Atoms, Molecules, and Compounds: What's the Difference?

Updated on December 30, 2018
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K S Lane is a student of chemistry and is passionate about educating others on her favourite aspects of science.

Chemistry can be a confusing subject to study. Keeping all of the different terms, theories, and ideas separate in your mind is a real challenge. Even after years of studying the discipline I still find myself mixing up the basics every now and then. To help you out, this article explains the differences between some of the most fundamental units in chemistry: the atom, the molecule, and the compound. Whether you're a school student just starting to delve into the weird and wonderful world of chemistry or a professional looking to return to the basics, this article is sure to help you out!

This article discusses the difference between atoms, molecules, and compounds.
This article discusses the difference between atoms, molecules, and compounds.

What is an Atom?

Dictionary definition: the smallest unit of matter that can take part in a chemical reaction and which cannot be broken down chemically into anything simpler.

Breaking down the definition: In short, atoms are the tiny pieces of matter that make up our universe. They can't chemically be broken down (the process of atom-splitting is not a chemical one) into smaller parts, but they are constituted of sub-atomic particles, including electrons, protons and neutrons. As explained by the diagram below, protons and neutrons make up the nucleus (centre) of an atom while electrons form clouds (also known as orbitals) on the outside. Atoms as classified as each belonging to a certain element based on the number of protons that they have. For example, an atom with one proton is always hydrogen. The numbers of neutrons and even electrons may change, but at the core the atom will always be hydrogen. Trying to call an atom with one proton anything else than hydrogen would be like trying to call a child with living parents an orphan or a married man a bachelor; it's an intrinsic impossibility.

Atoms consist of a positively charged nucleus surrounded by a cloud of negatively charged electrons
Atoms consist of a positively charged nucleus surrounded by a cloud of negatively charged electrons | Source

What is a Molecule?

Dictionary definition: a group of two or more atoms bonded together.

Breaking down the definition: arguably the simplest molecule in existence, H2, is two hydrogen atoms bonded together. The term 'bonded' is loose and may lead to confusion, so to clear things up we'll define 'bond' in this sense as being a chemical bridge between two atoms that does not involve a positive or negative charge (we'll get to that idea later). This means that the atoms in molecules are tied together by something called a covalent bond; a relationship in which two atoms share their outer-most electrons with each other.

Molecules consist of two or more atoms held together by covalent bonds
Molecules consist of two or more atoms held together by covalent bonds | Source

What is a Compound?

Dictionary definition: a chemical substance made up of two or more elements bonded together, so that they cannot be separated by physical means.

Breaking down the definition: this definition is so similar to that of a molecule that it's almost unhelpful, however there is a subtle distinction which serves to separate the two terms. Molecules, as discussed earlier, are held together through covalent bonds. Compounds, on the other hand, are held together through ionic bonds. Ionic bonds involve an electrical attraction between positive and negatively charged atoms. Ionic bonds are stronger than covalent bonds, which is why the phrase 'cannot be separated by physical means' is added to the definition. Unlike molecules, compounds must be made up of two or more distinct elements. Molecules cannot be compounds, and compounds cannot be molecules, because the atoms in each are held together by different types of attractions. For example, one of the most commonly used compounds, NaCl or table salt, could never be described as a molecule. Even if they seem similar, they're totally different things!

Compounds like NaCl are held together by ionic bonds.
Compounds like NaCl are held together by ionic bonds. | Source

Summary of the Differences:

What is it?
The fundamental 'building blocks' of life- a collection of protons and neutrons surrounded by a cloud of electrons.
Atoms are the thing that make up molecules and compounds.
Two or more atoms joined together with covalent bonds
Molecules contain two or more atoms and are held together by covalent bonds, whereas compounds are held together by ionic bonds.
Two or more elements bonded together through ionic attraction.
Compounds contain two or more atoms and are held together by ionic bonds, whereas molecules are held together by covalent bonds. The atoms within a compound also must be different from eachother, whereas a molecule can consist of only one element.


Atoms are the smallest units of matter that chemists work with; collections of subatomic particles that can be categorised into unique elements based on the number of protons in their nucleus. Molecules are a step up; a collection of two or more atoms held together by covalent bonds, such as hydrogen gas or H2. Compounds are very similar to molecules, but instead of utilising covalent bonding contain atoms held together with ionic bonds; an attraction between a positive and negative electrical charge.

Questions & Answers

    © 2018 K S Lane


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        4 months ago

        What about elements???


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