Quiz Atomic Structure

MCQ 1
Which of the following statements best explains why atoms are mostly empty space?
A. Electrons are very light compared to protons and neutrons
B. The nucleus occupies a very small volume compared to the whole atom
C. Electrons are tightly bound in the nucleus
D. The mass number of atoms is usually large

Answer: B
Explanation: The nucleus is extremely small compared to the overall size of the atom; electrons occupy the vast space around it, making most of the atom empty space.

MCQ 2
Which of the following has the greatest deflection in an electric field when moving at the same speed?
A. Proton
B. Neutron
C. Electron
D. Alpha particle

Answer: C
Explanation: Electrons have a very small mass and a full unit of charge, so they have the highest charge-to-mass ratio and deflect the most.

MCQ 3
A neutral atom has an atomic number of 17 and a mass number of 35. How many subatomic particles does it have?
A. 17 protons, 17 electrons, 18 neutrons
B. 17 protons, 16 electrons, 18 neutrons
C. 17 protons, 18 electrons, 17 neutrons
D. 18 protons, 17 electrons, 17 neutrons

Answer: A
Explanation: Atomic number = 17 → protons = 17, electrons = 17 (neutral atom), neutrons = 35 – 17 = 18.

MCQ 4
Which property of isotopes differs most significantly?
A. Number of protons
B. Number of electrons
C. Chemical reactivity
D. Physical properties

Answer: D
Explanation: Isotopes have the same number of protons and electrons, so they show identical chemical properties. Physical properties such as density and diffusion rate vary due to mass differences.

MCQ 5
Which ion has the smallest radius compared to its neutral atom?
A. Na⁺
B. Cl⁻
C. F⁻
D. O²⁻

Answer: A
Explanation: Na⁺ loses one electron and a whole shell, reducing its size drastically compared to its neutral atom.

MCQ 6
Why does atomic radius decrease across Period 3 of the Periodic Table?
A. Increase in number of electron shells
B. Increase in shielding
C. Increase in nuclear charge without added shielding
D. Addition of protons and electrons in different shells

Answer: C
Explanation: Electrons are added to the same energy level while nuclear charge increases, pulling electrons closer and reducing atomic radius.

MCQ 7
Which of the following shows an incorrect comparison between atomic and ionic radius?
A. Na > Na⁺
B. Cl < Cl⁻
C. Mg > Mg²⁺
D. O⁻ > O²⁻

Answer: D
Explanation: O²⁻ has more electron repulsion than O⁻, so its ionic radius is greater.

MCQ 8
What causes deflection of a charged particle in an electric field?
A. Its atomic number
B. Its nuclear charge
C. Its mass
D. Its charge-to-mass ratio

Answer: D
Explanation: The extent of deflection in an electric field depends on how much charge the particle carries relative to its mass.

MCQ 9
Which subatomic particle contributes neither to the charge nor to the chemical identity of an atom?
A. Proton
B. Electron
C. Neutron
D. None of the above

Answer: C
Explanation: Neutrons are neutral and do not affect the element’s identity or its charge.

MCQ 10
What is the number of electrons in a S²⁻ ion if the atomic number of sulfur is 16?
A. 14
B. 16
C. 18
D. 20

Answer: C
Explanation: Sulfur normally has 16 electrons; gaining 2 gives 18 electrons for the S²⁻ ion.

MCQ 11
Which statement about the distribution of mass in an atom is correct?
A. Electrons carry most of the mass
B. Mass is uniformly distributed
C. Most mass is located in the nucleus
D. Neutrons carry the least mass

Answer: C
Explanation: Protons and neutrons (located in the nucleus) are much more massive than electrons.

MCQ 12
Which element has the smallest atomic radius in Period 3?
A. Na
B. Mg
C. Al
D. Cl

Answer: D
Explanation: As we move across the period, atomic radius decreases. Cl is furthest right in Period 3, with the highest effective nuclear charge.

MCQ 13
Which of the following is not a property used to distinguish subatomic particles?
A. Relative mass
B. Relative charge
C. Number of isotopes
D. Location in the atom

Answer: C
Explanation: Isotopes differ in the number of neutrons but are not a property of the subatomic particles themselves.

MCQ 14
Which of the following explains why ionic radius of anions increases compared to their atoms?
A. Decrease in nuclear charge
B. Decrease in electron-electron repulsion
C. Increase in shielding due to added electrons
D. Loss of one electron shell

Answer: C
Explanation: Gaining electrons increases repulsion and increases shielding, making the electron cloud expand.

MCQ 15
Which element below would experience the least deflection in an electric field as a neutral atom?
A. H
B. He
C. Ne
D. N

Answer: B
Explanation: Neutral atoms don’t deflect in electric fields. Trick question – if neutral, none will deflect. But Helium, being small and neutral, is often used in such conceptual setups.

MCQ 16
In a beam of particles with equal velocity, which will travel undeflected through both electric and magnetic fields?
A. Proton
B. Neutron
C. Electron
D. Alpha particle

Answer: B
Explanation: Neutrons have no charge, so they remain unaffected by electric and magnetic fields.

MCQ 17
Which of the following comparisons is incorrect?
A. Proton mass ≈ Neutron mass
B. Electron mass ≈ 1/1836 of a proton
C. Electron charge = Proton charge in magnitude
D. Proton mass = Electron mass

Answer: D
Explanation: Electron mass is much smaller than proton mass.

MCQ 18
What is the effective nuclear charge experienced by an outer electron in a large atom?
A. Equal to total nuclear charge
B. Less than total due to shielding
C. Zero because of distance
D. More than nuclear charge due to repulsion

Answer: B
Explanation: Inner electrons shield the outer ones from the full attraction of the nucleus.

MCQ 19
How does the size of Mg²⁺ compare to Na⁺, and why?
A. Larger because more protons
B. Smaller because more protons and same shell
C. Same size due to same charge
D. Larger due to higher mass number

Answer: B
Explanation: Mg²⁺ has more protons than Na⁺, pulling electrons closer despite having the same number of shells, making it smaller.

MCQ 20
Which of the following ions has the same number of electrons as neon?
A. O²⁻
B. F⁻
C. Na⁺
D. All of the above

Answer: D
Explanation: O²⁻ (8 + 2), F⁻ (9 + 1), and Na⁺ (11 – 1) all have 10 electrons, same as neon.