An atom is the smallest unit of particles existing as a simple substance, being able to intervene in a chemical combination. Throughout the centuries, the limited knowledge that was had about the atom, was only the subject of conjecture and assumptions, so that concrete data could not be obtained until many years later. In the 18th and 19th centuries, the English scientist John Dalton suggested the existence of atoms as an extremely small unit, of which all matter would be composed, and assigned them mass and represented them as solid and indivisible spheres.
What is an atom
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It is the minimum unit of matter, of which solids, liquids and gases are composed. Atoms are grouped together, being able to be of the same type or different, to form molecules, which, in turn, constitute the matter of which existing bodies are composed. However, scientists have determined that only 5% of the matter in the universe is made up of atoms, since dark matter (which occupies more than 20% of the universe) is made up of unknown particles, as well as dark energy (which occupies 70%).
Its name comes from the Latin atomus, which means "indivisible", and those who gave it this terminology were the Greek philosophers Democritus (460-370 BC) and Epicurus (341-270 BC).
These philosophers, who without having experimented, in the search for an answer to the question of what we are composed of and the explanation of reality, concluded that it was impossible to divide matter infinitely, that there should be a "top", which meant that it would have reached the minimum limit of what all things are composed of. They called this "top" an atom, since that minimum particle could no longer be divided and the universe would be composed of that, it should be added that this concept is still preserved today when talking about what an atom is.
It is made up of a nucleus, where there is at least one proton and the same number of neutrons (whose union is called a “nucleon), and at least 99.94% of its mass is found in said nucleus. The remaining 0.06% is made up of the electrons that orbit the nucleus. If the number of electrons and protons is the same, the atom is electrically neutral; if it has more electrons than protons, its charge will be negative and it is determined as an anion; and if the number of protons exceeds the electrons, their charge will be positive, and called a cation.
Its size is so small (approximately ten billionth of a meter) that if an object were divided a considerable number of times, there would no longer be any of the material from which it was composed, but the atoms of the elements would remain that, in combination, they formed it, and these are practically invisible. However, not all types of atoms have the same shape and size, as it will depend on several factors.
Elements of an atom
Atoms have other components that make them up called subatomic particles, which cannot exist independently, unless under special and controlled conditions. These particles are: electrons, which have a negative charge; protons, which are positively charged; and neutrons, whose charge is equal, which makes them electrically neutral. Protons and neutrons are found in the nucleus (the center) of the atom, forming what is known as a nucleon, and electrons orbit the nucleus.
Protons
This particle is found in the nucleus of the atom, forming part of the nucleons, and its charge is positive. They contribute about 50% of the mass of the atom, and their mass is equivalent to 1836 times that of an electron. However, they have slightly less mass than neutrons. The proton is not an elementary particle, since it is composed of three quarks (which is a type of fermion, one of the two existing elementary particles).
The number of protons in an atom is decisive in defining the type of element. For example, the carbon atom has six protons, while a hydrogen atom has only one proton.
Electrons
They are the negative particles that are orbiting the nucleus of the atom. Its mass is so tiny that it is considered disposable. Normally, the number of electrons in an atom is the same as that of protons, so both charges cancel each other.
The electrons of different atoms are linked by the Coulomb force (electrostatic), and when shared and exchanged from one atom to another, it causes the chemical bonds. There are electrons that can be free, without being attached to some atom; and those that are linked to one, can have orbits of different sizes (the greater the orbital radius, the greater the energy contained in it).
The electron is an elementary particle, since it is a type of fermion (leptons), and it is not constituted by any other element.
Neutrons
It is the subatomic neutral particle of the atom, that is, it has the same amount of positive and negative charge. Its mass is slightly higher than that of protons, with which it forms the nucleus of the atom.
Like protons, neutrons are composed of three quarks: two descending or down with charge -1/3 and one ascending or up with charge +2/3, resulting in a total charge of zero, which gives it neutrality. A neutron by itself cannot exist outside the nucleus, since its average life outside the nucleus is about 15 minutes.
The amount of neutrons in an atom does not determine its nature, unless it is an isotope.
Isotopes
They are a type of atoms, whose nuclear composition is not equitable; that is, it has the same number of protons but a different number of neutrons. In this case, the atoms that make up the same element will be different, differentiated by the number of neutrons they contain.
There are two types of isotopes:
- Natural, found in nature, such as the hydrogen atom, which has three (protium, deuterium and tritium); or the carbon atom, which also has three (carbon-12, carbon-13, and carbon-14; each with different utilities).
- Artificial, which are produced in controlled environments, in which subatomic particles are bombarded, being unstable and radioactive.
There are stable isotopes, but this stability is relative, since, although they are radioactive in the same way, their period of disintegration is long compared to the existence of the planet.
How the elements of an atom are defined
An atom will be differentiated or defined by several factors, namely:
- Amount of protons: the variation in this number can result in a different element, as it determines which chemical element it belongs to.
- Number of neutrons: specifies the isotope of the element.
The force with which protons attract electrons is electromagnetic; while the one that attracts the protons and neutrons is the nuclear one, whose intensity is greater than the first one, which repels the positively charged protons among themselves.
If the number of protons in an atom is high, the electromagnetic force that repels them will become stronger than the nuclear one, there is a probability that the nucleons will be expelled from the nucleus, producing nuclear disintegration, or what is also known as radioactivity; to later result in nuclear transmutation, which is the conversion of one element into another (alchemy).
What is an atomic model
It is a scheme that helps define what an atom is, its composition, its distribution and the characteristics it presents. Since the birth of the term, different atomic models have been developed, which allowed us to better understand the structuring of matter.
The most representative atomic models are:
Bohr's atomic model
The Danish physicist Niels Bohr (1885-1962), after studies with his professor, the chemist and also physicist Ernest Rutherford, was inspired by the latter's model to expose his own, taking the hydrogen atom as a guide.
Bohr's atomic model consists of a kind of planetary system, in which the nucleus is in the center and electrons move around it like planets, in stable and circular orbits, where the larger one stores more energy. It includes the absorption and emission of gases, Max Planck's quantization theory and the photoelectric effect of
Albert Einstein
Electrons can jump from one orbit to another: if it goes from one of lower energy to another of higher energy, it will increase a quantum of energy for each orbit it reaches; The opposite happens when it goes from higher to lower energy, where it not only decreases, but also loses it in the form of radiation such as light (photon).
However, Bohr's atomic model had flaws, as it was not applicable for other types of atoms.
Dalton atomic model
John Dalton (1766-1844), mathematician and chemist, pioneered the publication of an atomic model with scientific basis, in which he stated that the atoms were similar to billiard balls, that is, spherical.
Dalton's atomic model establishes in his approach (which he called "atomic theory") that atoms cannot be divided. It also establishes that the atoms of the same element are of identical qualities, including their weight and mass; that although they can be combined, they remain indivisible with simple relationships; and that they can be combined in different proportions with other types of atoms to create various compounds (union of two or more types of atoms).
This Dalton atomic model was inconsistent, because it did not explain the presence of the subatomic particles, since the presence of the electron and the proton was unknown. Nor could it explain the phenomena of radioactivity or the current of electrons (cathode rays); furthermore, it does not take into account isotopes (atoms of the same element with different mass).
Rutherford atomic model
Raised by the physicist and chemist Ernest Rutherford (1871-1937), this model is an analogy to the solar system. Rutherford's atomic model establishes that the highest percentage of the mass of the atom and its positive part are found in its nucleus (center); and the negative part or electrons, revolve around it in elliptical or circular orbits, with a vacuum between them. Thus, it became the first model to separate the atom into nucleus and shell.
The physicist carried out experiments, in which he calculated the angle of dispersion of the particles when they hit a gold foil, and noticed that some bounced at incongruous angles, with which he concluded that their nucleus must be small but of great density. Thanks to Rutherford, who was a student of JJ Thomson, the first notion about the presence of neutrons was also had. Another achievement was raising questions about how positive charges in the nucleus could stay together in such a small volume, which later led to the discovery of one of the fundamental interactions: the strong nuclear force.
Rutherford's atomic model was inconsistent, as it contradicted Maxwell's laws on electromagnetism; nor did it explain the phenomena of energy radiation in the transition of an electron from a high to low energy state.
Thomson's atomic model
It was exposed by the scientist and winner of the 1906 Nobel Prize in Physics, Joseph John Thomson (1856-1940). Thomson's atomic model describes the atom as a positively charged spherical mass with electrons inserted into it, like a raisin pudding. The number of electrons in this model were sufficient to neutralize the positive charge, and the distribution of the positive mass and electrons was random.
He experimented with cathode rays: in a vacuum tube he passed current rays with two plates, producing an electric field that deflected them. Thus he determined that electricity was composed of another particle; discovering the existence of electrons.
However, Thomson's atomic model was brief, never having academic acceptance. His description of the internal structure of the atom was incorrect, as well as the distribution of charges, it did not take into account the existence of neutrons and it was not known about protons. Nor does it explain the regularity of the Periodic Table of the Elements.
Despite this, their studies served as the basis for later discoveries, since from this model, it was known about the existence of subatomic particles.
Atomic mass
Represented with the letter A, the total mass of protons and neutrons contained in an atom is called atomic mass, without taking electrons into account, since their mass is so small that it can be discarded.
Isotopes are variations of atoms of the same element with the same number of protons, but a different number of neutrons, so their atomic mass will be different even when they are very similar.
Atomic number
It is represented by the letter Z, and refers to the number of protons contained in an atom, which is the same number of electrons in it. Mendeleev's Periodic Table of the Elements of 1869, is ordered from smallest to largest according to atomic number.
