, Chapter 1 – Structure of Matter
Q1.
What is the primary factor that determines the chemical identity of an atom?
A. Number of neutrons
B. Number of electrons
C. Atomic mass number
D. Number of protons
✅ Correct Answer: D. Number of protons
Rationale: The number of protons in the nucleus defines the atomic number (Z),
which uniquely determines the element’s identity. Neutrons affect isotopic form,
and electrons may change due to ionization but do not change the element itself.
Keywords: Atomic number, protons, element identity
Q2.
Which subatomic particle contributes most significantly to the atomic mass of an
atom?
A. Proton
B. Neutron
C. Electron
D. Positron
✅ Correct Answer: B. Neutron
Rationale: Protons and neutrons both contribute ~1 amu each, but neutrons add
to mass without affecting charge. Electrons are much lighter (~1/1836 of a
proton). Since isotopes differ by neutron number, neutrons are the main drivers
of mass variation.
Keywords: Neutron, atomic mass, isotopes
,Q3.
In medical physics, why is the electron binding energy of inner shells
important?
A. It determines the color of emitted visible light.
B. It influences x-ray attenuation and photoelectric interactions.
C. It affects neutron capture probability.
D. It alters isotope stability.
✅ Correct Answer: B. It influences x-ray attenuation and photoelectric
interactions.
Rationale: High binding energies of inner shells (K, L) are crucial in determining
photoelectric effect probabilities, which directly impact imaging and radiation
absorption in tissues.
Keywords: Binding energy, photoelectric effect, x-ray interactions
Q4.
What is the most stable configuration for an atom in terms of electron shells?
A. Fully filled outer shell
B. Equal numbers of protons and neutrons
C. Half-filled d orbitals
D. One electron in the valence shell
✅ Correct Answer: A. Fully filled outer shell
Rationale: Atoms with filled valence shells (noble gases) are chemically inert and
stable. This underlies the drive for chemical bonding in other atoms.
Keywords: Stability, electron shell, valence
Q5.
Which type of radiation primarily interacts with electron orbital binding energy
rather than nuclear forces?
A. Alpha particles
B. X-rays
,C. Neutrons
D. Protons
✅ Correct Answer: B. X-rays
Rationale: X-rays and gamma rays interact with orbital electrons (via
photoelectric effect, Compton scattering, pair production). Alpha, protons, and
neutrons primarily interact through nuclear or Coulomb forces.
Keywords: X-rays, electron binding, photon interactions
Q6.
What is the mass defect in nuclear physics?
A. The difference between the weight of protons and neutrons
B. The loss of electron mass during ionization
C. The difference between nuclear mass and the sum of individual nucleons’
masses
D. The reduction of charge in radioactive decay
✅ Correct Answer: C. The difference between nuclear mass and the sum of
individual nucleons’ masses
Rationale: Binding energy accounts for the missing mass when nucleons
combine into a nucleus. This mass defect is directly related to nuclear stability.
Keywords: Mass defect, binding energy, nuclear stability
Q7.
Which of the following increases as the atomic number (Z) increases?
A. Electron-to-proton mass ratio
B. Binding energy of inner-shell electrons
C. Neutron decay probability
D. Half-life of isotopes
✅ Correct Answer: B. Binding energy of inner-shell electrons
,Rationale: Higher Z increases nuclear charge, which strongly binds inner-shell
electrons (especially K-shell). This trend is critical in diagnostic radiology.
Keywords: Atomic number, binding energy, K-shell
Q8.
Why are isotopes of the same element chemically similar but physically
different?
A. They differ in proton number.
B. They differ in neutron number.
C. They differ in electron orbital shape.
D. They differ in binding energies only.
✅ Correct Answer: B. They differ in neutron number.
Rationale: Isotopes share the same atomic number (protons/electrons →
chemical properties) but have different neutron numbers, altering nuclear
stability and mass.
Keywords: Isotopes, neutrons, nuclear stability
Q9.
What is the typical scale of binding energy per nucleon in stable nuclei?
A. 1–2 eV
B. 10–20 keV
C. 6–8 MeV
D. 100–200 MeV
✅ Correct Answer: C. 6–8 MeV
Rationale: Stable nuclei (e.g., Fe-56) have binding energies in the range of ~6–8
MeV/nucleon, much higher than electron binding energies (keV).
Keywords: Binding energy, nucleons, stability
Q10.
, In radiation therapy, why is atomic number (Z) significant for dose deposition?
A. It determines wavelength of therapeutic photons.
B. It influences the probability of photoelectric absorption.
C. It prevents radioactive decay.
D. It dictates isotopic half-life.
✅ Correct Answer: B. It influences the probability of photoelectric absorption.
Rationale: Photoelectric effect probability ~ Z³/E³, making high-Z materials (like
bone or contrast agents) absorb radiation more effectively.
Keywords: Atomic number, photoelectric absorption, radiation dose
Q11.
What is the role of neutrons in nuclear stability?
A. They add positive charge to the nucleus.
B. They balance electrostatic repulsion between protons.
C. They decrease the binding energy.
D. They control chemical reactivity.
✅ Correct Answer: B. They balance electrostatic repulsion between protons.
Rationale: Neutrons provide nuclear strong force without adding charge,
offsetting proton-proton repulsion and stabilizing the nucleus.
Keywords: Neutrons, nuclear stability, strong force
Q12.
Which orbital has the highest probability density closest to the nucleus?
A. 2p
B. 3d
C. 1s
D. 4f
✅ Correct Answer: C. 1s
Q1.
What is the primary factor that determines the chemical identity of an atom?
A. Number of neutrons
B. Number of electrons
C. Atomic mass number
D. Number of protons
✅ Correct Answer: D. Number of protons
Rationale: The number of protons in the nucleus defines the atomic number (Z),
which uniquely determines the element’s identity. Neutrons affect isotopic form,
and electrons may change due to ionization but do not change the element itself.
Keywords: Atomic number, protons, element identity
Q2.
Which subatomic particle contributes most significantly to the atomic mass of an
atom?
A. Proton
B. Neutron
C. Electron
D. Positron
✅ Correct Answer: B. Neutron
Rationale: Protons and neutrons both contribute ~1 amu each, but neutrons add
to mass without affecting charge. Electrons are much lighter (~1/1836 of a
proton). Since isotopes differ by neutron number, neutrons are the main drivers
of mass variation.
Keywords: Neutron, atomic mass, isotopes
,Q3.
In medical physics, why is the electron binding energy of inner shells
important?
A. It determines the color of emitted visible light.
B. It influences x-ray attenuation and photoelectric interactions.
C. It affects neutron capture probability.
D. It alters isotope stability.
✅ Correct Answer: B. It influences x-ray attenuation and photoelectric
interactions.
Rationale: High binding energies of inner shells (K, L) are crucial in determining
photoelectric effect probabilities, which directly impact imaging and radiation
absorption in tissues.
Keywords: Binding energy, photoelectric effect, x-ray interactions
Q4.
What is the most stable configuration for an atom in terms of electron shells?
A. Fully filled outer shell
B. Equal numbers of protons and neutrons
C. Half-filled d orbitals
D. One electron in the valence shell
✅ Correct Answer: A. Fully filled outer shell
Rationale: Atoms with filled valence shells (noble gases) are chemically inert and
stable. This underlies the drive for chemical bonding in other atoms.
Keywords: Stability, electron shell, valence
Q5.
Which type of radiation primarily interacts with electron orbital binding energy
rather than nuclear forces?
A. Alpha particles
B. X-rays
,C. Neutrons
D. Protons
✅ Correct Answer: B. X-rays
Rationale: X-rays and gamma rays interact with orbital electrons (via
photoelectric effect, Compton scattering, pair production). Alpha, protons, and
neutrons primarily interact through nuclear or Coulomb forces.
Keywords: X-rays, electron binding, photon interactions
Q6.
What is the mass defect in nuclear physics?
A. The difference between the weight of protons and neutrons
B. The loss of electron mass during ionization
C. The difference between nuclear mass and the sum of individual nucleons’
masses
D. The reduction of charge in radioactive decay
✅ Correct Answer: C. The difference between nuclear mass and the sum of
individual nucleons’ masses
Rationale: Binding energy accounts for the missing mass when nucleons
combine into a nucleus. This mass defect is directly related to nuclear stability.
Keywords: Mass defect, binding energy, nuclear stability
Q7.
Which of the following increases as the atomic number (Z) increases?
A. Electron-to-proton mass ratio
B. Binding energy of inner-shell electrons
C. Neutron decay probability
D. Half-life of isotopes
✅ Correct Answer: B. Binding energy of inner-shell electrons
,Rationale: Higher Z increases nuclear charge, which strongly binds inner-shell
electrons (especially K-shell). This trend is critical in diagnostic radiology.
Keywords: Atomic number, binding energy, K-shell
Q8.
Why are isotopes of the same element chemically similar but physically
different?
A. They differ in proton number.
B. They differ in neutron number.
C. They differ in electron orbital shape.
D. They differ in binding energies only.
✅ Correct Answer: B. They differ in neutron number.
Rationale: Isotopes share the same atomic number (protons/electrons →
chemical properties) but have different neutron numbers, altering nuclear
stability and mass.
Keywords: Isotopes, neutrons, nuclear stability
Q9.
What is the typical scale of binding energy per nucleon in stable nuclei?
A. 1–2 eV
B. 10–20 keV
C. 6–8 MeV
D. 100–200 MeV
✅ Correct Answer: C. 6–8 MeV
Rationale: Stable nuclei (e.g., Fe-56) have binding energies in the range of ~6–8
MeV/nucleon, much higher than electron binding energies (keV).
Keywords: Binding energy, nucleons, stability
Q10.
, In radiation therapy, why is atomic number (Z) significant for dose deposition?
A. It determines wavelength of therapeutic photons.
B. It influences the probability of photoelectric absorption.
C. It prevents radioactive decay.
D. It dictates isotopic half-life.
✅ Correct Answer: B. It influences the probability of photoelectric absorption.
Rationale: Photoelectric effect probability ~ Z³/E³, making high-Z materials (like
bone or contrast agents) absorb radiation more effectively.
Keywords: Atomic number, photoelectric absorption, radiation dose
Q11.
What is the role of neutrons in nuclear stability?
A. They add positive charge to the nucleus.
B. They balance electrostatic repulsion between protons.
C. They decrease the binding energy.
D. They control chemical reactivity.
✅ Correct Answer: B. They balance electrostatic repulsion between protons.
Rationale: Neutrons provide nuclear strong force without adding charge,
offsetting proton-proton repulsion and stabilizing the nucleus.
Keywords: Neutrons, nuclear stability, strong force
Q12.
Which orbital has the highest probability density closest to the nucleus?
A. 2p
B. 3d
C. 1s
D. 4f
✅ Correct Answer: C. 1s
