Molecular Structure.
Atoms, electrons, neutrons and protons



The atom is a structure in which matter is organized in the physical world or in nature. The atoms form the molecules, while the atoms in turn are formed by subatomic constituents such as protons (with positive charge), neutrons (without charge) and electrons. (with negative charge).

In a graphical way, what is an atom? Let's imagine that we have a piece of iron. We split it. We still have two pieces of iron but smaller ones. We will start them again, again ... Each time we will have more smaller pieces until a moment will come, in which if we go back to splitting what we would have left would not be iron, it would be another element of the periodic table. At this moment, we can say that what we have left is an atom, an atom of iron.

Definition of atom

In a more formal way, we define atom as the smallest particle in which an element can be divided without losing its chemical properties.

The origin of the word atom comes from the Greek, which means indivisible. At the time these particles were baptized, it was believed that they could not really be divided, although today we know that atoms are made up of even smaller particles, distributed in the two parts of the atom, called subatomic particles.

Parts of an atom

Definition of atom

The structure of an atom is very simple. We distinguish two parts of an atom: core and cortex.

The nucleus is the central part of the structure of the atom. In the part of the nucleus are protons (particles with positive charge) and neutrons (particles without electric charge).

In the cortex, the outer part of the atom is the electrons (particles with negative electrical charge).

Protons, neutrons and electrons are the subatomic particles that make up the structure of the atom. What differentiates one atom from another is the relationship established between them.

The negatively charged electrons are the lightest subatomic particles. The protons, positively charged, weigh about 1,836 times more than electrons. Neutrons, the only ones that have no electrical charge, weigh approximately the same as protons.

The protons and neutrons are grouped in the central part of the atom formed the atomic nucleus. For this reason they are also called nucleons.

In this way, the central part of the atom, the atomic nucleus, has a positive charge in which almost all its mass is concentrated, while in the foreshortening around the atomic nucleus there is a certain number of electrons, negatively charged. The total charge of the atomic nucleus (positive) is equal to the negative charge of the electrons, so that the total electric charge of the atom is neutral.

Bohr model

This description of the electrons orbiting the atomic nucleus corresponds to the simple Bohr model. According to quantum mechanics, each particle has a wave function that occupies all the space and the electrons are not located in orbits although the probability of presence is higher at a certain distance from the nucleus.

Properties of atoms

The basic units of chemistry are atoms. During chemical reactions the atoms are conserved as such, they are not created or destroyed, but they are organized differently creating different bonds between one atom and another.

The atoms are grouped forming molecules and other types of materials. Each type of molecule is the combination of a certain number of atoms linked together in a specific way.

According to the composition of each atom, the different chemical elements represented in the periodic table of the chemical elements are differentiated. In this table we can find the atomic number and the mass number of each element:

  • Atomic number, represented by the letter Z, indicates the number of protons present in an atom, which is equal to that of electrons. All atoms with the same number of protons belong to the same element and have the same chemical properties. For example all the atoms with a proton will be of hydrogen (Z = 1), all the atoms with two protons will be of helium (Z = 2).
  • Mass number, is represented by the letter A, and refers to the sum of protons and neutrons contained in the element. Isotopes are two atoms with the same number of protons, but different numbers of neutrons. The isotopes of the same element have chemical and physical properties very similar to each other.

Periodic table of elements.  Characteristics of atoms.

Atoms and their isotopes

It happens that the atoms of an element do not all have the same number of neutrons in the nucleus. This is called an isotope. Isotopes have (almost) the same chemical properties, but other physical properties. More than one isotope of virtually all elements is known. In addition, it is possible to produce new atoms with nuclear reactions, but they are often unstable and suffer from radioactive decay.

Isotopes are very important in the nuclear energy industry since manipulating them can generate more unstable variants (isotopes) that favor nuclear fission reactions. The enrichment of uranium is, precisely, the conversion of an isotope of uranium into another isotope of more unstable uranium.

History of the atomic theory

The concept of atom is very old. Even Demokritos suggested that everything is made of atoms and emptiness, and because there is no vacuum in the atoms, they are indivisible, because only the void can separate the songs from each other.

The concept of the atom in chemistry was introduced by John Dalton at the beginning of the 19th century. With this, he explained in particular the law of multiple weight coefficients. This law says that if two elements form more than one compound, then the quantities of one element that can coincide with the same amount of another element are in a simple whole proportion, usually small.

In the early nineteenth century, this theory and chemical reactions allowed us to determine very closely the relationship between the masses of the atoms of different elements. However, it was not yet known how large their masses were in a fraction of a gram, only their relative masses were known. Therefore, it was necessary to introduce a special unit of atomic mass that was initially defined as the mass of the hydrogen atom. (Today it is defined as 1/12 of the mass of the carbon-12 atom).

At the same time, thermo-theory and the theory of closely related kinetic gas, which also required gas consisting of molecules, also developed rapidly. Avogadro law, based on the theory of kinetic gas. It was also useful to determine the atomic masses of gaseous elements.

However, in the nineteenth century, atomic theory was of particular importance in organic chemistry. Through various chemical reactions, far-reaching conclusions were drawn about the structure of organic molecules and the order in which the atoms are located among themselves. When much later the structures of the organic molecules could also be studied with X-rays, the conclusions made by the chemists proved to be correct in most cases.

The atomic theory was quickly accepted by scientists, although there were still doubts at the end of the 19th century, for example Ernst Mach. Only the results of the radioactivity and the explanation of the movement of Albert by Albert Einstein in 1905 finally resolved the dispute and allowed to determine the relationship between the unit of atomic mass and the gram.

Like democracy, Dalton and the scientific community long after him kept the atom indivisible. However, the study of electrolysis to the conclusion that the atom can obtain an electrical charge, that is, ionized, gradually led to the assumption that there are also smaller electrically charged particles.

Atomic models

A little more than 2,000 years ago, the philosopher Plato introduced the atoms into various elements of the timios. He combined a regular polygon, the so-called Platonic piece, into each classical element: earth, air, fire and water, so that the earth was equivalent to a cube, an octahedron of air, a icosahedron of water and a tetrahedron of fire. Plato thought that each element was formed by its own atoms, as current theories suppose.

Of the current atomic models, based on scientific findings, the first is the bulb pattern of electron finder Joseph Thomson. It had been found that the atom was electrically neutral but was made up of charged particles of different marks. According to classical theory, the only possible permanent atomic model was one in which positive and negative particles are evenly distributed to the atom.

However, Ernest Rutherford did an experiment in which he bombarded a thin sheet of gold with alpha particles. To his great surprise, he discovered that a small part of the particles bounced through the others, as if most of the atom were empty and only a small core contained the entire mass. Rutherford ended up in a solar system model where electrons circulate a positive nucleus in the same way as the planets of the sun. However, Rutherford's atomic model would not be stable, according to classical physics, because electrons in the circular motion would soon radiate their energy.

Niels Bohr solved the problem by arguing that electrons circulate the nucleus only in certain permanent stationary paths. In Bohr's model, electrons are only emitted when they move from one track to another when absorbing or emitting a photon. The weaknesses of the model are related to the fact that in no way explains this quantum.

Finally, physicists like Erwin Schrödinger received developed quantum mechanics. The atomic model in which electrons form probability clouds around the nucleus: you never know for sure where the electron is, but it is as if it extended through space. Due to the complexity and rationality of quantum mechanics, the simple models of Rutherford and Bohr are still used in teaching, and most people still think of atoms as small solar systems. However, the quantum mechanical atomic model has proven to be valid in many extreme experiments.

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Last review: October 28, 2016