The electrons in an atom are arranged in shells, also called energy levels. Electrons fill the shell closest to the nucleus first, then the next shell out, and so on. For the first 20 elements, the first shell holds a maximum of 2 electrons, and the second and third shells hold a maximum of 8 electrons each. Potassium and calcium then start a fourth shell.
The atomic number of an element is the number of protons, and a neutral atom has the same number of electrons. To find the electronic configuration, write the electrons in each shell as numbers separated by full stops, starting with the shell nearest the nucleus. Sodium has 11 electrons: 2 in the first shell, 8 in the second and 1 in the third, so its configuration is 2.8.1 and it has 3 shells. Chlorine has 17 electrons, giving 2.8.7. Calcium has 20 electrons, giving 2.8.8.2.
In a diagram, each shell is drawn as a circle around the nucleus and the electrons are shown as dots or crosses on the circles. Fill the inner circle with 2, then the next with 8, then the next with 8, and place any remaining electrons in the fourth circle.
