:
Answer(b) Na⁺, a cation
Step by step
11 p, 11 e⁻ → 11 p, 10 e⁻.
Net charge +1: Na⁺.
Why the others are wrong
(a) gaining an electron would give an anion.
(c) isotopes differ in neutrons, not electrons.
(d) the proton number is unchanged, so it is still sodium.
