#### Niels Bohr was a Danish physicist who proposed a new model to explain the structure of the atom and its behavior.

For this decisive contribution to knowledge was awarded the Nobel Prize in Physics in 1922.

From the time of the ancient Greeks it was thought that matter was made up of extremely small and indivisible particles, that is, they could not be divided into smaller ones.

However, the first scientific approaches came with Dalton’s works towards the beginning of the 19th century. This basic model, which laid the foundations for chemical stoichiometry, was then perfected with the contributions of Thomson (1897) and later Rutherford (1911), who had already suggested the existence of smaller particles than the electrically charged atom. It was Bohr who understood and described the organization of these negatively charged particles, which are the electrons.

### Characteristics Of Bohr’s Atomic Model

- The electrons surround the nucleus not as a disorganized cloud, but in various circular orbits that determine different energy levels.
- Each of these orbits, which corresponds to a given energy level, is called the main quantum number and is represented by the letter “n”. (In the case of the hydrogen atom, which is the one investigated in depth Bohr, these range from 1 to 7).
- Each layer can only accommodate a number of electrons equivalent to 2n². Thus, the maximum number of electrons for the first four layers is 2, 8, 18 and 32. The elements of the periodic table located in the same column have the same number of electrons in their last layer.
- The electrons revolve in these stationary orbits without emitting energy (First postulate of Bohr).
- The only allowed orbits for an electron are those for which the angular momentum L of the electron is an integer multiple of h / 2π, with “h” being Planck’s constant (Second Bohr postulate). The final expression is: m * r * v = n * h / 2π, where m = mass of the electron; r = radius of the orbit, v = velocity of the electron; n = main quantum number and h = Planck constant (6.6256 · 10-34 J s). As n increases, the energy of the electron also increases and, on average, the electron is further away from the nucleus.
- When an electron jumps from a more internal level to a more external level there is absorption of energy, whereas when an electron goes from an external orbit to an inner one there is emission of energy. These jumps occur spontaneously and there is no passage of the electron through any intermediate orbit, so it is said that electrons can only give quantum jumps.
- In both cases, what is absorbed or emitted is electromagnetic energy in the form of photons of light (Third postulate of Bohr). The energy absorbed or released responds to the formula: E2-E1 = h * v.
- Bohr’s atomic model was based conceptually on the atomic model of Rutherford and on the first ideas related to quantization introduced some years before by Max Planck and Albert Einstein.
- The fourth hypothesis or fourth postulate assumes that the minimum value of n is 1. This value corresponds to a minimum radius of the electron orbit of 0.0529 nm, distance designated as the Bohr radius.
- Erwin Schrödinger perfected the model of Bohr, who introduced the concept of orbital, which is somewhat different from that of orbit. In the current model, each energy level is subdivided according to the orbital angular momentum that describes it and to which a letter (s, p, d y, f) is assigned. However, given its simplicity, the Bohr model is often used as a simplification to explain the structure of matter