What are Atoms?

Everything is made of atoms, but what exactly are atoms? And how do they work? Ask Science explains.

Lee Falin, PhD
3-minute read
Episode #9

What are Atoms?

The atom is known as the fundamental building block of all matter. But what is an atom and how can it act like a building block?

Atoms by the Numbers

All matter is made of atoms. An atom consists of a central nucleus made of protons and neutrons, surrounded by zero or more electrons.

In order to classify an atom, we need to know how many protons it has, how many neutrons, and how many electrons.

The number of protons an atom has determines which element it is. An oxygen atom will always have 8 protons. A carbon atom will always have 6 protons. An atom of gold will always have 79 protons. If you’ve ever seen the periodic table of elements, you probably know that the elements are arranged on that table in order of the number of protons they have. The number of protons an atom has is called its atomic number.

Do These Neutrons Make Me Look Fat?

Aside from protons, the nucleus of an atom also contains neutrons. While every carbon atom in the universe has exactly 6 protons, they don’t all have the same number of neutrons. Most carbon atoms (around 98.9%) have 6 neutrons. However about 1.1% have 7 neutrons. The mass number of an atom is the total protons and neutrons an atom has. So the mass number of a carbon atom with 6 neutrons would be 12 (since carbon always has 6 protons).

Atoms that have the same number of protons but different numbers of neutrons are called isotopes. We usually refer to isotopes by their mass numbers. So a carbon atom with 6 protons and 7 neutrons would be called Carbon-13.

As I mentioned in the episode on radiation, sometimes an atom will have more neutrons in its nucleus than it can handle, making it unstable. Unstable isotopes, also called radioactive isotopes, will break down by splitting into pieces, or shedding particles and energy in the form of radiation. This breakdown is referred to as radioactive decay. Depending on the particular atom, that breakdown can occur in a few seconds, or it might take thousands of years.

See also: What is Radiation?

Rash and Unpredictable Electrons

The final components of an atom are electrons.  Way back in the ancient past of the early 1900s, scientists thought that electrons orbited the nucleus like planets orbiting the sun. Apparently the electrons didn’t like being tied down like that, because now we know that electrons don’t behave so predictably and can in fact be anywhere they want, thank you very much.

Fortunately, despite these claims of rash spontaneity, electrons can be expected to spend most of their time in specific areas of space around the nucleus called atomic orbitals. These orbitals have various shapes, depending on how many electrons an atom has, and a couple of other factors.  Electrons are a bit anti-social, so no more than two electrons can be in a given orbital at any time.

The number of electrons and protons an atom has determine its charge. Each proton is one positive charge and each electron is one negative charge. So an atom with 6 protons and six electrons would have a net charge of zero. If that atom is irresponsible and loses one of its electrons, then it will now have one more positive charge than it has negative charges, giving it a net charge of positive 1 or +1. Atoms that have a net charge other than zero are called ions.

The number of electrons an atom has and its net charge determine a lot of the chemical properties of an atom, such as how likely it is to bond with another atom, something I’ll discuss in more detail next week.


So there you have it. With just three numbers, the atomic number, the mass number, and the net charge, you can uniquely identify every kind of atom in the universe. Next week we’ll talk about how atoms combine together to make molecules.

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Atoms image courtesy of Shutterstock.

Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.

About the Author

Lee Falin, PhD

Dr. Lee Falin earned a B.S. in Computer Science from the University of Illinois, then went on to obtain a Ph.D. in Genetics, Bioinformatics, and Computational Biology from Virginia Tech.