Atoms and Atomic Theory - Study Guide

Facts, Problems, and Quiz

Atom, Illustration

Atoms are one of the first topics covered in a chemistry course because they are the fundamental building block of matter. Atoms bond to each other to form pure elements, compounds, and alloys. These substances exchange atoms with each other to form new products through chemical reactions.

Key Takeaways: Atoms

  • Atoms are the smallest unit of matter that cannot be divided using any chemical method. They do consist of smaller parts, but can only be broken by nuclear reactions.
  • The three parts of an atom are protons, neutrons, and electrons. Protons carry a positive electrical charge. Neutrons are electrically neutral. Electrons carry a negative charge, equal in magnitude to that of a proton.
  • Protons and neutrons stick together to form the atomic nucleus. Electrons orbit around the nucleus.
  • Chemical bonding and chemical reactions occur due to the electrons around atoms. An atom with too many or too few electrons is unstable and may bond with another atom to either share or essentially donate electrons.

Atom Overview

Chemistry is the study of matter and the interactions between different types of matter and energy. The fundamental building block of matter is the atom. An atom consists of three main parts: protons, neutrons, and electrons. Protons have a positive electrical charge. Neutrons have no electrical charge. Electrons have a negative electrical charge. Protons and neutrons are found together in what is called the nucleus of the atom. Electrons circle around the nucleus.

Chemical reactions involve interactions between the electrons of one atom and the electrons of another atom. Atoms which have different amounts of electrons and protons have a positive or negative electrical charge and are called ions. When atoms bond together, they can make larger building blocks of matter called molecules.

The word "atom" was coined by the early Greeks Democritus and Leucippus, but the nature of the atom wasn't understood until later. In the 1800s, John Dalton demonstrated atoms react with each other in whole ratios to form compounds. The discovery of the electron earned J.J. Thomson the 1906 Nobel Prize in Physics. The atomic nucleus was discovered in the gold foil experiment conducted by Geiger and Marsden under the supervision of Ernest Rutherford in 1909.

Important Atom Facts

All matter consists of particles called atoms. Here are some useful facts about atoms:

  • Atoms cannot be divided using chemicals. They do consist of parts, which include protons, neutrons, and electrons, but an atom is a basic chemical building block of matter.
  • Each electron has a negative electrical charge.
  • Each proton has a positive electrical charge. The charge of a proton and an electron are equal in magnitude, yet opposite in sign. Electrons and protons are electrically attracted to each other.
  • Each neutron is electrically neutral. In other words, neutrons do not have a charge and are not electrically attracted to either electrons or protons.
  • Protons and neutrons are about the same size as each other and are much larger than electrons.
  • The mass of a proton is essentially the same as that of a neutron. The mass of a proton is 1840 times greater than the mass of an electron.
  • The nucleus of an atom contains protons and neutrons. The nucleus carries a positive electrical charge.
  • Electrons move around outside the nucleus.
  • Almost all of the mass of an atom is in its nucleus; almost all of the volume of an atom is occupied by electrons.
  • The number of protons (also known as its atomic number) determines the element. Varying the number of neutrons results in isotopes. Varying the number of electrons results in ions. Isotopes and ions of an atom with a constant number of protons are all variations of a single element.
  • The particles within an atom are bound together by powerful forces. In general, electrons are easier to add or remove from an atom than a proton or neutron. Chemical reactions largely involve atoms or groups of atoms and the interactions between their electrons.

Study Questions and Answers

Try these practice problems to test your understanding of atomic theory.

  1. Write the nuclear symbols for three isotopes of oxygen in which there are 8, 9, and 10 neutrons, respectively. Answer
  2. Write the nuclear symbol for an atom with 32 protons and 38 neutrons. Answer
  3. Identify the number of protons and electrons in the Sc3+ ion. Answer
  4. Give the symbol of an ion which has 10 e- and 7 p+Answer


  • Lewis, Gilbert N. (1916). "The Atom and the Molecule". Journal of the American Chemical Society. 38 (4): 762–786. doi:10.1021/ja02261a002
  • Wurtz, Charles Adolphe (1881). The Atomic Theory. New York: D. Appleton and company. ISBN 978-0-559-43636-9.
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Helmenstine, Anne Marie, Ph.D. "Atoms and Atomic Theory - Study Guide." ThoughtCo, Apr. 5, 2023, Helmenstine, Anne Marie, Ph.D. (2023, April 5). Atoms and Atomic Theory - Study Guide. Retrieved from Helmenstine, Anne Marie, Ph.D. "Atoms and Atomic Theory - Study Guide." ThoughtCo. (accessed June 6, 2023).