In 1905, J. Thomson proposed the first model of the structure of the atom, according to which it is a positively charged ball, inside which are particles with a negative charge - electrons. The electrical neutrality of the atom was explained by the equality of the charges of the ball and all its electrons.
This theory was replaced in 1911 by the planetary model created by Rutherford: in the center, the star-core, which makes up the bulk of the entire atom, the planets electrons rotate around it. However, further the results of the experiments cast doubt on the correctness of this model. For example, it follows from Rutherford's formulas that the velocities of electrons and their radii can continuously change. In this case, continuous emission would be observed over the entire spectrum. However, the experimental results indicate the line spectra of atoms. There are also some other contradictions. Subsequently, N. Bohr proposed a quantum model of the structure of the atom. It is necessary to note the ground and excited state of the atom. This characteristic allows, in particular, to explain the valency of the element.
The excited state of an atom is an intermediate step between a state with a zero energy level and exceeding it. It is extremely unstable, therefore very fleeting - the duration is millionths of a second. An excited state of an atom occurs when additional energy is given to it. For example, its source can be influencing temperature and electromagnetic fields.
In a simplified form, the classical theory of the structure of the atom claims that negatively charged indivisible particles - electrons - rotate around the nucleus at certain distances in circular orbits. Each orbit is not a line, as it may seem, but an energy "cloud" with several electrons. Additionally, each electron has its own spin (rotates around its axis). The orbit radius of any electron depends on its energy level, therefore, in the absence of external influence, the internal structure is quite stable. Its violation — the excited state of the atom — occurs when external energy is communicated. As a result of this, in the last orbits, where the force of interaction with the nucleus is small, the pair spins of the electrons are vaporized and, as a result, they transition into unoccupied cells. In other words, in accordance with the law of conservation of energy, the transition of an electron to higher energy levels is accompanied by absorption of quanta.
Consider the excited state of an atom using the example of an arsenic atom (As). Its valency is three. Interestingly, this value is true only for the case when the element is in a free state. Since the valency is determined by the number of unpaired spins, when an atom receives external energy in the area of the last orbit, pairing is observed with the transition of the particle into a free cell. As a result, the orbit changes. Since the energy sublevels simply change places, the transition back (recombination) to the ground state of the atom is accompanied by the release of the equivalent of absorbed energy in the form of quanta. Returning to the example with arsenic: due to a change in the number of unpaired spins in an excited state, the valency of the element corresponds to five.
Schematically, all of the above is as follows: when an atom receives a portion of energy from outside, external electrons are displaced a greater distance from the nucleus (the radius of the orbits increases). However, since the proton remains in the nucleus, the total value of the internal energy of the atom becomes larger. In the absence of a continuous supply of external energy, the electron very quickly returns to its former orbit. In this case, an excess of its energy is released in the form of electromagnetic radiation.