A Bohr diagram shows the distribution of an atom's electrons among different energy levels, or electron shells. Each diagram also features the number of protons and neutrons in the atom's nucleus.
The Bohr model is one of two ways scientists represent the structure of an atom, the other being the much more complex quantum mechanical model. According to the Bohr model, negatively charged electrons orbit in different energy levels around the positively charged nucleus. The electrons closest to the nucleus have the least amount of energy. Energy levels increase in proportion to the distance away from the nucleus.
In its neutral state, an atom has the same number of electrons as it does protons. For example, a Bohr diagram of the element boron shows five protons and five electrons. A Bohr diagram starts with a simple circle to represent the nucleus, followed by a larger circle around the nucleus to represent the first energy level. An atom's first energy level holds up to two electrons under the Bohr model, represented by two dots. The next step is to draw a second energy level around the first. The second energy level holds up to eight electrons, so three dots represent the remaining three electrons in the case of boron.
The number of protons goes inside of the nucleus next to a plus sign, because protons have a positive charge. The number of neutrons goes below the number of protons to finish off the Bohr diagram.