High School Chemistry  ·  Practice by Skill

Atomic Structure Practice Problems

An atom is built from protons, neutrons, and electrons, and the numbers of each decide what the atom is and how it behaves. Most atomic structure questions come down to reading a symbol and counting particles correctly. Practice below covers particle counts, isotopes, ions, and average atomic mass.

What you’ll practice

  • Particle counts: find protons, neutrons, and electrons from atomic number and mass number.
  • Isotopes: tell isotopes apart and read notation like iron-56.
  • Ions: work out how gaining or losing electrons changes the charge and the electron count.
  • Average atomic mass: weight each isotope by how common it is.

Learn the method first →

A quick example: average atomic mass

Chlorine is 75.77% chlorine-35 (34.969 amu) and 24.23% chlorine-37 (36.966 amu). What is its average atomic mass?

Multiply each mass by its abundance as a decimal: 0.7577 × 34.969 = 26.50 and 0.2423 × 36.966 = 8.957.

Add the contributions: 26.50 + 8.957 = 35.45 amu.

3 free atomic structure problems

Give each one a real try before you open the solution. Your answer is checked as soon as you submit it, and the worked solution shows every step so you can see exactly where a setup went wrong.

#1Subatomic Particles
How many neutrons are in an atom of iron-56 (Fe-56)? (Atomic number of Fe = 26)
#2Isotopes
Which of the following best defines isotopes?
#3Isotopes + Ions
An atom of bromine-80 (Z = 35) gains one electron to form Br-. How many electrons does Br- have, and how does this compare to the number of neutrons in Br-80?

Questions you might have about atomic structure

How do I find the number of protons, neutrons, and electrons?
The atomic number is the number of protons, and in a neutral atom it is also the number of electrons. Subtract the atomic number from the mass number to get the neutrons.
What is an isotope?
Isotopes are atoms of the same element with different numbers of neutrons. They have the same atomic number but different mass numbers, like carbon-12 and carbon-14.
How do I calculate average atomic mass?
Multiply the mass of each isotope by its relative abundance written as a decimal, then add the results. The answer sits closest to the mass of the most abundant isotope.
What is the difference between mass number and atomic mass?
Mass number is a whole number: protons plus neutrons in one specific atom. Atomic mass is the weighted average mass of all of an element's isotopes, so it is usually a decimal.
High School Chemistry · Atomic Structure Practice Problems