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.

Ancient thinkers believed that atoms were indivisible. Furthermore, the word atom comes from the ancient Greek ἄτομος [átomos] meaning unbreakable. This idea appeared especially among the pre-Socratics of ancient Greece, in the 5th century BC. regarding written records which refers tothe existence of these small elements (molecules or atoms), they date from several centuries later. Between XVIIᵉ and XIXᵉ centuryof great writers mentioned these entities in their writings. Descartes, author ofThe world, and Galileo, by Il Saggiatore, is part of it. Nevertheless, it is onlyin 1908 to John Perrin managed to prove it matter consists of atoms.

An atom, what is it?

Atom is the name used by physicists to denote the smallest part of a simple body that can be chemically combined with another. Simply put, all solid, liquid or gaseous substances are basically made up of atoms.

What is an atom made of?

If in the past, scientists still believed itan atom is indivisibleToday, this is no longer the case. It is proved that an atom made by of protons, neutrons and of electrons. Each of these entities is itself a set of even smaller elements, called quarks. Nevertheless, we will focus on the atom and its three components.

Protons and neutronswhose set is also called nucleons, forms the nucleus of the atom. Protons is loaded positivelywhile neutronsas their name suggests, have a neutral load.

Compared to to electronsthe orbiting 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 units around a nucleus 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 equal number of protons and electrons. This number indicates Atomic number Note Zwhile that neutrons is symbolized by DOES NOT. The sum of Z and N indicates the amount of noted nucleons HAVE Who is mass number. This gives the following formula: A=Z+N.

chemical elements

Every atom belongs to a chemical element which has its own symbol (X), followed by its atomic number (ZX). Indeed the amount of protons (or electrons) in the atom as determine its chemical element. E.g, the hydrogen atom has a single proton and is symbolized by 1 H. Besides, the iron atomit has 26 protons and is represented by the symbol 26Fairy.

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


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

Hydrogenfor example gifts three natural isotopes: simple hydrogen, deuterium (one neutron) and tritium (two neutrons). The first isotope, which is also the most abundant, contains no neutrons.

E.g, uranium which has 92 protons two natural isotopes: uranium 235 and uranium 238. Here the numbers 235 and 238 denote the mass number (A) or the nucleons. Since A = Z + N, then 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 regarding uranium 235 and uranium 238this gives 238U and 235U.

Simple representation of an atom

Invisible, but very present

In the past it was believed so atoms did not existbecause they were there inability to observe them. Because, the diameter of an atom (core and electron cloud) measurement between 0.5, 10-10 m (hydrogen) to 4.3 .10-10 m (radium).

In other words, it should times 1 million the diameter of a hydrogen atom to get it offa strand of hair. If the atom is small, its nucleus is even smaller. Actually, 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

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 a neutron or a proton. Thereby, the majority of the mass of an atom is contained in the nucleus. It’s the same with the solar systemwhose the mass is concentrated in the Sun.

Or else, calculate mass of a core is quite simple as soon as we know it mass number. Simply times A by the mass of a nucleon which is approx 1.67.10-27 kg. Of course, this calculation is only an estimate of the mass of an atom. However, it is possible to quantify the mass precisely of it with one mass spectrometer.

Volume of the atom

Like its mass, an atom has an infinitesimal volume. In comparison, a iron pinhead includes approx 60 million billion iron atoms. However, the volume of the pin tip is only 1 mm3.

The density of the nucleus of an atom

that nucleus of an atom of iron present a diameter of the order of 10-14 m and a mass of approx. 9.3.10– 26 kg. With the formula: density = mass/volume, the result is 1.8.1017 kg/m3. Yes, a cubic centimeter of iron core weighs approx hundred million tons. On the extent of needle headsuch a concentrate of iron core would have a mass on it 180 thousand tons.

An atom in the universe

Dark matter, almost everywhere

What is between an atom’s nucleus and its electron cloud? The answer is “nothing”! Finally, there is what scientists have dubbed “black substance” and she stays still unknown. However, researchers think soit makes up about 95% of the entire universe.

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