Un atome dans deux mains

What is the atom in physics? The simple definition


According to chemists, atoms are the building blocks of matter. This hypothesis, now proven, was born several centuries ago.

One atom in two hands

Ancient thinkers believed that atoms were indivisible. Moreover, the word atom comes from the ancient Greek ἄτομος [átomos] which means unbreakable. This idea appeared in particular among the pre-Socratics of Ancient Greece, in the 5th century BC. As for the written records which deal withexistence of these tiny elements (molecules or atoms), they date from several centuries later. Between the XVIIᵉ and XIXᵉ centuryof great authors mentioned these entities in their writings. Descartes, author ofThe world, and Galileo, of the tester, are part of it. Nevertheless, it is onlyin 1908 What John Perrin succeeded in proving that matter is made up of atoms.

An atom, what is it?

Atom is the name used by physicists to designate the smallest part of a simple body that can combine chemically with another. In simple terms, all solid, liquid or gaseous substances are basically made up of atoms.

What is an atom made of?

If in the past, scientists still thought thatan atom is indivisibletoday, this is no longer the case. It has been proven that an atom made of of protons, neutrons and of electrons. Each of these entities is itself the set of even smaller elements, called quarks. Nevertheless, we will focus on the atom and its three constituents.

Protons and neutronsthe set of which is also called nucleons, form the core of the atom. Protons are loaded positivelywhile neutronsas their name suggests, have a neutral load.

Amount to electronsthey orbit around the nucleon mainly due to its negative charge. Specifically, their negative charge causes them to be attracted to the nucleus (positively charged). The agitation of these entities around a core forms an electronic cloud. Depending on their energy, the electrons can have a more or less intense movement.

What is the difference between one atom and another?

An atom has a neutral charge because it has an equal number of protons and electrons. This number designates the atomic number note Zwhile the one neutrons is symbolized by NOT. The sum of Z and N gives the quantity of nucleons noted A who is the mass number. This gives the following formula: A=Z+N.

chemical elements

Each atom belongs to a chemical element which has its own symbol (X), preceded by its atomic number (ZX). Indeed, it is the amount of protons (or electrons) in the atom which determine its chemical element. For instance, the hydrogen atom has a single proton and is symbolized by 1 H. In addition, the iron atomit has 26 protons and is represented by the symbol 26Fe.

This classification of atoms has been imagined by Dimitri Mendeleïev in 1869. It is based in particular on the mass and the chemical properties of the atoms. Over time, this method of classification evolved into its current form: the periodic table of elements.


All atoms of one chemical element have the same amount of protons. As to number of neutrons, it can vary. Atoms of the same chemical element with different amounts of neutrons are then called isotopes.

Hydrogenfor example, present three natural isotopes: simple hydrogen, deuterium (one neutron) and tritium (two neutrons). The first isotope which is also the most widespread does not contain any neutrons.

For instance, uranium which has 92 protons presents two natural isotopes: uranium 235 and uranium 238. Here the numbers 235 and 238 designate the mass number (A) or that of nucleons. As A = Z + N, so 235 = 92 + 143 and 238 = 92 + 146.

To classify the different isotopes, simply write the symbol of their chemical element with their number of nucleons. Regarding the case of uranium 235 and uranium 238this gives 238you and 235you.

Simple representation of an atom

Invisible, but very present

In the past, people thought that atoms did not existbecause they were in the inability to observe them. Because, the diameter of an atom (nucleus and electron cloud) measurement between 0.5 .10-ten m (hydrogen) to 4.3 .10-ten m (radium).

In other words, it should multiply by 1 million the diameter of a hydrogen atom to get that ofa strand of hair. If the atom is small, its nucleus is even smaller. In fact, the diameter of atomic nucleus is about 100,000 times lower to that of atomic cloud.

Today, these entities are observable thanks to scanning probe microscopes.

SEE ALSO: What is a molecule? The simple definition

The volume and mass of an atom

The mass

a neutron and a proton have an almost equal mass. However, the mass of an electron is 1,840 times less to that of a neutron or a proton. Thereby, the majority of the mass of an atom is contained in the nucleus. It is the same with the solar systemdon’t the mass is concentrated in the Sun.

Otherwise, calculate mass of a kernel is quite simple as soon as we know its mass number. Simply multiplier A by the mass of a nucleon which is approximately 1.67.10-27 kg. Of course, this calculation is only an estimate of the mass of an atom. However, it is possible to precisely quantify the mass of it with a mass spectrometer.

The volume of the atom

Just like its mass, an atom has an infinitely small volume. For comparison, a iron pinhead includes approximately 60 million billion atoms of iron. Yet the volume of the pin tip is only 1 mm3.

The density of the nucleus of an atom

The nucleus of an atom TO DO present a diameter of the order of ten-14 m and a mass of approximately 9.3.10-26 kg. With the formula: density = mass/volume, the result is 1.8.1017 kg/m3. Indeed, one cubic centimeter of iron core weighs approximately hundred million tons. At the scale of the pinheadsuch a concentrate of iron core would have a mass of 180 thousand tons.

An atom in the universe

Dark matter, almost everywhere

What is between the nucleus of an atom and its electron cloud? The answer is “nothing”! Finally, there is what scientists have dubbed ” black matter “ and she stays still unknown. However, researchers believe thatit constitutes about 95% of the entire Universe.

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