According to Dalton's atomic theory, atoms of the same element are

Identical in mass and properties

Rutherford's gold foil experiment led to the conclusion that

Nucleus is small, dense and positively charged

Atomic Structure MCQs 2026

76 questions with detailed answers · 27 from past papers · 8 quiz batches available

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Page 1 of 1 Questions 1–10 of 76

Q1 medium

Atoms having the same number of neutrons but different atomic numbers are called
1. A Isotopes
2. B Isobars
3. C Isotones
4. D Isomers
💡 Explanation:
Isotones share neutron number but differ in atomic number.
Q2 easy

According to Dalton's atomic theory, atoms of the same element are
1. A Different in mass
2. B Identical in mass and properties
3. C Always radioactive
4. D Always ions
💡 Explanation:
Dalton proposed all atoms of a given element are identical in mass and properties.
Q3 Past Paper · PPSC/FPSC/NTS easy

The subatomic particle discovered by J.J. Thomson using cathode ray tube experiments is the
1. A Proton
2. B Electron
3. C Neutron
4. D Positron
💡 Explanation:
Thomson identified the electron as a negatively charged particle present in all atoms.
Q4 Past Paper · PPSC/FPSC/NTS easy

Rutherford's gold foil experiment led to the conclusion that
1. A Atom is a solid sphere
2. B Electrons move in fixed orbits
3. C Neutron exists in nucleus
4. D Nucleus is small, dense and positively charged
💡 Explanation:
Most alpha particles passed through, but some deflected sharply, showing a tiny dense positive nucleus.
Q5 easy

The subatomic particle with no electric charge is the
1. A Neutron
2. B Proton
3. C Electron
4. D Positron
💡 Explanation:
Neutrons are electrically neutral particles found in the nucleus.
Q6 Past Paper · PPSC/FPSC/NTS easy

The neutron was discovered by
1. A J.J. Thomson
2. B James Chadwick
3. C Ernest Rutherford
4. D Niels Bohr
💡 Explanation:
Chadwick discovered the neutron in 1932 through bombardment experiments.
Q7 easy

The charge on a proton is
1. A -1
2. B 0
3. C +1
4. D +2
💡 Explanation:
A proton carries one unit of positive charge.
Q8 Past Paper · PPSC/FPSC/NTS easy

Canal rays, which led to the discovery of the proton, were observed by
1. A J.J. Thomson
2. B Eugen Goldstein
3. C Robert Millikan
4. D Henry Moseley
💡 Explanation:
Goldstein observed positively charged canal rays in discharge tube experiments.
Q9 easy

Robert Millikan's oil drop experiment was used to determine the
1. A Mass of the neutron
2. B Size of the nucleus
3. C Wavelength of X-rays
4. D Charge on an electron
💡 Explanation:
Millikan measured the precise charge of the electron using charged oil droplets.
Q10 Past Paper · PPSC/FPSC/NTS easy

Bohr's model of the atom proposed that electrons move in
1. A Random paths around the nucleus
2. B Fixed circular orbits of definite energy
3. C A continuous cloud
4. D Straight lines only
💡 Explanation:
Bohr proposed quantized circular orbits with fixed energy levels.
Q11 easy

The maximum number of electrons that can occupy the third principal energy level (n=3) is
1. A 2
2. B 8
3. C 16
4. D 18
💡 Explanation:
Using 2n^2, for n=3, the maximum is 2(3)^2 = 18 electrons.
Q12 medium

Isotopes of an element have the same number of ___ but different number of ___
1. A Protons; neutrons
2. B Neutrons; protons
3. C Electrons; protons
4. D Protons; electrons
💡 Explanation:
Isotopes share the same atomic number (protons) but differ in neutron count.
Q13 Past Paper · PPSC/FPSC/NTS medium

Atoms having the same mass number but different atomic numbers are called
1. A Isotopes
2. B Isobars
3. C Isotones
4. D Isomers
💡 Explanation:
Isobars have equal mass numbers but different numbers of protons.
Q14 Past Paper · PPSC/FPSC/NTS medium

The atomic number of an element represents the number of
1. A Protons in the nucleus
2. B Neutrons in the nucleus
3. C Total nucleons
4. D Valence shells
💡 Explanation:
Atomic number (Z) equals the number of protons in the nucleus.
Q15 medium

The mass number of an atom is the sum of
1. A Protons and neutrons
2. B Protons and electrons
3. C Neutrons and electrons
4. D Protons only
💡 Explanation:
Mass number (A) is the total count of protons and neutrons.
Q16 medium

According to the Aufbau principle, electrons fill orbitals
1. A In order of increasing energy
2. B Randomly
3. C In order of decreasing energy
4. D Only in the outermost shell
💡 Explanation:
The Aufbau principle states electrons occupy the lowest energy orbitals first.
Q17 Past Paper · PPSC/FPSC/NTS medium

The Pauli exclusion principle states that no two electrons in an atom can have
1. A The same principal quantum number
2. B The same shape orbital
3. C The same energy
4. D The same set of four quantum numbers
💡 Explanation:
Each electron in an atom must have a unique combination of all four quantum numbers.
Q18 medium

Hund's rule of maximum multiplicity states that electrons occupy degenerate orbitals
1. A Singly with parallel spins before pairing
2. B In pairs immediately
3. C Only in s-orbitals
4. D According to atomic mass
💡 Explanation:
Hund's rule minimizes electron repulsion by singly filling orbitals of equal energy first.
Q19 medium

The principal quantum number (n) determines the
1. A Shape of orbital
2. B Spin of electron
3. C Size and energy of the shell
4. D Orientation of orbital
💡 Explanation:
The principal quantum number defines the shell's overall size and energy level.
Q20 Past Paper · PPSC/FPSC/NTS medium

The azimuthal (angular momentum) quantum number determines the
1. A Energy of the shell only
2. B Shape of the subshell/orbital
3. C Spin direction
4. D Number of shells
💡 Explanation:
The azimuthal quantum number (l) defines the shape of the subshell, e.g. s, p, d, f.
Q21 medium

The magnetic quantum number determines the
1. A Energy of electron
2. B Shape of orbital
3. C Spin of electron
4. D Orientation of orbital in space
💡 Explanation:
The magnetic quantum number (ml) specifies the spatial orientation of an orbital.
Q22 medium

The spin quantum number can have values of
1. A +1/2 and -1/2
2. B 0 and 1
3. C -1, 0, +1
4. D 1, 2, 3
💡 Explanation:
Electron spin is quantized to only two possible values, +1/2 or -1/2.
Q23 Past Paper · PPSC/FPSC/NTS medium

The shape of a p-orbital is
1. A Spherical
2. B Cubical
3. C Circular
4. D Dumbbell-shaped
💡 Explanation:
p-orbitals have two lobes on opposite sides of the nucleus, forming a dumbbell shape.
Q24 medium

The shape of an s-orbital is
1. A Dumbbell
2. B Cloverleaf
3. C Spherical
4. D Elliptical
💡 Explanation:
s-orbitals are spherically symmetric around the nucleus.
Q25 medium

The maximum number of electrons a single orbital can hold is
1. A 1
2. B 3
3. C 2
4. D 6
💡 Explanation:
Each orbital holds at most two electrons with opposite spins.
Q26 Past Paper · PPSC/FPSC/NTS medium

The number of orbitals in a p-subshell is
1. A 1
2. B 3
3. C 5
4. D 7
💡 Explanation:
A p-subshell consists of three orbitals: px, py, and pz.
Q27 medium

The number of orbitals in a d-subshell is
1. A 3
2. B 1
3. C 5
4. D 7
💡 Explanation:
A d-subshell consists of five distinct orbitals.
Q28 medium

Heisenberg's uncertainty principle states that it is impossible to simultaneously determine, with perfect accuracy, an electron's
1. A Position and momentum
2. B Charge and mass
3. C Spin and charge
4. D Energy and charge
💡 Explanation:
The principle sets a fundamental limit on knowing both position and momentum precisely at once.
Q29 Past Paper · PPSC/FPSC/NTS medium

The de Broglie equation relates the wavelength of a particle to its
1. A Charge
2. B Momentum
3. C Volume
4. D Temperature
💡 Explanation:
De Broglie proposed wavelength = h/momentum, giving particles wave-like character.
Q30 medium

The quantum mechanical model of the atom was developed mainly by
1. A Bohr
2. B Rutherford
3. C Schrodinger
4. D Dalton
💡 Explanation:
Schrodinger's wave equation forms the basis of the modern quantum mechanical model.
Q31 medium

Electron configuration of chromium (Cr, Z=24) is an exception to the Aufbau principle because it is written as
1. A [Ar]3d^4 4s^2
2. B [Ar]3d^5 4s^1
3. C [Ar]3d^6 4s^0
4. D [Ar]4s^2 3d^4
💡 Explanation:
A half-filled 3d subshell offers extra stability, so one 4s electron shifts to 3d.
Q32 Past Paper · PPSC/FPSC/NTS medium

Electron configuration of copper (Cu, Z=29) is written as
1. A [Ar]3d^9 4s^2
2. B [Ar]4s^2 3d^9
3. C [Ar]3d^10 4s^1
4. D [Ar]3d^8 4s^2
💡 Explanation:
A fully filled 3d^10 subshell is more stable, so one 4s electron shifts to 3d.
Q33 medium

The number of valence electrons in an atom of chlorine (Cl, Z=17) is
1. A 7
2. B 5
3. C 8
4. D 1
💡 Explanation:
Chlorine's electron configuration 2,8,7 gives 7 electrons in its outermost shell.
Q34 medium

Moseley's experiments on X-ray spectra established the concept of
1. A Isotopes
2. B Electron spin
3. C Nuclear fission
4. D Atomic number as a fundamental property
💡 Explanation:
Moseley showed atomic number, not atomic mass, determines an element's position in the periodic table.
Q35 Past Paper · PPSC/FPSC/NTS medium

An atom that has gained one or more electrons becomes a
1. A Cation
2. B Neutral atom
3. C Anion
4. D Isotope
💡 Explanation:
Gaining electrons gives the atom a net negative charge, forming an anion.
Q36 medium

The K, L, M, N designations refer to
1. A Types of orbitals
2. B Types of quantum numbers
3. C Types of subatomic particles
4. D Principal energy shells (n=1,2,3,4)
💡 Explanation:
K, L, M, N are traditional labels for the shells corresponding to n=1,2,3,4.
Q37 medium

The relative atomic mass of an atom is expressed in
1. A Grams
2. B Kilograms
3. C Atomic mass units (amu)
4. D Moles
💡 Explanation:
Atomic masses are conventionally expressed in atomic mass units.
Q38 Past Paper · PPSC/FPSC/NTS medium

One atomic mass unit (amu) is defined as 1/12th the mass of an atom of
1. A Hydrogen-1
2. B Oxygen-16
3. C Carbon-12
4. D Helium-4
💡 Explanation:
The amu scale is defined relative to one-twelfth the mass of a carbon-12 atom.
Q39 medium

The number of unpaired electrons in a nitrogen atom (Z=7, 1s^2 2s^2 2p^3) is
1. A 1
2. B 2
3. C 0
4. D 3
💡 Explanation:
By Hund's rule, the three 2p electrons occupy separate orbitals singly, giving three unpaired electrons.
Q40 medium

Which of the following pairs represents isotopes
1. A Carbon-12 and Carbon-14
2. B Carbon-14 and Nitrogen-14
3. C Argon-40 and Potassium-40
4. D Sodium-23 and Magnesium-23
💡 Explanation:
Both are carbon atoms (same protons) with different neutron numbers, making them isotopes.
Q41 Past Paper · PPSC/FPSC/NTS hard

Which pair below is an example of isobars
1. A Carbon-12 and Carbon-14
2. B Hydrogen-1 and Hydrogen-2
3. C Oxygen-16 and Oxygen-18
4. D Argon-40 and Calcium-40
💡 Explanation:
Argon-40 and Calcium-40 share the same mass number but have different atomic numbers.
Q42 hard

The energy of an electron in the nth orbit of a hydrogen atom according to Bohr's model is proportional to
1. A n
2. B n^3
3. C 1/n
4. D -1/n^2
💡 Explanation:
Bohr's formula gives orbital energy as proportional to -1/n^2.
Q43 hard

The phenomenon in which spectral lines split under a magnetic field is called the
1. A Zeeman effect
2. B Compton effect
3. C Photoelectric effect
4. D Doppler effect
💡 Explanation:
The Zeeman effect describes spectral line splitting in the presence of a magnetic field.
Q44 Past Paper · PPSC/FPSC/NTS hard

The total number of subshells present in the third principal shell (n=3) is
1. A 2
2. B 3
3. C 4
4. D 1
💡 Explanation:
The n=3 shell contains three subshells: 3s, 3p, and 3d.
Q45 hard

An electron in a 2p orbital has an azimuthal quantum number (l) equal to
1. A 0
2. B 3
3. C 1
4. D 2
💡 Explanation:
p-orbitals always correspond to an azimuthal quantum number of 1.
Q46 hard

The number of radial nodes in a 2s orbital is
1. A 0
2. B 1
3. C 2
4. D 3
💡 Explanation:
Radial nodes equal n-l-1, so for 2s (n=2, l=0) there is 1 radial node.
Q47 Past Paper · PPSC/FPSC/NTS hard

Which scientist proposed that electrons behave both as particles and waves
1. A Rutherford
2. B Bohr
3. C Chadwick
4. D de Broglie
💡 Explanation:
De Broglie introduced the concept of wave-particle duality for matter.
Q48 hard

The electron configuration of a neutral sodium atom (Z=11) is
1. A 1s^2 2s^2 2p^5 3s^2
2. B 1s^2 2s^2 2p^6 3s^1
3. C 1s^2 2s^2 2p^6 3s^2
4. D 1s^2 2s^2 2p^4 3s^3
💡 Explanation:
Sodium has 11 electrons filling up to a single 3s electron after a complete 2p subshell.
Q49 hard

The effective nuclear charge experienced by valence electrons generally ___ across a period from left to right
1. A Increases
2. B Decreases
3. C Remains constant
4. D Becomes zero
💡 Explanation:
Added protons increase nuclear charge faster than shielding increases across a period.
Q50 hard

The shielding (screening) effect is caused mainly by
1. A Inner-shell electrons repelling outer electrons from the nucleus's pull
2. B Neutrons in the nucleus
3. C Protons attracting outer electrons
4. D Photons emitted by the nucleus
💡 Explanation:
Inner electrons partially block the nuclear attraction felt by outer electrons.
Q51 Past Paper · PPSC/FPSC/NTS medium

The atomic number of an element represents the number of
1. A Protons in the nucleus
2. B Neutrons in the nucleus
3. C Total nucleons
4. D Valence shells
💡 Explanation:
Atomic number (Z) equals the number of protons in the nucleus.
Q52 medium

The mass number of an atom is the sum of
1. A Protons and neutrons
2. B Protons and electrons
3. C Neutrons and electrons
4. D Protons only
💡 Explanation:
Mass number (A) is the total count of protons and neutrons.
Q53 medium

According to the Aufbau principle, electrons fill orbitals
1. A In order of increasing energy
2. B Randomly
3. C In order of decreasing energy
4. D Only in the outermost shell
💡 Explanation:
The Aufbau principle states electrons occupy the lowest energy orbitals first.
Q54 Past Paper · PPSC/FPSC/NTS medium

The Pauli exclusion principle states that no two electrons in an atom can have
1. A The same principal quantum number
2. B The same shape orbital
3. C The same energy
4. D The same set of four quantum numbers
💡 Explanation:
Each electron in an atom must have a unique combination of all four quantum numbers.
Q55 medium

Hund's rule of maximum multiplicity states that electrons occupy degenerate orbitals
1. A Singly with parallel spins before pairing
2. B In pairs immediately
3. C Only in s-orbitals
4. D According to atomic mass
💡 Explanation:
Hund's rule minimizes electron repulsion by singly filling orbitals of equal energy first.
Q56 medium

The principal quantum number (n) determines the
1. A Shape of orbital
2. B Spin of electron
3. C Size and energy of the shell
4. D Orientation of orbital
💡 Explanation:
The principal quantum number defines the shell's overall size and energy level.
Q57 Past Paper · PPSC/FPSC/NTS medium

The azimuthal (angular momentum) quantum number determines the
1. A Energy of the shell only
2. B Shape of the subshell/orbital
3. C Spin direction
4. D Number of shells
💡 Explanation:
The azimuthal quantum number (l) defines the shape of the subshell, e.g. s, p, d, f.
Q58 medium

The magnetic quantum number determines the
1. A Energy of electron
2. B Shape of orbital
3. C Spin of electron
4. D Orientation of orbital in space
💡 Explanation:
The magnetic quantum number (ml) specifies the spatial orientation of an orbital.
Q59 medium

The spin quantum number can have values of
1. A +1/2 and -1/2
2. B 0 and 1
3. C -1, 0, +1
4. D 1, 2, 3
💡 Explanation:
Electron spin is quantized to only two possible values, +1/2 or -1/2.
Q60 Past Paper · PPSC/FPSC/NTS medium

The shape of a p-orbital is
1. A Spherical
2. B Cubical
3. C Circular
4. D Dumbbell-shaped
💡 Explanation:
p-orbitals have two lobes on opposite sides of the nucleus, forming a dumbbell shape.
Q61 medium

The shape of an s-orbital is
1. A Dumbbell
2. B Cloverleaf
3. C Spherical
4. D Elliptical
💡 Explanation:
s-orbitals are spherically symmetric around the nucleus.
Q62 medium

The maximum number of electrons a single orbital can hold is
1. A 1
2. B 3
3. C 2
4. D 6
💡 Explanation:
Each orbital holds at most two electrons with opposite spins.
Q63 Past Paper · PPSC/FPSC/NTS medium

The number of orbitals in a p-subshell is
1. A 1
2. B 3
3. C 5
4. D 7
💡 Explanation:
A p-subshell consists of three orbitals: px, py, and pz.
Q64 medium

The number of orbitals in a d-subshell is
1. A 3
2. B 1
3. C 5
4. D 7
💡 Explanation:
A d-subshell consists of five distinct orbitals.
Q65 medium

Heisenberg's uncertainty principle states that it is impossible to simultaneously determine, with perfect accuracy, an electron's
1. A Position and momentum
2. B Charge and mass
3. C Spin and charge
4. D Energy and charge
💡 Explanation:
The principle sets a fundamental limit on knowing both position and momentum precisely at once.
Q66 Past Paper · PPSC/FPSC/NTS medium

The de Broglie equation relates the wavelength of a particle to its
1. A Charge
2. B Momentum
3. C Volume
4. D Temperature
💡 Explanation:
De Broglie proposed wavelength = h/momentum, giving particles wave-like character.
Q67 medium

The quantum mechanical model of the atom was developed mainly by
1. A Bohr
2. B Rutherford
3. C Schrodinger
4. D Dalton
💡 Explanation:
Schrodinger's wave equation forms the basis of the modern quantum mechanical model.
Q68 medium

Electron configuration of chromium (Cr, Z=24) is an exception to the Aufbau principle because it is written as
1. A [Ar]3d^4 4s^2
2. B [Ar]3d^5 4s^1
3. C [Ar]3d^6 4s^0
4. D [Ar]4s^2 3d^4
💡 Explanation:
A half-filled 3d subshell offers extra stability, so one 4s electron shifts to 3d.
Q69 Past Paper · PPSC/FPSC/NTS medium

Electron configuration of copper (Cu, Z=29) is written as
1. A [Ar]3d^9 4s^2
2. B [Ar]4s^2 3d^9
3. C [Ar]3d^10 4s^1
4. D [Ar]3d^8 4s^2
💡 Explanation:
A fully filled 3d^10 subshell is more stable, so one 4s electron shifts to 3d.
Q70 medium

The number of valence electrons in an atom of chlorine (Cl, Z=17) is
1. A 7
2. B 5
3. C 8
4. D 1
💡 Explanation:
Chlorine's electron configuration 2,8,7 gives 7 electrons in its outermost shell.
Q71 medium

Moseley's experiments on X-ray spectra established the concept of
1. A Isotopes
2. B Electron spin
3. C Nuclear fission
4. D Atomic number as a fundamental property
💡 Explanation:
Moseley showed atomic number, not atomic mass, determines an element's position in the periodic table.
Q72 Past Paper · PPSC/FPSC/NTS medium

An atom that has gained one or more electrons becomes a
1. A Cation
2. B Neutral atom
3. C Anion
4. D Isotope
💡 Explanation:
Gaining electrons gives the atom a net negative charge, forming an anion.
Q73 medium

The K, L, M, N designations refer to
1. A Types of orbitals
2. B Types of quantum numbers
3. C Types of subatomic particles
4. D Principal energy shells (n=1,2,3,4)
💡 Explanation:
K, L, M, N are traditional labels for the shells corresponding to n=1,2,3,4.
Q74 medium

The relative atomic mass of an atom is expressed in
1. A Grams
2. B Kilograms
3. C Atomic mass units (amu)
4. D Moles
💡 Explanation:
Atomic masses are conventionally expressed in atomic mass units.
Q75 Past Paper · PPSC/FPSC/NTS medium

One atomic mass unit (amu) is defined as 1/12th the mass of an atom of
1. A Hydrogen-1
2. B Oxygen-16
3. C Carbon-12
4. D Helium-4
💡 Explanation:
The amu scale is defined relative to one-twelfth the mass of a carbon-12 atom.
Q76 medium

The number of unpaired electrons in a nitrogen atom (Z=7, 1s^2 2s^2 2p^3) is
1. A 1
2. B 2
3. C 0
4. D 3
💡 Explanation:
By Hund's rule, the three 2p electrons occupy separate orbitals singly, giving three unpaired electrons.

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