What is the atom in physics? The simple definition

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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] which means unbreakable. This idea appeared especially among the pre-Socratics of ancient Greece, in the 5th century BC. J.-C. As for the written traces that deal with the existence of these tiny elements (molecules or atoms), they date from several centuries later. Between the 17th and 19th centuries, great authors mentioned these entities in their writings. Descartes, author of Le Monde, and Galileo, of Il Saggiatore, are part of it. However, it was not until 1908 that Jean Perrin managed to demonstrate 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 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 that an atom could not be broken, today it is no longer the case. It has been verified that an atom is made up of protons, neutrons and 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 neutrons, which together are also called nucleons, make up the nucleus of the atom. Protons are positively charged, while neutrons, as the name suggests, have a neutral charge. As for the electrons, they orbit around the nucleon mainly thanks to their negative charge. Specifically, their negative charge causes them to be attracted to the nucleus (positively charged). The agitation of these entities around a nucleus constitutes 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 has a neutral charge because it has the same number of protons and electrons. This number designates the atomic number noted Z, while that of neutrons is symbolized by N. The sum of Z and N gives the amount of nucleons noted A which is the mass number. This gives the following formula: A = Z + N. The chemical elements Each atom belongs to a chemical element that has its own symbol (X), preceded by its atomic number (ZX). In fact, it is the amount of protons (or electrons) in the atom that determines its chemical element. For example, the hydrogen atom has a single proton and is symbolized by 1 H. In addition, the iron atom has 26 protons and is represented by the symbol 26Fe. This classification of atoms was devised by Dimitri Mendeleev in 1869. It is based especially on the mass and chemical properties of atoms. Over time, this classification method evolved into its current form: the Periodic Table of the Elements. Isotopes All atoms of the same chemical element have the same number of protons. As for the number of neutrons, it can vary. Atoms of the same chemical element with different amounts of neutrons are called isotopes. Hydrogen, for example, has three naturally occurring isotopes: simple hydrogen, deuterium (one neutron), and tritium (two neutrons). The first isotope which is also the most widespread contains no neutrons. For example, uranium, which has 92 protons, has two naturally occurring isotopes: uranium 235 and uranium 238. Here, the numbers 235 and 238 denote the mass number (A) or nucleon number. As A = Z + N, so 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. In the case of uranium 235 and uranium 238, this gives 238U and 235U. Invisible, but very present In the past, men thought that atoms did not exist, because they could not observe them. For good reason, the diameter of an atom (nucleus and electron cloud) measures between 0.5 x 10-10 m (hydrogen) and 4.3 x 10-10 m (radius). In other words, the diameter of a hydrogen atom would have to be multiplied by 1 million to get that of a hair. If the atom is small, its nucleus is even smaller. In fact, the diameter of the atomic nucleus is about 100,000 times smaller than that of the atomic cloud. Today, these entities can be observed using scanning probe microscopes. SEE ALSO: What is a molecule? The simple definition Volume and mass of an atom Mass A neutron and a proton have almost the same mass. On the other hand, the mass of an electron is 1,840 times smaller than that of a neutron or a proton. Therefore, most of the mass of an atom is contained in the nucleus. The same happens with the solar system, whose mass is concentrated in the Sun. Also, calculating the mass of a nucleus is quite simple once we know its mass number. Simply multiply 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 accurately quantify its mass with a mass spectrometer. The volume of the atom Like its mass, an atom has an infinitely small volume. For comparison, a head of iron contains about 60 million iron atoms. However, the volume of the pin tip is only 1 mm3. Density of the nucleus of an atom The nucleus of an iron atom has a diameter of the order of 10-14 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. In fact, one cubic centimeter of iron core weighs about one hundred million tons. On a pinhead scale, this iron core concentrate would have a mass of 180 thousand tons. Dark matter, almost everywhere What lies between the nucleus of an atom and its electron cloud? The answer is “nothing”! Finally, there is what scientists have dubbed “dark matter” and it is still unknown. However, researchers estimate that it makes up around 95% of the entire Universe.
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