HESI A2 Chemistry Exam
2025/2026 – Verified
Questions & Correct Answers
| Updated for Real Exam
Question 1
What is the primary component of the nucleus in an atom?
A. Electrons B. Protons and neutrons C. Neutrons only D. Positrons
B. Protons and neutrons
Rationale: The nucleus consists of protons (positively charged) and neutrons (neutral), making
up most of the atom's mass; electrons orbit the nucleus, explaining atomic structure and stability
in chemical bonding.
Question 2
In atomic structure, the atomic number represents the number of?
A. Neutrons B. Electrons in the outer shell C. Protons D. Isotopes
C. Protons
Rationale: Atomic number (Z) defines the element by proton count, determining electron
configuration and periodic table placement; this is fundamental to understanding elemental
properties and reactions.
Question 3
Isotopes of an element differ in their number of?
A. Protons B. Electrons C. Neutrons D. Nucleons
C. Neutrons
,Rationale: Isotopes have the same protons (same element) but varying neutrons, affecting mass
number and stability; e.g., carbon-12 vs. carbon-14, relevant in radioactive decay reactions.
Question 4
The electron configuration of sodium (Na) is?
A. 1s² 2s² 2p⁶ 3s¹ B. 1s² 2s² 2p⁵ C. 1s² 2s² 2p⁶ 3s² D. 1s² 2s¹
A. 1s² 2s² 2p⁶ 3s¹
Rationale: Sodium (Z=11) fills orbitals per Aufbau principle up to 3s¹, explaining its valence
electron and reactivity in ionic bonding reactions like NaCl formation.
Question 5
What force holds the nucleus together despite proton repulsion?
A. Electromagnetic force B. Strong nuclear force C. Weak nuclear force D. Gravitational force
B. Strong nuclear force
Rationale: The strong force binds quarks into protons/neutrons and overcomes electrostatic
repulsion; this atomic structure principle prevents nuclear instability in everyday elements.
Question 6
In the Bohr model, electrons orbit in?
A. Random paths B. Fixed energy levels (shells) C. Continuous waves D. Probabilistic clouds
B. Fixed energy levels (shells)
Rationale: Quantized orbits explain spectral lines; transitions emit/absorb photons, foundational
to understanding atomic spectra and chemical reactions involving energy.
Question 7
The mass number of an atom is the sum of?
A. Protons only B. Protons and electrons C. Protons and neutrons D. Neutrons and electrons
C. Protons and neutrons
, Rationale: Mass number (A) approximates atomic mass in u; used to calculate isotopes' weights
in stoichiometry for reaction balancing.
Question 8
Which subatomic particle has no charge?
A. Proton B. Electron C. Neutron D. Positron
C. Neutron
Rationale: Neutrons contribute to mass without charge, stabilizing nuclei; excess or deficit leads
to radioactive decay reactions.
Question 9
The Pauli exclusion principle states that no two electrons can have the same?
A. Principal quantum number B. Four quantum numbers C. Orbital shape D. Energy level
B. Four quantum numbers
Rationale: n, l, m_l, m_s uniquely identify electrons, explaining orbital filling and periodic table
electron configurations for bonding predictions.
Question 10
In atomic structure, valence electrons determine?
A. Atomic mass B. Chemical reactivity C. Nuclear stability D. Isotopic variation
B. Chemical reactivity
Rationale: Outer shell electrons participate in bonding; e.g., alkali metals (1 valence e⁻) form +1
ions in reactions.
Question 11
The ground state of an atom means electrons are in?
A. Highest energy levels B. Lowest possible energy levels C. Excited states D. Ionized form
B. Lowest possible energy levels
2025/2026 – Verified
Questions & Correct Answers
| Updated for Real Exam
Question 1
What is the primary component of the nucleus in an atom?
A. Electrons B. Protons and neutrons C. Neutrons only D. Positrons
B. Protons and neutrons
Rationale: The nucleus consists of protons (positively charged) and neutrons (neutral), making
up most of the atom's mass; electrons orbit the nucleus, explaining atomic structure and stability
in chemical bonding.
Question 2
In atomic structure, the atomic number represents the number of?
A. Neutrons B. Electrons in the outer shell C. Protons D. Isotopes
C. Protons
Rationale: Atomic number (Z) defines the element by proton count, determining electron
configuration and periodic table placement; this is fundamental to understanding elemental
properties and reactions.
Question 3
Isotopes of an element differ in their number of?
A. Protons B. Electrons C. Neutrons D. Nucleons
C. Neutrons
,Rationale: Isotopes have the same protons (same element) but varying neutrons, affecting mass
number and stability; e.g., carbon-12 vs. carbon-14, relevant in radioactive decay reactions.
Question 4
The electron configuration of sodium (Na) is?
A. 1s² 2s² 2p⁶ 3s¹ B. 1s² 2s² 2p⁵ C. 1s² 2s² 2p⁶ 3s² D. 1s² 2s¹
A. 1s² 2s² 2p⁶ 3s¹
Rationale: Sodium (Z=11) fills orbitals per Aufbau principle up to 3s¹, explaining its valence
electron and reactivity in ionic bonding reactions like NaCl formation.
Question 5
What force holds the nucleus together despite proton repulsion?
A. Electromagnetic force B. Strong nuclear force C. Weak nuclear force D. Gravitational force
B. Strong nuclear force
Rationale: The strong force binds quarks into protons/neutrons and overcomes electrostatic
repulsion; this atomic structure principle prevents nuclear instability in everyday elements.
Question 6
In the Bohr model, electrons orbit in?
A. Random paths B. Fixed energy levels (shells) C. Continuous waves D. Probabilistic clouds
B. Fixed energy levels (shells)
Rationale: Quantized orbits explain spectral lines; transitions emit/absorb photons, foundational
to understanding atomic spectra and chemical reactions involving energy.
Question 7
The mass number of an atom is the sum of?
A. Protons only B. Protons and electrons C. Protons and neutrons D. Neutrons and electrons
C. Protons and neutrons
, Rationale: Mass number (A) approximates atomic mass in u; used to calculate isotopes' weights
in stoichiometry for reaction balancing.
Question 8
Which subatomic particle has no charge?
A. Proton B. Electron C. Neutron D. Positron
C. Neutron
Rationale: Neutrons contribute to mass without charge, stabilizing nuclei; excess or deficit leads
to radioactive decay reactions.
Question 9
The Pauli exclusion principle states that no two electrons can have the same?
A. Principal quantum number B. Four quantum numbers C. Orbital shape D. Energy level
B. Four quantum numbers
Rationale: n, l, m_l, m_s uniquely identify electrons, explaining orbital filling and periodic table
electron configurations for bonding predictions.
Question 10
In atomic structure, valence electrons determine?
A. Atomic mass B. Chemical reactivity C. Nuclear stability D. Isotopic variation
B. Chemical reactivity
Rationale: Outer shell electrons participate in bonding; e.g., alkali metals (1 valence e⁻) form +1
ions in reactions.
Question 11
The ground state of an atom means electrons are in?
A. Highest energy levels B. Lowest possible energy levels C. Excited states D. Ionized form
B. Lowest possible energy levels
