Skip to main content

What is an atom?

A science lesson about atoms provided by, here you will find: definition, origin, synonyms, antonyms, history, structure, properties (mass; shape; size), the current state and more...

Definition of atom

What is an atom? An atom is the smallest distinct unit of an element that has the properties of a chemical element and can take part in a chemical reaction. An atom is composed of a nucleus and one or more electrons bound to the nucleus. Atoms are invisible to the naked eye. Atoms are the building blocks of molecules.

Origin and synonyms of the word "atom"

According to almost all etymological dictionaries found nowadays, the word "atom" appeared in the late 15th century with the meaning of a hypothetical indivisible body that is also the building block of the universe, "atom" comes from the Latin word "atomus" (indivisible particle) and from the Greek word "atomos" (meaning: uncut; indivisible).

Synonyms used in physics and chemistry: elementary particle, chemical element or fundamental particle.

Synonyms with nontechnical usage: bit, jot, dot, grain, iota, speck, particle, material, a tiny piece of anything.

Antonyms with nontechnical usage: lot or whole.

A short history of atomic theory

The concept that matter is made up of something smaller dates back to ancient times, when philosophers of the 4th century BCE theorized that nature consists of two fundamental principles: atom and void. However, these ideas were unproven, because they couldn't show any evidence at that time.

It was not until the year 1808 when the first evidence-based theory appeared and the idea that atoms exist was embraced and refined by scientists from all over the world. The British chemist John Dalton was the first scientist who made a scientific discovery related to atoms and also the first evidence-based theory which he called it the law of multiple proportions in his first volume of his "New System of Chemical Philosophy" published in the year 1808.

Between the years 1827 and 1908, scientists such as Robert Brown, Albert Einstein and Jean Perrin, through analysis and experiments, have provided additional validation for Dalton's atomic theory.

The first subatomic particle to be discovered was the electron by J. J. Thomson through his work on cathode rays, in the year 1897.

The nucleus was discovered by Ernest Rutherford, a former student of J. J. Thomson, through Geiger–Marsden experiment, in the year 1909. The experiment conducted in 1919 by Ernest Rutherford made him the first person to deliberately transmute one element into another and also the discoverer of the proton which he later named.

Ernest Rutherford in the year 1920 theorized about the existence of a new subatomic particle called the neutron, but the discovery of the neutron which has about the same mass as the proton and no net electric charge was made 12 years later by another physicist by the name of James Chadwick.

In the 1950s, with the development of particle accelerators and particle detectors scientists discovered that neutrons and protons are composed of even smaller particles called quarks.

Structure of the atom

The structure of an atom can be simply described as a nucleus and one or more electrons bound to the nucleus. Because, in the past decades scientists have discovered that atoms are more than just a nucleus and one or more electrons, the more detailed description gets really complicated. Further we will analyze more thoroughly the structure of the atom.

For a better understanding of the structure of the atom, we tried to make a list of facts which can be more easily remembered:
- The nucleus of an atom is formed by protons and neutrons, they are also called nucleons.
- Protons have a positive electrical charge and a free mass of 1.6726×10−27 kg (1,836 times the mass of the electron).
- The number of protons is called atomic number and is equal to the number of electrons (non-ionized atom).
- Neutrons have no electrical charge and a free mass of 1.6929×10−27 kg (1,839 times the mass of the electron).
- Not all atoms of a particular chemical element are exactly the same, even if they aren't chemically or physically changed, they may have a number of natural variations of the neutron number in the nucleus, this type of atoms are called isotopes.
- Electrons, protons and neutrons are fermions, because they obey the Pauli exclusion principle.
- The protons and neutrons are subatomic composite particles, because they are composed of elementary particles called quarks.
- Between the constituent particles of an atom, electrons are truly elementary particles with no internal structure.
- Electrons have a negative electrical charge and a mass of 9.11×10−31 kg.
- Electrons have the properties of both a particle and a wave, just like any other particles.
- If an atom has more or fewer electrons than its atomic number, then it becomes respectively negatively or positively charged. This type of atoms are called ions and they are created via ionization (by either chemical or physical means).
- Electrons keep moving in shells outside the nucleus, each shell consists of one or more sub-shells, and each sub-shell consists of one or more atomic orbitals. A shell contains a fixed number of electrons. The outermost shell of an atom, is called the valence shell.
- The electrons that travel farthest from the nucleus determine how an atom reacts chemically.

Properties: mass, shape and size

The atomic mass (ma) is commonly expressed in unified atomic mass units (u), 1 unified atomic mass unit is defined as 1/12 of the mass of a single carbon-12 atom. Because, nucleons account for almost all of the mass, the atomic mass (u) has nearly the same value as the mass number (A).

Atoms lack a well-defined outer boundary. The smallest atom is helium with a radius of 32 pm, while one of the largest is cesium at 225 pm.

Origin and the current state

Origin: The first nuclei were formed about three minutes after the Big Bang, through the process called Big Bang nucleosynthesis. The current state: Atoms form about 4.9% of the total mass–energy content of the universe and 10% of the total mass–energy content of the Milky Way galaxy (the rest of the mass–energy content is dark matter and dark energy).

Atom: Questions & Answers

Below we have answered some of the most frequently asked questions about atoms.

How do people really know that atoms exist? Can we see atoms? Scientists use a scanning tunneling microscope (STM) to see and manipulate individual atoms. This instrument for imaging surfaces at the atomic level has amazing resolution (0.1 nm lateral resolution and 0.01 nm depth resolution). The scanning tunneling microscope is based on the concept of quantum tunneling.


Learn Something New Every Day! - education, gaming, information and research. A website with a large variety of articles and resources on various topics. All the articles found on this website are made by Andrei Ionita (creator and owner of, the entire content found on this website is fact checked and original, when I post something, I try to double check all the information provided against all the reliable and trustworthy sources that I can find. Now, about the gaming part. Beside, being a website owner and content writer, I'm also a gamer and a game developer. I create video games, animations, images and videos, this is why, from time to time, I will also post content about entertainment and media industry (a broad terminology, just to cover everything in the category). on the internet

YoutubeGoogle PlayItch.ioEdvog Explorer SteamImgur

The header image has been made by me using GIMP