Aleks Placement Test: Chemistry Questions and
Answers and Detailed Explanation 2025/2026
Question 1
Which subatomic particle has a positive charge?
A) Electron
B) Neutron
C) Proton
D) Positron
Answer: C) Proton
Explanation: Protons have a positive charge (+1) and are located in the nucleus of
an atom. Electrons have a negative charge, and neutrons are neutral. The number of
protons determines the element's identity .
Question 2
The atomic number of an element is defined as the number of:
A) Neutrons
B) Protons
C) Electrons in the outermost shell
D) Protons plus neutrons
Answer: B) Protons
,Explanation: The atomic number equals the number of protons in an atom's
nucleus. It uniquely identifies the element. In a neutral atom, the number of
electrons equals the number of protons .
Question 3
How many protons, neutrons, and electrons are in an atom of ⁴⁰₂₀Ca?
A) 20 protons, 20 neutrons, 20 electrons
B) 20 protons, 20 neutrons, 18 electrons
C) 20 protons, 40 neutrons, 20 electrons
D) 40 protons, 20 neutrons, 40 electrons
Answer: A) 20 protons, 20 neutrons, 20 electrons
Explanation: The subscript 20 is the atomic number (protons). The mass number
40 minus the atomic number 20 equals 20 neutrons. A neutral calcium atom has 20
electrons .
Question 4
Which of the following represents a pair of isotopes?
A) ¹⁴C and ¹⁴N
B) ³²S and ³²S²⁻
C) ³⁵Cl and ³⁷Cl
D) H₂O and D₂O
Answer: C) ³⁵Cl and ³⁷Cl
,Explanation: Isotopes are atoms of the same element (same number of protons)
with different numbers of neutrons. ³⁵Cl and ³⁷Cl are both chlorine isotopes .
Question 5
Which scientist conducted the gold foil experiment and discovered the nucleus?
A) J.J. Thomson
B) Ernest Rutherford
C) John Dalton
D) Robert Boyle
Answer: B) Ernest Rutherford
Explanation: Rutherford's gold foil experiment (shooting alpha particles at gold foil)
proved there was a small, dense, positively charged nucleus. He also discovered
that protons are positive .
Question 6
What is the Plum Pudding Model?
A) A model depicting a sphere of positive charge with electrons scattered
throughout
B) A model with electrons in fixed orbits around the nucleus
C) A model with a dense positive nucleus
D) A model with electrons in a cloud
Answer: A) A model depicting a sphere of positive charge with electrons
scattered throughout
, Explanation: J.J. Thomson proposed the Plum Pudding Model, which depicted the
atom as a sphere of positive charge with electrons scattered throughout like
"plums in pudding." This was before Rutherford discovered the nucleus .
Question 7
Elements located on the left side of the periodic table that want to lose electrons
are called:
A) Nonmetals
B) Metalloids
C) Metals
D) Noble gases
Answer: C) Metals
Explanation: Metals are located on the left side of the periodic table and tend to
lose electrons to form positive ions (cations). Characteristics include malleability,
ductility, conductivity, and luster .
Question 8
Which of the following is NOT a characteristic of metals?
A) Malleable
B) Ductile
C) Conductive
D) Brittle
Answer: D) Brittle
Answers and Detailed Explanation 2025/2026
Question 1
Which subatomic particle has a positive charge?
A) Electron
B) Neutron
C) Proton
D) Positron
Answer: C) Proton
Explanation: Protons have a positive charge (+1) and are located in the nucleus of
an atom. Electrons have a negative charge, and neutrons are neutral. The number of
protons determines the element's identity .
Question 2
The atomic number of an element is defined as the number of:
A) Neutrons
B) Protons
C) Electrons in the outermost shell
D) Protons plus neutrons
Answer: B) Protons
,Explanation: The atomic number equals the number of protons in an atom's
nucleus. It uniquely identifies the element. In a neutral atom, the number of
electrons equals the number of protons .
Question 3
How many protons, neutrons, and electrons are in an atom of ⁴⁰₂₀Ca?
A) 20 protons, 20 neutrons, 20 electrons
B) 20 protons, 20 neutrons, 18 electrons
C) 20 protons, 40 neutrons, 20 electrons
D) 40 protons, 20 neutrons, 40 electrons
Answer: A) 20 protons, 20 neutrons, 20 electrons
Explanation: The subscript 20 is the atomic number (protons). The mass number
40 minus the atomic number 20 equals 20 neutrons. A neutral calcium atom has 20
electrons .
Question 4
Which of the following represents a pair of isotopes?
A) ¹⁴C and ¹⁴N
B) ³²S and ³²S²⁻
C) ³⁵Cl and ³⁷Cl
D) H₂O and D₂O
Answer: C) ³⁵Cl and ³⁷Cl
,Explanation: Isotopes are atoms of the same element (same number of protons)
with different numbers of neutrons. ³⁵Cl and ³⁷Cl are both chlorine isotopes .
Question 5
Which scientist conducted the gold foil experiment and discovered the nucleus?
A) J.J. Thomson
B) Ernest Rutherford
C) John Dalton
D) Robert Boyle
Answer: B) Ernest Rutherford
Explanation: Rutherford's gold foil experiment (shooting alpha particles at gold foil)
proved there was a small, dense, positively charged nucleus. He also discovered
that protons are positive .
Question 6
What is the Plum Pudding Model?
A) A model depicting a sphere of positive charge with electrons scattered
throughout
B) A model with electrons in fixed orbits around the nucleus
C) A model with a dense positive nucleus
D) A model with electrons in a cloud
Answer: A) A model depicting a sphere of positive charge with electrons
scattered throughout
, Explanation: J.J. Thomson proposed the Plum Pudding Model, which depicted the
atom as a sphere of positive charge with electrons scattered throughout like
"plums in pudding." This was before Rutherford discovered the nucleus .
Question 7
Elements located on the left side of the periodic table that want to lose electrons
are called:
A) Nonmetals
B) Metalloids
C) Metals
D) Noble gases
Answer: C) Metals
Explanation: Metals are located on the left side of the periodic table and tend to
lose electrons to form positive ions (cations). Characteristics include malleability,
ductility, conductivity, and luster .
Question 8
Which of the following is NOT a characteristic of metals?
A) Malleable
B) Ductile
C) Conductive
D) Brittle
Answer: D) Brittle
