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Bohr model in Chemistry can be defined as, “The theory of atomic structure in which the hydrogen atom called the bohr atom is assumed to consist of a proton as nucleus with a single electron moving in distinct circular orbits around it”.

In the above definition, each orbit corresponds to specific quantised energy states. The theory was proposed by Niels Bohr and Ernest Rutherford in 1913 and it was later extended to explain the other atoms.

Significance of Bohr’s model
The theory is also used to predict reactivity in elements. Reactivity is how likely an element is to  form a compound with another element. This is determined by the valence electrons which are the electrons on the last energy level. Electrons in outer levels have higher average energy. This makes them play an important role in how atoms reacts chemically. The pull by the nucleus on the outer level is weaker and can be broken easily. The reactivity of an element is dependent on its electronic configuration.

Therefore, the elements with complete valence shells are most chemically non-reactive and the elements with one electron in the valence shell or just missing one electron in the valence shell are most reactive.

How to draw a bohr model

Follow the steps given below to draw the bohr model.

  1. Draw the nucleus
  2. Write down the number of protons and neutrons in the nucleus
  3. Now draw the first energy level or shell around the nucleus. The electron energy levels are maned K, L, M, N, O, P and Q or 1,2,3,4,5,6 and 7 moving from the innermost to the outermost level.

The rules for drawing the energy levels are as follows:

  • The first energy level that is the shell closest to the nucleus can hold 2 electrons
  • The second energy level or shell can hold a maximum of 8 electrons
  • The third energy level or shell can hold a maximum of 18 electrons
  • The fourth energy level or shell can hold upto a maximum of 32 electrons
  • The general formula is that the  nth energy can hold 2n2 number of electrons.

The electrons circle around the nucleus. Note that the electrons have to be drawn in pairs and you have to fill the first level before moving to the next level.

Example of Bohr diagram
According to the bohr diagram the electrons circle around the nucleus. Consider an atom with 16 protons, 16 neutrons and 16 electrons. Draw the nucleus and write down the number of protons and neutrons. The first energy level or the K shell will have 2 electrons, the next energy level or the L shell has 8 electrons and the remaining 6 electrons are in the third shell or the M shell.  

The electronic configuration is K(2), L(8) and M(6).

Bohr Diagram | Image source: socratic.org

The bohr diagram for all elements
You can draw the bohr model for any element in the same way. To make the bohr diagram of a particular element look at the periodic table. Every element in the table has an atomic number. The atomic number is the number of protons in an element. As you know, the number of protons, neutrons and the electrons is the same. You have to place the protons and the neutrons in the nucleus and place the electrons in the energy levels around the nucleus.

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