What Are Atoms? - Definition & Structure

Instructor: Jennifer Beddoe

Jennifer has an MS in Chemistry and a BS in Biological Sciences.

An atom is the basic building block of all matter. Everything in existence is made up of atoms. Learn more about the atom, its history and structure, then test your knowledge with quiz questions.


Atoms are incredibly small particles that make up everything in the universe. As far as we know, there are 118 different elements; around 92 that occur naturally and another 26 that scientists have created in laboratories (although this number fluctuates as scientists discover and create new elements). Each element is comprised of atoms that are unique to that element; hydrogen atoms are different from gold atoms, which are different from neon atoms.


The word 'atom' derives from a Greek word that means 'indivisible,' and it was first used to describe the idea that there was a point at which matter could not be made any smaller. These tiny particles were called atoms, and even later when it was discovered that the atom had pieces as well and was not the smallest unit of matter, the name was kept to avoid confusion.

In the early 1900s, two scientists developed a model of the atom that was based on the solar system, with a nucleus in the center (like the sun) and bodies orbiting around it (like the planets). Scientists have since realized that this model is too simple to completely describe an atom's structure; however, the Rutherford-Bohr model of an atom is a useful starting point to understanding the structure of an atom.


Every atom is made up of three subatomic particles:

  • Electrons - the smallest part of an atom, these particles are too small to measure. They are the 'planets' in the Rutherford-Bohr model of the atom as they rotate around the nucleus in a cloud. Electrons are negatively charged.
  • Protons - found in the nucleus of an atom, these particles are the positively charged components of the atom.
  • Neutrons - also found in the nucleus, the neutrons have no charge. Both the neutrons and protons are almost 2,000 times larger than the electrons.

Each element is different based on the number of protons and electrons it contains. An atom of hydrogen contains only one proton and one electron, while an atom of carbon contains six of each, and an atom of silver contains 47 protons and 47 electrons.

Size and Shape

The mass of an atom comes mainly from its protons and neutrons. The amount of these particles is equal to the atom's mass number. The atomic number however, comes just from the number of protons. The atomic number are how atoms are arranged in the periodic table.

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