Atomic Structure Practice Questions
20 free Atomic Structure practice questions for the General Science. Tap an option to answer — you get instant feedback, the correct answer, and a detailed explanation for every question.
Which subatomic particle was the first to be discovered, leading to the rejection of the 'indivisible atom' theory?
- A Proton
- B Neutron
- C Electron
- D Alpha particle
Correct answer: Electron
The electron was discovered by J.J. Thomson in 1897 through his experiments with cathode ray tubes. This discovery proved that atoms were not indivisible spheres but contained smaller, negatively charged particles.
In Rutherford’s gold foil experiment, why did a small fraction of alpha particles deflect at large angles?
- A They were repelled by the electron cloud.
- B They struck the dense, positive nucleus.
- C They were absorbed by the gold atoms.
- D They collided with other alpha particles.
Correct answer: They struck the dense, positive nucleus.
Rutherford concluded that the atom's positive charge and most of its mass are concentrated in a tiny, dense center called the nucleus. Alpha particles, which are positively charged, were deflected only when they came close to or hit this central nucleus.
An atom of an element has 17 protons, 18 neutrons, and 17 electrons. What is its mass number?
- A 17
- B 18
- C 34
- D 35
Correct answer: 35
The mass number of an atom is the sum of its protons and neutrons ($17 + 18 = 35$). Electrons are not included in the mass number calculation because their mass is negligible compared to nucleons.
Which of the following describes isotopes of the same element?
- A Same atomic number, different mass number
- B Different atomic number, same mass number
- C Same number of neutrons, different number of protons
- D Same mass number, same number of neutrons
Correct answer: Same atomic number, different mass number
Isotopes are atoms of the same element that have the same number of protons (atomic number) but a different number of neutrons. This results in different mass numbers while maintaining the same chemical properties.
According to the Bohr model, what happens when an electron moves from a lower energy shell to a higher energy shell?
- A Energy is emitted.
- B Energy is absorbed.
- C The atom becomes a neutron.
- D Electron mass increases.
Correct answer: Energy is absorbed.
Moving an electron to a higher energy level (an excited state) requires the input of energy. Conversely, when an electron falls back to a lower level, it releases that energy in the form of a photon.
Which quantum number determines the shape of an atomic orbital?
- A Principal quantum number (n)
- B Azimuthal quantum number (l)
- C Magnetic quantum number (ml)
- D Spin quantum number (ms)
Correct answer: Azimuthal quantum number (l)
The angular momentum quantum number ($l$) defines the subshell and the shape of the orbital (e.g., $l=0$ is s-spherical, $l=1$ is p-dumbbell). The principal quantum number ($n$) relates to the size and energy of the orbital.
What is the maximum number of electrons that can occupy the third principal energy level (n = 3)?
- A 8
- B 10
- C 18
- D 32
Correct answer: 18
The maximum number of electrons in a principal energy level is calculated using the formula $2n^2$. For $n = 3$, the calculation is $2(3)^2 = 2(9) = 18$.
Which rule or principle states that electrons must occupy the lowest energy orbital available first?
- A Pauli Exclusion Principle
- B Hund's Rule
- C Aufbau Principle
- D Heisenberg Uncertainty Principle
Correct answer: Aufbau Principle
The Aufbau Principle (from the German word for 'building up') dictates the order in which electron shells are filled. It ensures that the most stable, lowest-energy configuration is achieved before higher energy levels are used.
The region in space where there is a high probability of finding an electron is known as a(n):
- A Orbit
- B Orbital
- C Nucleus
- D Photon
Correct answer: Orbital
Unlike the Bohr model's fixed 'orbits,' quantum mechanics uses 'orbitals' to describe the 3D regions around the nucleus. These are mathematical probability maps defined by Schrödinger's wave equations.
How many valence electrons are present in an atom of Magnesium (atomic number 12)?
- A 2
- B 8
- C 10
- D 12
Correct answer: 2
Magnesium has an electron configuration of $1s^2 2s^2 2p^6 3s^2$. The valence electrons are those in the outermost shell ($n=3$), which contains two electrons.
Which subatomic particle determines the chemical identity of an atom?
- A Electron
- B Neutron
- C Proton
- D Positron
Correct answer: Proton
The number of protons in the nucleus is the atomic number, which defines the element. If the number of protons changes, the atom becomes a different element entirely.
Which statement correctly describes the charge and mass of a neutron?
- A Positive charge, mass of 1 amu
- B Negative charge, negligible mass
- C Neutral charge, mass of 1 amu
- D Neutral, negligible mass
Correct answer: Neutral charge, mass of 1 amu
Neutrons are electrically neutral particles located in the nucleus. They have a mass of approximately 1 atomic mass unit (amu), which is nearly equal to the mass of a proton.
Which of the following electron configurations represents a noble gas?
- A 1s² 2s² 2p⁶
- B 1s² 2s² 2p⁶ 3s¹
- C 1s² 2s² 2p⁵
- D 1s² 2s²
Correct answer: 1s² 2s² 2p⁶
The configuration $1s^2 2s^2 2p^6$ corresponds to Neon, which has a full valence shell (8 electrons in $n=2$). This 'octet' configuration provides the chemical stability characteristic of noble gases.
According to Hund's Rule, how are electrons distributed in orbitals of the same energy (degenerate orbitals)?
- A They pair up immediately.
- B They fill orbitals singly with parallel spins.
- C They only occupy the first available orbital in the set.
- D They avoid orbitals of the same energy.
Correct answer: They fill orbitals singly with parallel spins.
Hund's Rule states that for degenerate orbitals, such as the three p-orbitals, electrons will fill each orbital singly before any pairing occurs. This minimizes electron-electron repulsion within the subshell.
Which of the following best defines the 'Atomic Mass Unit' (amu)?
- A The mass of one hydrogen atom.
- B 1/12th the mass of a carbon-12 atom.
- C The mass of a single electron.
- D The average mass of all isotopes of oxygen.
Correct answer: 1/12th the mass of a carbon-12 atom.
The atomic mass unit is a standard unit of mass defined as exactly 1/12th the mass of a carbon-12 atom. It provides a relative scale to compare the masses of different atoms and subatomic particles.
The Heisenberg Uncertainty Principle states that it is impossible to simultaneously know which two properties of a particle?
- A Mass and Velocity
- B Charge and Position
- C Position and Momentum
- D Energy and Electron Spin
Correct answer: Position and Momentum
Heisenberg's principle asserts that the more precisely the position of a particle is determined, the less precisely its momentum (velocity) can be known, and vice versa. This is a fundamental limit of quantum systems.
In the symbol ⁴⁰₂₀Ca, what does the number 20 represent?
- A The mass number
- B The number of neutrons
- C The atomic number
- D The number of nucleons
Correct answer: The atomic number
In standard nuclear notation ($^A_Z X$), the subscript ($Z$) is the atomic number, representing the number of protons. The superscript ($A$) is the mass number, representing the total number of protons and neutrons.
Which subshell has a maximum capacity of 10 electrons?
- A s
- B p
- C d
- D f
Correct answer: d
The d-subshell contains 5 orbitals. Since each orbital can hold a maximum of 2 electrons with opposite spins, the total capacity of the d-subshell is 10 electrons.
What is the identity of an ion with 11 protons, 12 neutrons, and 10 electrons?
- A Neutral Sodium (Na)
- B Sodium cation (Na⁺)
- C Magnesium cation (Mg²⁺)
- D Neon (Ne)
Correct answer: Sodium cation (Na⁺)
11 protons identify the element as Sodium. Since there is one more proton (positive) than electrons (negative), the atom has a net charge of $+1$, making it a $Na^+$ ion.
Which transition in a hydrogen atom would emit a photon with the highest energy?
- A n=2 to n=1
- B n=3 to n=2
- C n=4 to n=3
- D n=5 to n=4
Correct answer: n=2 to n=1
Energy levels in an atom are not evenly spaced; the gap between $n=1$ and $n=2$ is much larger than the gaps between higher levels. Therefore, an electron falling to the ground state ($n=1$) releases the most energy.