Qu’est-ce que l’atome en physique ? La définition simple

Un atome dans deux mains

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

Ancient thinkers believed that atoms were indivisible. Also, the word atom comes from the ancient Greek ἄτομος [átomos] who wants to say unbreakable. This idea appeared in particular among the pre-Socratics of Ancient Greece, in the 5th century BC. written records they deal withexistence of these tiny elements (molecules or atoms), date from several centuries later. Between 17th and 19th centuryof great authors they mentioned these entities in their writings. Descartes, author ofThe world, and Galileo, of Il Saggiatore, they are part of it. However, it is onlyin 1908 that John Perrin managed to show that The matter is made of atoms.

An atom, what is it?

Atom is the name physicists use to designate the smallest part of a simple body that can be chemically combined 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 indivisibleThis day, This is not the case. It has been shown that an atom consists of protons, neutrons Y of electrons. Each of these entities is itself the set of even smaller elements, called quarks. However, we will focus on the atom and its three constituents.

protons and neutronswhose set is also called nucleons, form the core of the atom protons they are loaded affirmativelywhile neutronsAs their name suggests, they have a neutral charge.

As to to electronsthey orbit around the nucleon mainly due to its negative charge. Specifically, their negative charge makes them feel attracted to the nucleus (positively charged). The agitation of these entities around a nucleus form an electron 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 is neutrally charged because it has an equal number of protons and electrons. This number designates the atomic number Note Zwhile neutrons is symbolized by NO. The sum of Z and N gives the number of annotated nucleons HAS who is he 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). In fact, it is the number of protons (or electrons) in the atom that determine its chemical element. For example, the hydrogen atom has only one proton and is symbolized by 1 H. Also, the iron atomhas 26 protons and is represented by the symbol 26Faith.

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


all atoms of one chemical element have the same number of protons. As for the number of neutrons, can vary. Atoms of the same chemical element with different numbers of neutrons are called isotopes.

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

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

To classify the different isotopes, it is enough to write the symbol of its chemical element with its number of nucleons. As for the case of uranium 235 Y uranium 238is all 238your Y 235your.

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. For cause, the diameter of an atom (nucleus and electron cloud) measurement between 0.5 .10-10 meter (hydrogen) to 4.3 .10-10 meter (radio).

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


a neutron Y a proton have an almost equal mass. Nevertheless, the mass of an electron is 1,840 times less to that of a neutron or a proton. Thus, most of the mass of an atom is content in the core. it’s the same with the solar systemwhose the mass is concentrated in the sun.

Secondly, calculate the mass of a kernel is quite simple as soon as we know its mass number. Enough with multiply a by the mass of a nucleon which is approximately 1.67.10-27 kg. Of course, this calculation is just an estimate of the mass of an atom. However, it is possible accurately quantify mass of it with a Mass spectrometer.

The volume of the atom

Like its mass, an atom has an infinitesimally small volume. For comparison, a iron pin head includes approximately 60 trillion trillion iron atoms. However, the volume of the pin tip is only 1mm.3.

The density of the nucleus of an atom.

the nucleus of an atom of iron Present a diameter of the order of 10-14 meter and a mass of about 9.3.10-26 kg. With the formula: density = mass/volume, the result is 1.8.1017 kg/m33. Indeed, one cubic centimeter of iron core weighs approximately hundred million tons. On the scale of pinheadsuch an iron core concentrate 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 called “black matter” and she stays an unknown. However, the researchers believe thatmakes up about 95% of the entire Universe.

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