HESI A2 Chemistry Study Guide: Key Topics to Review
Preparing for the HESI A2 Chemistry section can feel intimidating, especially if it has been a while since your last chemistry course. Breaking the material into smaller topics can make studying much more manageable.
This guide reviews several foundational concepts you should understand, including atomic structure, the periodic table, molecular and molar mass, chemical bonds, and common reaction types.
Atoms, elements, and subatomic particles
The atom is the basic unit of an element. Atoms contain three primary subatomic particles:
- Protons: Positively charged particles located in the nucleus
- Neutrons: Particles with no electrical charge located in the nucleus
- Electrons: Negatively charged particles found in orbitals surrounding the nucleus
An element is a substance composed of atoms that all have the same number of protons. The number of protons determines the element’s atomic number and identity.
In a neutral atom, the number of electrons equals the number of protons. If an atom gains or loses electrons, it becomes an ion:
- An atom that loses electrons becomes a positively charged ion, or cation.
- An atom that gains electrons becomes a negatively charged ion, or anion.
Atoms of the same element can have different numbers of neutrons. These variations are called isotopes.
How to read the periodic table
The periodic table organizes elements according to their atomic numbers and recurring chemical properties. For each element, you will typically see:
- The element’s name
- Its chemical symbol
- Its atomic number
- Its average atomic mass
The atomic number equals the number of protons in the atom.
The mass number is the total number of protons and neutrons in a specific atom or isotope. Because the mass number refers to one isotope, it is a whole number.
The average atomic mass shown on the periodic table is a weighted average of the naturally occurring isotopes of an element. This is why it is usually written as a decimal.
The periodic table is broadly divided into main-group elements, transition metals, lanthanides, and actinides. Elements in the same group often have similar chemical properties because they have similar arrangements of valence electrons.
Atomic mass, molecular mass, and molar mass
These chemistry terms are related, but they do not mean exactly the same thing:
- Atomic mass: The mass of an individual atom, usually expressed in atomic mass units
- Molecular mass: The combined atomic masses of all atoms in one molecule
- Molar mass: The mass of one mole of a substance, expressed in grams per mole
How to calculate molecular mass
To calculate the molecular mass of a substance:
- Identify each element in the chemical formula.
- Determine how many atoms of each element are present.
- Multiply each element’s atomic mass by the number of atoms present.
- Add the results.
For example, one molecule of water, H2O, contains two hydrogen atoms and one oxygen atom:
(2 × 1.008) + (1 × 15.999) = approximately 18.015 atomic mass units
What is a mole?
A mole is a unit used to count particles such as atoms, molecules, or ions. One mole contains approximately 6.022 × 1023 particles. This value is known as Avogadro’s number.
Molar mass tells you how many grams are present in one mole of a substance. It is calculated using the same numerical process as molecular mass, but the unit is grams per mole.
For example, oxygen gas has the formula O2. Because it contains two oxygen atoms, its molar mass is:
2 × 16.00 = 32.00 g/mol
This means that one mole of oxygen gas has a mass of approximately 32 grams.
Valence electrons and chemical bonds
Atoms form chemical bonds through interactions involving their electrons. The outermost electrons, called valence electrons, are especially important in determining how an atom bonds with other atoms.
Three common types of chemical bonds and attractions include:
- Ionic bonds: Form when electrons are transferred from one atom to another. This creates oppositely charged ions that attract each other. Ionic bonding commonly occurs between metals and nonmetals.
- Covalent bonds: Form when atoms share electrons. Covalent bonding commonly occurs between nonmetals and is common in organic molecules.
- Hydrogen bonds: Weak attractions that form between a partially positive hydrogen atom and an electronegative atom, such as oxygen, nitrogen, or fluorine. Although individually weak, hydrogen bonds play an important role in water and biological molecules.
Hydrogen bonds are intermolecular or intramolecular attractions rather than bonds formed through the direct sharing or transfer of electrons.
Common types of chemical reactions
You should be able to recognize common reaction patterns and determine how reactants are rearranged to create products.
- Synthesis: Two or more substances combine to form a new compound.
A + B → AB - Decomposition: One compound breaks down into two or more simpler substances.
AB → A + B - Single-replacement reaction: One element replaces another element in a compound.
AB + C → AC + B - Double-replacement reaction: Ions in two compounds exchange partners to form new compounds.
AB + CD → AD + CB - Acid-base neutralization: An acid reacts with a base, typically producing water and a salt.
- Combustion: A substance reacts with oxygen and releases energy, usually as heat or light.
- Oxidation-reduction: Electrons are transferred between substances. Oxidation involves losing electrons, while reduction involves gaining electrons.
How to study for HESI A2 Chemistry
Reading definitions is a useful starting point, but you also need to practice applying chemistry concepts. As you prepare:
- Practice identifying protons, neutrons, and electrons.
- Review how atomic number, mass number, and average atomic mass differ.
- Use the periodic table to calculate molecular and molar mass.
- Practice converting between moles, particles, and grams.
- Compare ionic, covalent, and hydrogen bonding.
- Learn to recognize common chemical-reaction patterns.
- Use practice questions to find the topics that need more review.
When you miss a question, review why the correct answer works and why the other options do not. This turns each practice question into an opportunity to strengthen your understanding.
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