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What is an Atom?


Just as cells are the basic unit of biological organisms, atoms are the basic unit of all matter. Everything in the universe, except energy, is made up of atoms. The word “atom” comes from the Greek word “atomos” which means “that which cannot be split”. We now know that it is actually possible to split an atom, but atoms are the smallest possible unit of a particular element (because if an atom splits it turns into a different type of atom).


The idea of atoms dates back as far as ancient Greece. A philosopher named Democritus postulated that if a stone were cut in half as many times as possible, eventually you would get a piece of stone so small that it could no longer be divided. While this idea may seem very simple, it became the basis for atomic theory.


Atoms are made up of three kinds of particles: Protons, neutrons, and electrons. We call these “subatomic particles” because they are smaller than an atom. Protons are positively charged, electrons are negatively charged, and neutrons have no charge.


The center of an atom is called the “nucleus” and is made up of protons and neutrons. Since protons are the only particle in the nucleus to have a charge, the nucleus is overall positively charged. Protons and neutrons have a very similar size and weight and most atoms have an equal or similar number of protons and neutrons.










​(Pettycon, 2016)

Electrons are much smaller and lighter than protons or neutrons. Rather than standing still, they orbit the nucleus in a constantly shifting cloud. You can imagine electrons kind of like moons orbiting a central planet, if the planet was over 1,800 times larger than the moon.   


Moons orbit planets due to gravitational force, but electrons orbit the nucleus because the positive and negative charges of the protons and electrons are attracted to each other. This attraction is what holds an atom together. In a normal state all atoms are neutral, with an equal number of protons and electrons.


The number of protons in an atom’s nucleus determines what type of atom, or element, it is. For example, an atom of gold has 79 protons in its nucleus. If a proton were to be added or subtracted, it would become a different element.


Changing the number of neutrons or electrons in an atom may change its properties, but not what element it is. An atom with a higher or lower number of neutrons than normal is called an isotope. Adding or removing an electron from an atom creates an ion, or an atom with a positive or negative charge.


Atomic theory allows us to better understand many scientific topics, from modern chemistry to the generation of nuclear energy.



Image Citation:


Pettycon (artist). (2016). Atom Chemistry Science [digital art]. Retrieved from

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