An electron dot diagram is a method of writing the chemical symbol of an element by surrounding it with dots to indicate the number of valence electrons. Valence electrons are found in an atom's outer shell and are the ones involved in chemical reactions.
Hydrogen, for example, is the simplest atom and contains a single electron. This electron is part of the outer shell, so hydrogen is written as an H with a single dot. Helium has two electrons, both of which are in its outer shell, so it is written as He with two dots around it.
A more complicated example is halogens. These types of elements have an outer shell containing seven electrons. To write these elements using the electron dot diagram, the letter symbol for the element should be surrounded by seven dots. The atom may have more than seven electrons in total, but only the valence electrons are included in the diagram.
Only valence electrons are included because these are the electrons that can be lost or gained in a reaction. During a chemical reaction, the valence electrons are the least tightly bound to the atom. This means that they can move between atoms, allowing new elements to be formed in the process.