structure of atoms — BSSC Chemistry

50 Questions • 30 Minutes • Chemistry Mock Test in Hindi and English

Sample Questions from this Test

Question 1:

Match the following fundamental particles with their respective discoverers:

Column I (Particle)
A. Electron
B. Proton
C. Neutron
D. Positron

Column II (Discoverer)
1. James Chadwick
2. Carl Anderson
3. J.J. Thomson
4. Eugen Goldstein
A.A-4, B-3, C-1, D-2
B.A-3, B-4, C-1, D-2
C.A-3, B-1, C-4, D-2
D.A-1, B-4, C-3, D-2

• Key Fact
J.J. Thomson discovered the electron (1898), E. Goldstein discovered the proton (1886) via canal rays, James Chadwick discovered the neutron (1932), and Carl Anderson discovered the positron (1932).

• Supporting Detail
Positron is the anti-particle of an electron, having the same mass but a positive charge.

• Related Concept
The charge on the electron was measured by Robert Millikan using the Oil Drop Experiment.

• Why wrong options are wrong
Other options incorrectly swap the discoverers of protons, electrons, or neutrons.

• Exam Trick
Remember the mnemonic "PEN = RTC" (Proton-Rutherford/Goldstein, Electron-Thomson, Neutron-Chadwick).

• Additional Info
Rutherford named the proton, but Goldstein is credited with discovering positive canal rays.

Question 2:

Assertion (A): The atom is mostly empty space with a very dense, positively charged nucleus at the center.
Reason (R): In Rutherford's gold foil experiment, most alpha particles passed straight through the foil, but a few were deflected at large angles (up to 180°).
A.Both A and R are true, and R is the correct explanation of A
B.Both A and R are true, but R is NOT the correct explanation of A
C.A is true but R is false
D.A is false but R is true

• Key Fact
Rutherford's 1911 experiment concluded that an atom's mass and positive charge occupy a tiny volume (nucleus).

• Supporting Detail
He used a gold foil approximately 1000 atoms thick and bombarded it with alpha particles (doubly-charged helium ions).

• Related Concept
The atomic radius is ~10⁻¹⁰ m, while the nuclear radius is much smaller, ~10⁻¹⁵ m.

• Why wrong options are wrong
Both statements are factually perfect, and the scattering directly proves the concentrated nucleus.

• Exam Trick
"Deflection" = Positively charged nucleus; "Straight pass" = Empty space.

• Additional Info
This experiment led to the downfall of Thomson's "Plum Pudding" model.

Question 3:

Consider the following statements regarding the rules for filling electrons in orbitals:
1. The word 'Aufbau' is a German word meaning 'building up'.
2. Hund's Rule states that no two electrons in an atom can have the same set of all four quantum numbers.
3. Pauli's Exclusion Principle states that pairing of electrons in a subshell starts only after each orbital is singly occupied.
Which of the above statements is/are correct?
A.1 only
B.1 and 2 only
C.2 and 3 only
D.All of the above

• Key Fact
The Aufbau principle is named after a German word meaning "building up," meaning electrons fill the lowest energy orbitals first.

• Supporting Detail
Statements 2 and 3 have their definitions swapped. Pauli's Exclusion Principle is about quantum numbers, and Hund's Rule is about the pairing of electrons.

• Related Concept
Hund's rule insists on maximizing parallel spins before pairing occurs.

• Why wrong options are wrong
Options B, C, and D are incorrect because statements 2 and 3 interchange the definitions of Pauli's and Hund's rules.

• Exam Trick
Remember "Pauli restricts Pair" (No 4 quantum numbers same), "Hund Hunts single first" (Single occupancy before pairing).

• Additional Info
The order of filling according to Aufbau is 1s → 2s → 2p → 3s → 3p → 4s → 3d.

Question 4:

An element has an atomic number 29. Its short electronic configuration is exceptional due to the extra stability of completely filled d-subshells. What is its correct electronic configuration?
A.[Ar] 3d⁹ 4s²
B.[Ar] 3d¹⁰ 4s¹
C.[Kr] 3d¹⁰ 4s¹
D.[Ar] 3d⁸ 4s²

• Key Fact
Atomic number 29 corresponds to Copper (Cu). Its actual configuration is [Ar] 3d¹⁰ 4s¹ instead of the expected [Ar] 3d⁹ 4s².

• Supporting Detail
Half-filled (d⁵) and fully-filled (d¹⁰) d-subshells provide extra stability. Hence, one 4s electron jumps to the 3d orbital.

• Related Concept
Chromium (Atomic No. 24) is another exception, having the configuration [Ar] 3d⁵ 4s¹.

• Why wrong options are wrong
Option A is the theoretical (but incorrect) configuration. Option C uses Krypton ([Kr]), which is wrong since Cu is in the 4th period.

• Exam Trick
For Cu (29) and Cr (24), always take 1 electron from '4s' and give it to '3d' for stability.

• Additional Info
The symbol [Ar] represents the Argon core (18 electrons: 1s² 2s² 2p⁶ 3s² 3p⁶).

Question 5:

Which of the following elements is known as the "odd one out" because it is the only element that does not contain any neutron in its nucleus?
A.Carbon
B.Helium
C.Hydrogen
D.Lithium

• Key Fact
The common isotope of Hydrogen (Protium, ¹H) contains 1 proton, 1 electron, and 0 neutrons.

• Supporting Detail
A hydrogen ion (H⁺) has lost its only electron, so it is essentially just a bare proton.

• Related Concept
Other isotopes of hydrogen do have neutrons: Deuterium (1 neutron) and Tritium (2 neutrons).

• Why wrong options are wrong
Carbon, Helium, and Lithium all contain neutrons in their nuclei.

• Exam Trick
Mass Number (A) = Protons + Neutrons. For Hydrogen, A=1 and Z=1, so N = 1 - 1 = 0.

• Additional Info
Hydrogen is also the only element whose isotopes have completely different names (Protium, Deuterium, Tritium).

Question 6:

Arrange the following atomic models in the chronological order of their proposal (earliest to latest):
1. Rutherford's Nuclear Model
2. John Dalton's Atomic Theory
3. Schrödinger's Quantum Mechanical Model
4. Thomson's Plum Pudding Model
5. Niels Bohr's Atomic Model
A.2, 4, 1, 5, 3
B.2, 1, 4, 3, 5
C.4, 2, 1, 5, 3
D.2, 4, 5, 1, 3

• Key Fact
The correct sequence is: Dalton (1808) → Thomson (1898) → Rutherford (1911) → Bohr (1913) → Quantum Mechanical Model (post-1926).

• Supporting Detail
Dalton proposed indivisible atoms; Thomson discovered electrons; Rutherford discovered the nucleus; Bohr introduced quantized energy orbits.

• Related Concept
The Quantum Mechanical model replaced fixed orbits with 'orbitals' (regions of probability).

• Why wrong options are wrong
Any sequence placing Rutherford before Thomson or Bohr before Rutherford is historically inaccurate.

• Exam Trick
Remember the timeline order: D-T-R-B-Q (Dalton, Thomson, Rutherford, Bohr, Quantum).

• Additional Info
Democritus (400 BCE) gave the ancient philosophical concept of "atomos" before Dalton's scientific theory.

Question 7:

An atom of an element contains 7 electrons in its M-shell and has 18 neutrons in its nucleus. What is its mass number?
A.35
B.17
C.25
D.18

• Key Fact
The electron distribution follows K, L, M shells. If M has 7 electrons, the configuration is K=2, L=8, M=7.

• Supporting Detail
Total electrons = 2 + 8 + 7 = 17. For a neutral atom, number of protons (Z) = number of electrons = 17.

• Related Concept
Mass number (A) = Number of protons (Z) + Number of neutrons (N) = 17 + 18 = 35.

• Why wrong options are wrong
Option B is the atomic number (17), not mass number. Option D is just the neutron count.

• Exam Trick
Always fully write the shell configuration (2, 8, ...) until you reach the outermost shell mentioned in the question to find total electrons.

• Additional Info
This element is Chlorine (Cl), which has an atomic number of 17 and a mass number of 35.

Question 8:

Match the following radioactive isotopes with their medical or industrial uses:

Column I (Isotope)
A. Cobalt-60
B. Iodine-131
C. Sodium-24
D. Arsenic-74

Column II (Use)
1. Treatment of thyroid disorders
2. Cancer treatment (radiotherapy)
3. Study of tumors (PET scan)
4. Blood circulation information
A.A-2, B-1, C-4, D-3
B.A-1, B-2, C-3, D-4
C.A-2, B-4, C-1, D-3
D.A-3, B-1, C-4, D-2

• Key Fact
Cobalt-60 is used for cancer treatment, Iodine-131 for thyroid disorders, Sodium-24 for blood circulation, and Arsenic-74 for studying tumors.

• Supporting Detail
Radioisotopes emit high-energy radiation (like gamma rays from Co-60) which can destroy cancer cells.

• Related Concept
Phosphorus-32 is used in biological research. Iron-59 tracks iron in the bloodstream.

• Why wrong options are wrong
Only option A correctly matches every isotope to its established application in medical science.

• Exam Trick
"Iodine = Thyroid", "Sodium = Blood Circulation", "Cobalt = Cancer".

• Additional Info
Uranium-235 is heavily used as fuel in nuclear reactors.

Question 9:

Statement I: X-rays are not deflected by electric and magnetic fields.
Statement II: X-rays travel with the velocity of light.
A.Both Statements are correct and Statement II is the correct explanation of Statement I
B.Both Statements are correct but Statement II is NOT the correct explanation of Statement I
C.Statement I is correct but Statement II is incorrect
D.Statement I is incorrect but Statement II is correct

• Key Fact
X-rays are electromagnetic waves, hence they carry no electrical charge. Therefore, they are not deflected by electric or magnetic fields.

• Supporting Detail
Because X-rays are electromagnetic radiation, they travel at the speed of light (3 × 10⁸ m/s) in a vacuum.

• Related Concept
In contrast, cathode rays (electrons) and canal rays (protons) are deflected by electric/magnetic fields because they possess charge.

• Why wrong options are wrong
Both statements are true. However, traveling at the speed of light is NOT the reason they are not deflected; the lack of charge is the reason.

• Exam Trick
If a ray is "Electromagnetic" (like X-rays or Gamma rays), it has zero charge and zero rest mass.

• Additional Info
Gamma rays from radioactive decay are also uncharged electromagnetic waves.

Question 10:

Consider the following statements regarding fundamental particles and properties:
1. Atomic number is always equal to the number of protons and electrons in a neutral atom.
2. Mass number can never be equal to the atomic number.
3. The specific charge (e/m ratio) of anode rays does not depend upon the nature of the gas taken in the discharge tube.
Which of the above statements is/are incorrect?
A.1 and 2 only
B.2 and 3 only
C.1 and 3 only
D.All of the above

• Key Fact
Statement 2 is incorrect because in ordinary Hydrogen (Protium, ¹H), atomic number (1) equals mass number (1) since it has 0 neutrons.

• Supporting Detail
Statement 3 is incorrect because the e/m ratio of ANODE (canal) rays DOES depend on the nature of gas, unlike cathode rays.

• Related Concept
Statement 1 is perfectly correct; in a neutral atom, Protons = Electrons = Atomic Number.

• Why wrong options are wrong
The question asks for INCORRECT statements. Since 2 and 3 are wrong, option B is the correct answer.

• Exam Trick
Read carefully if the question asks for "Correct" or "Incorrect". Cathode rays = independent of gas. Anode rays = dependent on gas.

• Additional Info
The e/m ratio of an electron was calculated by J.J. Thomson as -1.76 × 10¹¹ C/kg.

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