HESI A2 Chemistry Study Guide: Key Topics to Review

The HESI A2 Chemistry section assesses your understanding of foundational chemistry concepts. Break the material into smaller topics and practice applying each concept to calculations, formulas, and reaction patterns.

Atomic structure

Atoms contain three primary subatomic particles:

  • Protons: Positively charged particles in the nucleus
  • Neutrons: Neutral particles in the nucleus
  • Electrons: Negatively charged particles in orbitals around the nucleus

The atomic number equals the number of protons. In a neutral atom, the numbers of protons and electrons are equal. An atom that gains or loses electrons becomes an ion.

The periodic table

The periodic table organizes elements by atomic number and recurring chemical properties.

  • Mass number: Total number of protons and neutrons in a specific isotope
  • Average atomic mass: Weighted average of an element’s naturally occurring isotopes
  • Valence electrons: Outermost electrons involved in chemical bonding

Elements within the same group often have similar chemical properties because they have similar valence-electron arrangements.

Molecular and molar mass

To calculate the molecular mass of a compound:

  1. Identify each element in the formula.
  2. Count the atoms of each element.
  3. Multiply each atomic mass by the number of atoms.
  4. Add the results.

For H2O:

(2 × 1.008) + (1 × 15.999) = approximately 18.015 atomic mass units

Molar mass uses the same numerical value but is expressed in grams per mole.

Moles and Avogadro’s number

One mole contains approximately 6.022 × 1023 particles. Use molar mass to convert between grams and moles and Avogadro’s number to convert between moles and particles.

Chemical bonds

  • Ionic bonds: Form through electron transfer and attraction between oppositely charged ions
  • Covalent bonds: Form when atoms share electrons
  • Hydrogen bonds: Relatively weak attractions involving a partially positive hydrogen atom and an electronegative atom

Acids, bases, and pH

The pH scale describes how acidic or basic a solution is:

  • A pH below 7 is acidic.
  • A pH of 7 is neutral.
  • A pH above 7 is basic or alkaline.

Because the pH scale is logarithmic, a change of one pH unit represents a tenfold change in hydrogen-ion concentration.

Chemical reactions

Practice recognizing these reaction types:

  • Synthesis
  • Decomposition
  • Single replacement
  • Double replacement
  • Acid-base neutralization
  • Combustion
  • Oxidation-reduction

Review how to identify reactants and products, balance equations, and apply the law of conservation of mass.

How to study HESI A2 Chemistry

  • Use the periodic table during practice.
  • Work calculations without copying examples.
  • Keep units visible throughout each calculation.
  • Practice balancing equations and identifying reaction types.
  • Review missed concepts again after several days.

Prepare for HESI A2 Chemistry with Pocket Prep. Practice questions and detailed explanations can help you build chemistry knowledge across repeated study sessions.