What are Atoms & Molecules? - Definition & Differences

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  • 0:03 Atoms
  • 1:13 Molecules, Compounds &…
  • 1:53 Ionic & Covalent Bonds
  • 2:50 Lesson Summary
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Lesson Transcript
Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

There are lots of terms in chemistry that can be easy to mix up: atoms, molecules, compounds, mixtures, and more. In this lesson, we'll go through some of the basics of atoms and molecules, and define all of the important terms you need to know.


Atoms are the basic building blocks from which everything around us is built. This includes the computer before you, the floor beneath you, the air you breathe, and the stars you see at night. All of them are made up of atoms. But what exactly are atoms?

Atoms are made up of three types of tiny particles. Protons (or positively charged particles) and neutrons (or particles that contain no charge) are found in the nucleus of an atom. Electrons (or negatively charged particles) are found outside the nucleus. There are 118 elements, or substances composed of a single type of atom, on the periodic table, including hydrogen, carbon, lead, gold, lithium, oxygen, and more.

What makes each element different is the number of positively charged protons in the nucleus of the atom. Typically, these elements have the same number of negatively charged electrons orbiting around the outside of the nucleus. Ions are atoms that have lost or gained electrons. The number of neutrons varies and is necessary for keeping an atom stable.

Molecules, Compounds, & Mixtures

Atoms can bond together into groups and form a molecule. If the molecule contains atoms of different types bonded together, we call it a compound. For example, two hydrogen atoms and one oxygen atom bonded together creates the compound water. Bonding is vital for life; you wouldn't be reading this lesson without it.

Mixtures are what you get when you combine several compounds. No bonding or chemical reactions are involved in creating a mixture. For example, if you take water and mix it with a packet of flavoring, you get a refreshing drink. But, the molecules are really just interspersed between each other; no bonding has taken place.

Ionic & Covalent Bonds

So how do bonds form to create molecules and compounds? There are actually two ways that bonds can form between atoms: ionic bonds and covalent bonds.

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