75 free MCQs on Structure of Atom with worked answers and explanations. Explore how electrons are arranged inside an atom. Covers Bohr's model, quantum numbers, Heisenberg's uncertainty principle, shapes of s, p, d orbitals, and electronic configuration rules.
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Bohr's model: electrons occupy fixed circular shells (K, L, M) around the nucleus, with each shell holding up to 2n2 electrons.
Q1.
According to Planck's quantum theory, the energy of a photon is directly proportional to its:
Answer: A. frequency
Why: Planck's relation E = h x frequency shows that photon energy is directly proportional to frequency (and inversely proportional to wavelength).
Q2.
The photoelectric effect provides experimental evidence for the ... nature of electromagnetic radiation.
Answer: B. particle
Why: The photoelectric effect can only be explained if light behaves as a stream of particles (photons), supporting the particle nature of radiation.
Q3.
The Lyman series of the hydrogen emission spectrum is found in which region of the electromagnetic spectrum?
Answer: C. Ultraviolet
Why: Lyman series lines (transitions ending at n = 1) have the highest energies in the hydrogen spectrum and lie in the ultraviolet region.
Q4.
The wave (dual) nature of matter, relating wavelength to momentum, was first proposed by:
Answer: B. de Broglie
Why: Louis de Broglie proposed that every moving particle has an associated wavelength given by lambda = h/mv, establishing the dual nature of matter.
Q5.
The approximate value of Planck's constant (h) is:
Answer: A. 6.626 x 10<sup>-34</sup> J s
Why: Planck's constant h = 6.626 x 10<sup>-34</sup> J s. The other values are the speed of light, the electronic charge, and the electron mass respectively.
Q6.
The center of an atom is called the:
Answer: B. Nucleus
Why: The nucleus is the central part of an atom and contains protons and neutrons.
Q7.
Which subatomic particle has a positive charge?
Answer: C. Proton
Why: A proton carries a positive charge and is present in the nucleus of an atom.
Q8.
Which subatomic particle has a negative charge?
Answer: B. Electron
Why: An electron carries a negative charge and moves around the nucleus.
Q9.
Which subatomic particle has no charge?
Answer: C. Neutron
Why: A neutron is electrically neutral and is found in the nucleus.
Q10.
Atomic number is equal to the number of:
Answer: B. Protons
Why: Atomic number is defined as the number of protons in the nucleus.
Q11.
Mass number is equal to:
Answer: A. Protons + Neutrons
Why: Mass number is the total number of protons and neutrons in the nucleus.
Q12.
In a neutral atom, the number of protons is equal to the number of:
Answer: B. Electrons
Why: A neutral atom has equal positive and negative charges, so protons equal electrons.
Q13.
Electrons are found mainly in:
Answer: B. Shells around the nucleus
Why: Electrons occupy shells or energy levels around the nucleus.
Q14.
The first electron shell can hold a maximum of:
Answer: A. 2 electrons
Why: The first shell, K shell, can hold a maximum of 2 electrons.
Q15.
The second electron shell can hold a maximum of:
Answer: C. 8 electrons
Why: The second shell, L shell, can hold a maximum of 8 electrons.
Q16.
The charge on an electron is:
Answer: B. Negative
Why: Electrons are negatively charged particles.
Q17.
Most of the mass of an atom is present in the:
Answer: C. Nucleus
Why: Protons and neutrons are much heavier than electrons, and both are in the nucleus.
Q18.
An atom that gains or loses electrons becomes a:
Answer: B. Ion
Why: An ion is formed when an atom gains or loses electrons and becomes charged.
Q19.
Atoms of the same element always have the same number of:
Answer: B. Protons
Why: The number of protons identifies the element, so atoms of the same element have the same proton number.
Q20.
The symbol Z is commonly used for:
Answer: B. Atomic number
Why: Z represents atomic number, which is the number of protons.
Q21.
The subatomic particle that carries a negative charge is the:
Answer: C. electron
Why: The electron carries a unit negative charge; the proton is positive and the neutron is neutral.
Q22.
The maximum number of electrons that the L shell (n = 2) can hold is:
Answer: C. 8
Why: Capacity of a shell is 2n². For n = 2 that is 2 × 4 = 8 electrons.
Q23.
Isotopes of an element have the same number of:
Answer: B. protons
Why: Isotopes share the same atomic number (protons) but differ in the number of neutrons, hence different mass numbers.
Q24.
The mass number of an atom is equal to the number of:
Answer: C. protons and neutrons
Why: Mass number A = number of protons + number of neutrons (the nucleons).
Q25.
Which quantum number describes the shape of an orbital?
Answer: C. azimuthal (l)
Why: The azimuthal (l) quantum number fixes the subshell and hence the orbital shape (s, p, d, f).
Q26.
The energy of a photon of radiation of frequency 5 x 10<sup>14</sup> Hz is (h = 6.6 x 10<sup>-34</sup> J s):
Answer: A. 3.3 x 10<sup>-19</sup> J
Why: E = h x frequency = 6.6 x 10<sup>-34</sup> x 5 x 10<sup>14</sup> = 3.3 x 10<sup>-19</sup> J.
Q27.
If the radius of the first Bohr orbit of hydrogen is 0.529 A, the radius of its third orbit (n = 3) is:
Answer: B. 4.76 A
Why: r<sub>n</sub> is proportional to n<sup>2</sup>, so r<sub>3</sub> = 0.529 x 3<sup>2</sup> = 0.529 x 9 = 4.76 A.
Q28.
When an excited electron in a hydrogen atom falls from n = 4 to n = 1, the maximum number of spectral lines that can be emitted is:
Answer: C. 6
Why: Maximum number of lines = n(n-1)/2 = 4 x 3 / 2 = 6 spectral lines.
Q29.
According to the (n + l) rule, which of the following orbitals is filled first?
Answer: D. 4s
Why: For 4s, (n + l) = 4 + 0 = 4, which is lower than 3d (3 + 2 = 5) and 4p (4 + 1 = 5), so 4s fills first.
Q30.
For an electron present in a 3d orbital, the number of possible values of the magnetic quantum number (m<sub>l</sub>) is:
Answer: B. 5
Why: For a d subshell, l = 2, so m<sub>l</sub> ranges from -2 to +2, giving 5 possible values (5 d-orbitals).
Q31.
An atom has atomic number 11 and mass number 23. How many neutrons does it have?
Answer: B. 12
Why: Neutrons = mass number - atomic number = 23 - 11 = 12.
Q32.
Which electronic configuration is correct for sodium, atomic number 11?
Answer: A. 2,8,1
Why: Sodium has 11 electrons. They fill as 2 in K shell, 8 in L shell, and 1 in M shell.
Q33.
Which electronic configuration is correct for oxygen, atomic number 8?
Answer: A. 2,6
Why: Oxygen has 8 electrons, arranged as 2 in the first shell and 6 in the second shell.
Q34.
Isotopes are atoms of the same element with different numbers of:
Answer: C. Neutrons
Why: Isotopes have the same atomic number but different mass numbers because they have different neutrons.
Q35.
Which pair represents isotopes?
Answer: A. 12C and 14C
Why: Carbon-12 and carbon-14 are isotopes because both are carbon atoms but have different mass numbers.
Q36.
The maximum number of electrons in a shell is given by:
Answer: C. 2n<sup>2</sup>
Why: The maximum number of electrons in the nth shell is 2n<sup>2.</sup>
Q37.
How many electrons can the M shell hold at maximum?
Answer: B. 18
Why: For M shell, n = 3. Maximum electrons = 2n<sup>2</sup> = 2 x 3<sup>2</sup> = 18.
Q38.
The valence shell is the:
Answer: B. Outermost occupied shell
Why: The valence shell is the outermost shell that contains electrons.
Q39.
Valence electrons mainly decide an element's:
Answer: B. Chemical properties
Why: Valence electrons participate in bonding, so they largely determine chemical properties.
Q40.
Which scientist proposed that electrons revolve in fixed energy levels?
Answer: C. Bohr
Why: Bohr proposed fixed energy levels or shells around the nucleus.
Q41.
Which scientist discovered the nucleus through the gold foil experiment?
Answer: B. Rutherford
Why: Rutherford's alpha-particle scattering experiment led to the discovery of the atomic nucleus.
Q42.
Which particle was discovered by J.J. Thomson?
Answer: B. Electron
Why: J.J. Thomson discovered the electron using cathode ray experiments.
Q43.
Which particle was discovered by James Chadwick?
Answer: C. Neutron
Why: James Chadwick discovered the neutron in 1932.
Q44.
An atom with 17 protons and 18 electrons has what charge?
Answer: B. -1
Why: It has one extra electron compared with protons, so its charge is -1.
Q45.
The number of neutrons in chlorine-35 with atomic number 17 is:
Answer: B. 18
Why: Neutrons = mass number - atomic number = 35 - 17 = 18.
Q46.
The maximum number of electrons a d subshell can accommodate is:
Answer: C. 10
Why: A d subshell has five orbitals, each holding 2 electrons, so 10 in all.
Q47.
For the principal quantum number n = 3, the allowed values of the azimuthal quantum number l are:
Answer: C. 0, 1 and 2
Why: l ranges from 0 to n − 1, so for n = 3 the values are 0 (s), 1 (p) and 2 (d).
Q48.
The electronic configuration of the chloride ion (Cl⁻, Z = 17) is the same as that of:
Answer: A. argon
Why: Cl⁻ gains one electron to reach 18 electrons, matching the configuration of argon.
Q49.
According to Hund’s rule, electrons occupy degenerate orbitals:
Answer: C. singly with parallel spins
Why: Hund’s rule: each orbital of equal energy is singly filled with parallel spins before any pairing begins.
Q50.
The total number of orbitals present in the n = 2 shell is:
Answer: C. 4
Why: n = 2 has one 2s orbital and three 2p orbitals, giving four orbitals in total.
Q51.
In a hydrogen atom the ground-state energy is -13.6 eV. The energy of the electron in the n = 2 level is:
Answer: A. -3.4 eV
Why: E<sub>n</sub> = -13.6/n<sup>2</sup> eV, so for n = 2, E = -13.6/4 = -3.4 eV.
Q52.
The wave number of the first line of the Balmer series (n = 3 to n = 2) of hydrogen is (R<sub>H</sub> = 1.097 x 10<sup>7</sup> m<sup>-1</sup>):
Answer: C. 1.52 x 10<sup>6</sup> m<sup>-1</sup>
Why: Wave number = R<sub>H</sub>(1/2<sup>2</sup> - 1/3<sup>2</sup>) = 1.097 x 10<sup>7</sup> x (5/36) = 1.52 x 10<sup>6</sup> m<sup>-1</sup>.
Q53.
The de Broglie wavelength of an electron (mass 9.1 x 10<sup>-31</sup> kg) moving at 2.2 x 10<sup>6</sup> m s<sup>-1</sup> is about (h = 6.6 x 10<sup>-34</sup> J s):
Answer: A. 3.3 x 10<sup>-10</sup> m
Why: lambda = h/mv = 6.6 x 10<sup>-34</sup> / (9.1 x 10<sup>-31</sup> x 2.2 x 10<sup>6</sup>) = 3.3 x 10<sup>-10</sup> m.
Q54.
Which of the following sets of quantum numbers is allowed for an electron in an atom?
Answer: C. n=3, l=2, m<sub>l</sub>=-2, m<sub>s</sub>=+1/2
Why: For n = 3, l can be 0,1,2; with l = 2, m<sub>l</sub> can be -2, so this set is valid. The others violate the rule that l < n and that m<sub>l</sub> ranges from -l to +l.
Q55.
The uncertainty in the position of an electron is 1 x 10<sup>-10</sup> m. The minimum uncertainty in its momentum is about (use delta-x times delta-p >= h/4pi, h = 6.6 x 10<sup>-34</sup> J s):
Answer: D. 5.3 x 10<sup>-25</sup> kg m s<sup>-1</sup>
Why: delta-p >= h/(4 pi delta-x) = 6.6 x 10<sup>-34</sup> / (4 x 3.14 x 1 x 10<sup>-10</sup>) = 5.3 x 10<sup>-25</sup> kg m s<sup>-1</sup>.
Q56.
Which set of quantum numbers is not possible?
Answer: C. n=2, l=2, m=0
Why: For a given n, l can have values from 0 to n-1. If n=2, l can only be 0 or 1, not 2.
Q57.
Which quantum number mainly determines the shape of an orbital?
Answer: B. Azimuthal quantum number
Why: The azimuthal quantum number, l, determines the orbital shape such as s, p, d, or f.
Q58.
How many orbitals are present in the p subshell?
Answer: B. 3
Why: A p subshell has three orbitals, usually named px, py, and pz.
Q59.
How many electrons can a single orbital hold?
Answer: B. 2
Why: One orbital can hold a maximum of two electrons with opposite spins.
Q60.
Which rule says no two electrons in an atom can have the same four quantum numbers?
Answer: C. Pauli exclusion principle
Why: The Pauli exclusion principle states that no two electrons in an atom can have identical four quantum numbers.
Q61.
According to Hund's rule, electrons fill equal-energy orbitals by:
Answer: B. Occupying singly first
Why: Hund's rule says electrons occupy degenerate orbitals singly with parallel spins before pairing.
Q62.
Which electronic configuration is correct for chlorine, atomic number 17?
Answer: A. 2,8,7
Why: Chlorine has 17 electrons arranged as 2,8,7.
Q63.
The number of unpaired electrons in a nitrogen atom, 1s<sup>2</sup> 2s<sup>2</sup> 2p<sup>3</sup>, is:
Answer: C. 3
Why: The three 2p electrons occupy three separate p orbitals singly, so nitrogen has 3 unpaired electrons.
Q64.
Which subshell can hold a maximum of 10 electrons?
Answer: C. d
Why: A d subshell has 5 orbitals, and each orbital holds 2 electrons, so d holds 10 electrons.
Q65.
How many orbitals are present in the third shell, n=3?
Answer: C. 9
Why: The total number of orbitals in a shell is n<sup>2.</sup> For n=3, orbitals = 9.
Q66.
Which isotope of hydrogen has one proton and one neutron?
Answer: B. Deuterium
Why: Deuterium has one proton and one neutron, so its mass number is 2.
Q67.
Which isotope of hydrogen has one proton and two neutrons?
Answer: C. Tritium
Why: Tritium has one proton and two neutrons, so its mass number is 3.
Q68.
What is the maximum number of electrons in the N shell?
Answer: C. 32
Why: For N shell, n=4. Maximum electrons = 2n<sup>2</sup> = 2 x 16 = 32.
Q69.
Which principle says the exact position and momentum of an electron cannot both be known exactly?
Answer: C. Heisenberg uncertainty principle
Why: Heisenberg's uncertainty principle limits simultaneous exact measurement of position and momentum.
Q70.
Which orbital is spherical in shape?
Answer: A. s orbital
Why: s orbitals are spherical, while p orbitals are dumbbell-shaped.
Q71.
Using E = −13.6/n² eV for hydrogen, the energy of the n = 2 level is:
Answer: C. −3.4 eV
Why: E = −13.6 / 2² = −13.6 / 4 = −3.4 eV.
Q72.
The de Broglie wavelength of a particle is inversely proportional to its:
Answer: B. momentum
Why: λ = h/p, so wavelength is inversely proportional to momentum - heavier or faster particles have shorter wavelengths.
Q73.
The number of radial nodes in a 3p orbital is:
Answer: C. 1
Why: Radial nodes = n − l − 1 = 3 − 1 − 1 = 1.
Q74.
How many electrons in an atom can have the quantum numbers n = 3, l = 2?
Answer: C. 10
Why: n = 3, l = 2 denotes the 3d subshell, which holds a maximum of 10 electrons.
Q75.
The Heisenberg uncertainty principle states that one cannot simultaneously know exactly a particle’s position and its:
Answer: B. momentum
Why: The principle links position and momentum: Δx·Δp ≥ h/4π, so both cannot be known exactly at once.