What Is an Atom? — Definition & Overview

Imagine you're ripping aluminum foil into smaller and smaller pieces. How small can you make the pieces until they are no longer aluminum?

The smallest pieces of matter that maintain their properties are called atoms. In fact, the word atom comes from the Greek word for "indivisible" or "impossible to cut," because it was once thought of as the smallest possible particle of matter. As it turns out, atoms themselves are actually made of even smaller particles, called protons, neutrons, and electrons.

The above image is a diagram of a helium atom. Helium, like aluminum, is an element, or a type of atom. This means that you could break aluminum foil into a piece the size of a single atom, and it would still be aluminum and have all of its properties. Water, which is chemically referred to as $H_{2}O$, is also made of atoms, but cannot be broken into "atoms of water," because water is a compound, a combination of hydrogen and oxygen atoms.

An atom's element is defined by the number of protons, or positively charged particles, it has. We call the number of protons the atomic number. We use it to find an element on the periodic table. For example, helium has two protons. So, it's the second element. It also has two neutrons, which are the unlabeled particles in the above diagram which do not have a charge.

Together, the protons and neutrons are known as the nucleus. Changing the nucleus of an atom is impossible under most normal conditions, which is why alchemists were unable to change other metals to gold. Any process where the nucleus is changed is referred to as "nuclear," which is where we get nuclear power and nuclear weapons.

In order for an atom to be neutral, it must contain the same amount of negative charges as positive charges. The negative charges in an atom are found in the electrons, which orbit around the positively charged nucleus. Neutral atoms contain an equal amount of protons and electrons. But, by adding energy, electrons can be easily added or removed. Now our atom has a charge! Charged atoms called ions. The vast majority of chemistry focuses on the sharing and swapping of electrons between atoms.

Atoms are the basic building blocks of matter. Everything around us is made up of atoms, even if we don't see them.

Particles in an Atom

Atoms consist of three particles: protons, neutrons, and electrons.

Protons have a positive electrical charge. Neutrons have no electrical charge; hence they are neutral. Electrons have a negative electrical charge.

Atomic Structure

Protons and neutrons join together in the central part of the atom, the nucleus, while the electrons orbit around the nucleus. Since opposite charges attract, the neutrons are in the nucleus to stabilize the atom and neutralize the charge.

Atoms compose all materials that we deal with on a day-to-day basis. They are incredibly small, ranging in size from about 0.1 nanometers in diameter to 0.5 nanometers. To put this in perspective, the average fingernail is approximately 0.4 mm thick. So we would need more than a million atoms put end to end just to have something as thick as a fingernail. Atoms come together and bond in order to form compounds and molecules, and are themselves composed of protons, neutrons, and electrons.

Shown above is a diagram of a helium atom. In the center is the nucleus. The nucleus contains protons and neutrons, and is positively charged. Whizzing around the nucleus are electrons. The type of atom, or element is determined only by the number of protons. Typically an atom will have the same number of electrons as protons, and since protons and electrons have equal and opposite charges, an atom is electrically neutral. If it does not have the same number of electrons as protons, it is called an ion. The number of neutrons can vary a little bit. This means there are multiple types of atoms for each element. The different types are called isotopes. If there are too many or too few neutrons it causes the atom to be unstable, or radioactive.

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$\color{red}{\text{Proton}}$: The electrically positive part of an atom. It is found in the nucleus. $\\$ $\color{red}{\text{Neutron}}$: The electrically neutral part of an atom. It is found in the nucleus. $\\$ $\color{red}{\text{Electron}}$: The electrically negative part of an atom. It is found outside the nucleus. $\\$ $\color{red}{\text{Isotope}}$: Atoms of the same element that differ in the number of neutrons. $\\$ $\color{red}{\text{Radioactive}}$: An atom that contains either too many or too few neutrons, and is thus unstable. $\\$

(Video) What Is an Atom and How Do We Know?

by Stated Clearly

This video gives a great explanation of what atoms are, how they make up elements, and how we can see them.