HESI A2: CHEMISTRY QUESTIONS WITH ANSWERS
AND EXPLANATIONS
1. What is the name for the number of protons in an atom?
A. atomic identity
B. atomic mass
C. atomic weight
D. atomic number
Explanation: The number of protons in an atom is the atomic number. Protons are the
fundamental positive unit of an atom. They are located in the nucleus. In a neutral atom (an atom
with neither positive nor negative charge), the number of protons in the nucleus is equal to the
number of electrons orbiting the nucleus. When it needs to be expressed, atomic number is
written as a subscript in front of the element’s symbol, for example in 13Al. Atomic mass,
meanwhile, is the average mass of the various isotopes of a given element. Atomic identity and
atomic weight are not concepts in chemistry.
,2. Which of the following elements is an alkali metal?
A. magnesium
B. rubidium
C. hydrogen
D. chlorine
Explanation: Rubidium is an alkali metal. The alkali metals are located in group 1 of the
periodic table. These soft substances melt at a low temperature and are typically white in color.
The alkali metals are lithium, sodium, potassium, rubidium, cesium, and francium. Rubidium,
cesium, and francium are not commonly encountered in the natural world. The alkali metals are
highly reactive, meaning that they easily engage in chemical reactions when combined with other
elements. These metals have a low density and tend to react violently with water.
1. Which of the following substances allows for the fastest diffusion?
A. gas
B. solid
, C. liquid
D. plasma
Explanation: Diffusion is fastest through gases. The next fastest medium for diffusion is liquid,
followed by plasma, and then solids. In chemistry, diffusion is defined as the movement of
matter by the random motions of molecules. In a gas or a liquid, the molecules are in perpetual
motion. For instance, in a quantity of seemingly immobile air, molecules of nitrogen and
oxygen are constantly bouncing off each other. There is even some miniscule degree of
diffusion in solids, which rises in proportion to the temperature of the substance.
2. What is the oxidation number of hydrogen in CaH2?
A. +1
B. 1
C. 0
D. +2
Explanation: The oxidation number of the hydrogen in CaH2 is –1. The oxidation number is the
positive or negative charge of a monoatomic ion. In other words, the oxidation number is the
AND EXPLANATIONS
1. What is the name for the number of protons in an atom?
A. atomic identity
B. atomic mass
C. atomic weight
D. atomic number
Explanation: The number of protons in an atom is the atomic number. Protons are the
fundamental positive unit of an atom. They are located in the nucleus. In a neutral atom (an atom
with neither positive nor negative charge), the number of protons in the nucleus is equal to the
number of electrons orbiting the nucleus. When it needs to be expressed, atomic number is
written as a subscript in front of the element’s symbol, for example in 13Al. Atomic mass,
meanwhile, is the average mass of the various isotopes of a given element. Atomic identity and
atomic weight are not concepts in chemistry.
,2. Which of the following elements is an alkali metal?
A. magnesium
B. rubidium
C. hydrogen
D. chlorine
Explanation: Rubidium is an alkali metal. The alkali metals are located in group 1 of the
periodic table. These soft substances melt at a low temperature and are typically white in color.
The alkali metals are lithium, sodium, potassium, rubidium, cesium, and francium. Rubidium,
cesium, and francium are not commonly encountered in the natural world. The alkali metals are
highly reactive, meaning that they easily engage in chemical reactions when combined with other
elements. These metals have a low density and tend to react violently with water.
1. Which of the following substances allows for the fastest diffusion?
A. gas
B. solid
, C. liquid
D. plasma
Explanation: Diffusion is fastest through gases. The next fastest medium for diffusion is liquid,
followed by plasma, and then solids. In chemistry, diffusion is defined as the movement of
matter by the random motions of molecules. In a gas or a liquid, the molecules are in perpetual
motion. For instance, in a quantity of seemingly immobile air, molecules of nitrogen and
oxygen are constantly bouncing off each other. There is even some miniscule degree of
diffusion in solids, which rises in proportion to the temperature of the substance.
2. What is the oxidation number of hydrogen in CaH2?
A. +1
B. 1
C. 0
D. +2
Explanation: The oxidation number of the hydrogen in CaH2 is –1. The oxidation number is the
positive or negative charge of a monoatomic ion. In other words, the oxidation number is the
