CHEM 120 midterm QUESTIONs, Verified Answers
& Detailed Rationales (2026/2027)
QUESTION 1
How many moles of oxygen atoms are present in 3.0 mol of aluminum
sulfate, Al2 (SO4 )3 ?
• A. 3.0 mol
• B. 12.0 mol
• C. 24.0 mol
• D. 36.0 mol
Correct Answer: D. 36.0 mol
Detailed Rationale: The chemical formula for aluminum sulfate is
Al2 (SO4 )3 . Each formula unit contains 3 sulfate groups, and each sulfate
group (SO4 ) contains 4 oxygen atoms, meaning there are 3 × 4 = 12
oxygen atoms per formula unit. Therefore, in 3.0 mol of the compound,
the total moles of oxygen atoms is 3.0 mol × 12 = 36.0 mol.
QUESTION 2
An atom of a specific element has 29 protons, 35 neutrons, and 29
electrons. What is the correct designation for this nuclide?
• A. 64
29Cu
• B. 35
29Cu
• C. 64
35Br
, • D. 58
29Ni
Correct Answer: A. 64
29Cu
Detailed Rationale: The atomic number (𝑍), which represents the
number of protons, defines the element (29 protons corresponds to
Copper, Cu). The mass number (𝐴) is the sum of protons and neutrons
(29 + 35 = 64). In nuclide notation, the mass number is written as a
superscript on the left, and the atomic number is written as a subscript
on the left: 64
29Cu.
QUESTION 3
What is the ground-state electron configuration of a neutral chlorine
atom (𝑍 = 17)?
• A. 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝5
• B. 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝6
• C. 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝4
• D. 1𝑠 2 2𝑠 2 2𝑝5 3𝑠 2 3𝑝6
Correct Answer: A. 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝5
Detailed Rationale: A neutral chlorine atom has 17 electrons. Filling the
orbitals in order of increasing energy (1𝑠, 2𝑠, 2𝑝, 3𝑠, 3𝑝), the capacity is
2 in 1𝑠, 2 in 2𝑠, 6 in 2𝑝, 2 in 3𝑠, and the remaining 5 electrons go into
the 3𝑝 orbital (2 + 2 + 6 + 2 + 5 = 17), yielding 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝5 .
QUESTION 4
Which of the following atoms has the largest atomic radius?
• A. F
, • B. Cl
• C. Br
• D. I
Correct Answer: D. I
Detailed Rationale: Atomic radius increases down a group on the
periodic table because additional electron shells are added, increasing
the distance of the valence electrons from the nucleus. Fluorine,
chlorine, bromine, and iodine are all in Group 17 (halogens), with iodine
located farthest down the group, giving it the largest atomic radius.
QUESTION 5
Which of the following elements has the highest first ionization energy?
• A. Na
• B. Mg
• C. P
• D. Cl
Correct Answer: D. Cl
Detailed Rationale: First ionization energy generally increases across a
period from left to right as the effective nuclear charge increases,
holding electrons more tightly. Sodium (Na), magnesium (Mg),
phosphorus (P), and chlorine (Cl) are all in Period 3. Since chlorine is
farthest to the right, it requires the most energy to remove its valence
electron.
QUESTION 6
, Which of the following substances contains both ionic and covalent
bonds?
• A. NaCl
• B. H2 O
• C. NH4 Cl
• D. CO2
Correct Answer: C. NH4 Cl
Detailed Rationale: Ammonium chloride (NH4 Cl) is composed of the
−
polyatomic ammonium ion (NH+ 4 ) and the chloride ion (Cl ), which are
held together by ionic bonds. Within the ammonium ion itself, the
nitrogen and hydrogen atoms are held together by covalent bonds.
Therefore, it contains both bond types.
QUESTION 7
According to VSEPR theory, what is the molecular geometry and
hybridization of a water molecule (H2 O)?
• A. Linear, 𝑠𝑝
• B. Trigonal planar, 𝑠𝑝2
• C. Tetrahedral (bent geometry), 𝑠𝑝3
• D. Trigonal pyramidal, 𝑠𝑝3
Correct Answer: C. Tetrahedral (bent geometry), 𝑠𝑝3
Detailed Rationale: The central oxygen atom in water has two single
covalent bonds and two lone pairs of electrons, giving it a steric number
of 4. This corresponds to an 𝑠𝑝3 hybridization and a tetrahedral
& Detailed Rationales (2026/2027)
QUESTION 1
How many moles of oxygen atoms are present in 3.0 mol of aluminum
sulfate, Al2 (SO4 )3 ?
• A. 3.0 mol
• B. 12.0 mol
• C. 24.0 mol
• D. 36.0 mol
Correct Answer: D. 36.0 mol
Detailed Rationale: The chemical formula for aluminum sulfate is
Al2 (SO4 )3 . Each formula unit contains 3 sulfate groups, and each sulfate
group (SO4 ) contains 4 oxygen atoms, meaning there are 3 × 4 = 12
oxygen atoms per formula unit. Therefore, in 3.0 mol of the compound,
the total moles of oxygen atoms is 3.0 mol × 12 = 36.0 mol.
QUESTION 2
An atom of a specific element has 29 protons, 35 neutrons, and 29
electrons. What is the correct designation for this nuclide?
• A. 64
29Cu
• B. 35
29Cu
• C. 64
35Br
, • D. 58
29Ni
Correct Answer: A. 64
29Cu
Detailed Rationale: The atomic number (𝑍), which represents the
number of protons, defines the element (29 protons corresponds to
Copper, Cu). The mass number (𝐴) is the sum of protons and neutrons
(29 + 35 = 64). In nuclide notation, the mass number is written as a
superscript on the left, and the atomic number is written as a subscript
on the left: 64
29Cu.
QUESTION 3
What is the ground-state electron configuration of a neutral chlorine
atom (𝑍 = 17)?
• A. 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝5
• B. 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝6
• C. 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝4
• D. 1𝑠 2 2𝑠 2 2𝑝5 3𝑠 2 3𝑝6
Correct Answer: A. 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝5
Detailed Rationale: A neutral chlorine atom has 17 electrons. Filling the
orbitals in order of increasing energy (1𝑠, 2𝑠, 2𝑝, 3𝑠, 3𝑝), the capacity is
2 in 1𝑠, 2 in 2𝑠, 6 in 2𝑝, 2 in 3𝑠, and the remaining 5 electrons go into
the 3𝑝 orbital (2 + 2 + 6 + 2 + 5 = 17), yielding 1𝑠 2 2𝑠 2 2𝑝6 3𝑠 2 3𝑝5 .
QUESTION 4
Which of the following atoms has the largest atomic radius?
• A. F
, • B. Cl
• C. Br
• D. I
Correct Answer: D. I
Detailed Rationale: Atomic radius increases down a group on the
periodic table because additional electron shells are added, increasing
the distance of the valence electrons from the nucleus. Fluorine,
chlorine, bromine, and iodine are all in Group 17 (halogens), with iodine
located farthest down the group, giving it the largest atomic radius.
QUESTION 5
Which of the following elements has the highest first ionization energy?
• A. Na
• B. Mg
• C. P
• D. Cl
Correct Answer: D. Cl
Detailed Rationale: First ionization energy generally increases across a
period from left to right as the effective nuclear charge increases,
holding electrons more tightly. Sodium (Na), magnesium (Mg),
phosphorus (P), and chlorine (Cl) are all in Period 3. Since chlorine is
farthest to the right, it requires the most energy to remove its valence
electron.
QUESTION 6
, Which of the following substances contains both ionic and covalent
bonds?
• A. NaCl
• B. H2 O
• C. NH4 Cl
• D. CO2
Correct Answer: C. NH4 Cl
Detailed Rationale: Ammonium chloride (NH4 Cl) is composed of the
−
polyatomic ammonium ion (NH+ 4 ) and the chloride ion (Cl ), which are
held together by ionic bonds. Within the ammonium ion itself, the
nitrogen and hydrogen atoms are held together by covalent bonds.
Therefore, it contains both bond types.
QUESTION 7
According to VSEPR theory, what is the molecular geometry and
hybridization of a water molecule (H2 O)?
• A. Linear, 𝑠𝑝
• B. Trigonal planar, 𝑠𝑝2
• C. Tetrahedral (bent geometry), 𝑠𝑝3
• D. Trigonal pyramidal, 𝑠𝑝3
Correct Answer: C. Tetrahedral (bent geometry), 𝑠𝑝3
Detailed Rationale: The central oxygen atom in water has two single
covalent bonds and two lone pairs of electrons, giving it a steric number
of 4. This corresponds to an 𝑠𝑝3 hybridization and a tetrahedral
