The Atom

Every atom in your body comes from a star that exploded. And the atoms in your left hand probably come from a different star than the one in your right hand. That's the most poetic thing I know about physics: you are stardust.

(Lawrence Maxwell Krauss)


The atom is the structure in which matter is organized in the physical world and on whose basis it is divided into fundamental elements. Multiple atoms form the molecules. While atoms are in turn formed by subatomic constituents such as protons (with positive charge), neutrons (without charge) and electrons (with negative charge).






It was so called because it was initially considered the smallest and most indivisible unit of matter. Towards the end of the nineteenth century, with the discovery of the electron it was shown that the atom was actually divisible. In fact, it is in turn composed of smaller particles, to which we refer with the term of subatomic particles.Leucus (5th century BC) founded a theory according to which he hypothesized the non-continuity of matter. Rather, he claimed that matter itself was made up of tiny and indivisible particles. This theory was called "atomism". The different "atoms" were supposed to be different in shape and size. Democritus proposed the "atomic theory", according to which matter is made up of tiny particles, different from each other, called atoms, whose union gives rise to all known substances . These particles were the smallest existing entity and could not be further divided: for this reason they were called atoms. Only at the beginning of the 19th century did John Dalton rework and propose Democritus's theory. So he founded modern atomic theory, with which he gave an explanation to chemical phenomena. He proposed a law according to which the different quantities by weight of an element that combine with the same quantity of another element. This procedure was used to form compounds different from one another in ratios expressed by small integers (law of multiple proportions). So he hypothesized that matter was made up of atoms.

The atom is the smallest part of matter, it is formed by the following particles: the electron, negatively charged


particle; the proton, positively charged particle; the neutron, uncharged particle whose mass is equal to that of the proton. The proton and the neutron have a relative mass equal to 1 while the electron has an irrelevant mass, therefore the mass of the atom is given by neutrons and electrons.
Generally in an atom the number of protons and the number of electrons is equal, so the atom is neutral. If the atom loses electrons it charges positively and becomes a positive ion, if an atom acquires electrons it charges negatively and becomes a negative ion.
Scientists over time have formulated different atomic models to explain the structure of the atom. In 1898 Thomson developed a model according to which the atom was similar to a sphere containing positive charges and negative charges in equal number and uniformly distributed (Thomson's atomic model).

Rutherford in 1910 established that the atom was constituted by a central zone, called nucleus, containing protons, around which electrons in the same number as the protons present in the nucleus rotated, at various distances, on different circular orbits. This model was also called planetary because similar to the solar system (Rutherford atomic model).
Rutherford's theory, however, showed the following defects:
1. the electrons during their motion should lose energy and fall into the nucleus with consequent destruction of the atoms, which in reality does not occur.
2. this model also does not explain the emission of energy in discontinuous form by atoms.
These defects were overcome by Bohr's atomic model which was developed between 1913 and 1915.

According to Bohr the atom of each element consists of a central nucleus, formed by protons and neutrons, around which the electrons rotate. The movement of electrons happens however on particular orbits called stationary orbits. The movement of electrons on the orbits happens without absorption or emission of energy, so the electron rotating within an orbit does not lose energy and cannot fall into the nucleus. Stationary orbits are considered energy levels.
Every time an electron moves from a higher energy level to a lower energy level it releases energy, vice versa it acquires energy when it moves from a lower energy level to a higher energy level. In these passages the electron yields or acquires energy in a quantized way, that is, according to well-defined quantities called energy quanta.
Bohr's atomic model, however, contrasted with Heisenberg's (1927) principle of indeterminacy according to which it is impossible to assign to the electron, in a given instant, a very precise position in its orbit.
It was then formulated the quantum-mechanical or atomic orbital model, according to which atoms are formed by a central nucleus containing protons and neutrons, around it rotate the electrons arranged in different levels of energy.
The electrons, however, do not occupy precise orbits around the nucleus but are found with greater frequency in certain areas of space around the nucleus, these areas are called atomic orbitals.
Atomic orbits are, therefore, the areas of space around the nucleus where there is the greatest probability of finding an electron.
The main difference between this theory and Bohr's theory is that according to Bohr the electrons are inside well-defined orbits, while according to the quantum-mechanical model the electrons are in the orbits that represent the area around the nucleus where there is the greatest probability to find the electron.


Quantum numbers. The characteristics of each orbital are described by the quantum numbers they are:
- Main quantum number n. The main quantum number n indicates the energy layer or level of the electron and can assume values from 1 to 7.
- Secondary quantum number l. The secondary quantum number indicates the sublevel of a given energy level. These sub-levels are indicated by the letters s, p, d, f.
- Magnetic quantum number m. The magnetic quantum number m determines the number of possible orbitals for each sublevel.
- Quantum spin number ms . the spin number indicates the direction of rotation of the electron around its axis. The spin can be clockwise or counterclockwise.
Pauli exclusion principle. Pauli's principle states that an atomic orbital can only contain two electrons that must have opposite spin.

Filling of atomic orbitals. The energy levels are filled by the electrons from the lowest level, and each level is occupied to the maximum of its capacity, before the next higher energy level is occupied.
The order in which the different orbitals are filled is as follows:

1s, 2s,2p,3s,3p, 4s, 3d,  4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d...
The rules you follow are as follows:
-the filling of the orbitals by the electrons takes place starting from the lowest energy level, then proceeding in the increasing order of energy.
-an orbital can contain a maximum of two electrons, with opposite spin.
-if more orbitals of equal energy are available, the electrons are arranged one for each of them, with parallel spins, only later complete the filling.



Atomic number: indicates the number of protons (and therefore also electrons) present in an element.
Mass number: indicates the sum of the number of protons and neutrons present in an element.
Isotopes: are isotopes the elements that have the same atomic number but different mass number.

For example if an atom has 10 protons and 12 neutrons, its atomic number will be 10 because it corresponds to the number of protons, its mass number will be 22 because it corresponds to the sum of protons and neutrons (10+12=22). This atom also has 10 electrons because in a neutral atom the number of electrons is equal to the number of protons
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