What Is an Atom? - Definition, Parts & Measurement

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.

Expert Contributor
Christianlly Cena

Christianlly has taught college physics and facilitated laboratory courses. He has a master's degree in Physics and is pursuing his doctorate study.

After completing this lesson, you will be able to explain what an atom is, what it is made of, and how we understand the atom through measurement. A short quiz will follow.

What Is an Atom?

Look around you. Everything you see is made out of atoms: The computer screen you're looking at, the chair you're sitting on - even the air you're breathing into your lungs. But what exactly IS an atom?

We often think of atoms as tiny little spheres. Like a bunch of marbles piles on top of each other. But atoms aren't that simple: they're made of smaller parts called protons, neutrons and electrons.

The protons and neutrons are found in the very center of the atom, which is called the nucleus. And the electrons orbit around the outside. We'll talk ore about each of those parts later in the lesson.

Structure of the Atom
Structure of the Atom

But if you look at the model of the atom, what do you notice? Well, one thing you might notice is that those protons, neutrons and electrons are all pretty small, at least if the diagram is drawn anything close to being to scale. In fact, atoms are mostly...nothing. They're mostly made of empty space.

So when you put your elbow down on your desk, why doesn't it go straight though? According to quantum mechanics, it's actually possible that it could go through. It's just very unlikely. Even though they're mostly empty space, there is a repulsive intermolecular force that you feel when you touch things. Intermolecular means between molecules. Those forces are strong enough that the desk still feels pretty solid.

The Parts of an Atom

Like we said, atoms are made of three parts: protons, neutrons and electrons. Those small PARTS are types of PARTicles. And particles have properties, or features about them that are important.

One of those properties is mass: how much stuff they contain. Protons and neutrons have masses of 1 atomic mass unit (AMU). And electrons have a tiny, tiny mass of 1/1836th of the protons and neutrons.

Another important property is charge. There are positive charges and negative charges: like charges repel, and opposite charges attract. Protons have a +1 charge. Electrons have a -1 charge. And neutrons are NEUtral. That's why we call them NEUtrons. So even though electrons are tiny in terms of mass, they have a charge that is just as big as a proton (but negative instead of positive).

Electrons are fundamental particles, because they can't be broken down any more. But protons and neutrons actually contain smaller particles inside them called quarks. You don't need to know much about them, unless you're a physics major in college. But just knowing they exist means you understand atoms better than most people.

The Quarks Inside a Proton
The Quarks Inside a Proton

Measuring Atoms

Okay, but if atoms are so tiny. How do we know they're there?

Well, for one thing, we can see the effects they produce. When chemical reactions happen, it's because atoms are rearranging the way they're connected to other atoms. And by understanding what atoms are made of, we can explain why those chemical reactions happen. For example, maybe one atom gives an electron to another atom. Or maybe two atoms start sharing electrons between them. We can also use a device called a mass spectrometer, to measure the masses of atoms.

A Mass Spectrometer
A Mass Spectrometer

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Additional Activities

Atom Crossword Puzzle

In this activity, you will check your knowledge regarding the definition, parts, and measurement of an atom as presented from the lesson.


Complete the crossword by filling in a word that fits each of the given clues. To do this, you can just right-click the crossword to save the image and print it. With a pencil and an eraser, neatly write your answers in the boxes provided.


2. A large fraction of the atom's _____ is concentrated in its nucleus, which contains protons and neutrons.

5. Two electrons will tend to repel each other because both have a _____ electrical charge.

7. It is an uncharged elementary particle that forms part of the nucleus of an atom.

9. It is the subatomic particle having a negative charge and orbiting the nucleus.

10. _____ are particles believed to be the building blocks of protons and neutrons.


1. Unlike protons, the electron is a fundamental particle that could never be _____ down.

3. A _____ is an analytical tool useful for measuring the mass of molecules present in a sample.

4. An electron microscope can be used to visualize objects in the size of an _____.

6. The massive, positively charged central part of an atom is known as the _____.

8. The absolute value of the _____ of an electron is equal to that of a proton.

Answer Key

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