Chemists and physicists use energy level diagrams to determine which electrons are available for bonding. The diagram also indicates how the atom will interact with other atoms in chemical reactions. The energy level diagram pictorially shows the relative energy levels that electrons may occupy. The position of the energy levels on the diagram shows the relative energy of the electrons with respect to all the other energy levels possible for electrons. Each energy level represents an orbital the electron may occupy. The "s" level has only one orbital, the "p" level has three orbitals available for occupation and the "d" orbitals include five orbitals.
Draw a diagram that illustrates the filling order of the orbitals according to the Aufbau principle. The table lists the orbitals available for each of the energy levels beginning at 1. Write them spaced apart slightly. The first line would list the 1s orbital and the next orbitals are the 2s and 2p. The s orbital only has one orbital to accept electrons, while the p orbitals have three distinct orbitals that will each accept two electrons. The s orbitals fill first as they are the lowest energy orbitals and then the p orbitals fill.
Draw a diagram that shows the nucleus of the atom with the energy axis at the nucleus representing zero. As the energy rises, the first orbital placed on the diagram is for the 1s electrons. As you move up in energy, the next orbital is for the 2s electrons. Slightly higher in energy are the 2p orbitals. The next orbital placed on the energy diagram is for the 3s electrons, followed by the three 3p orbitals and then slightly higher on the energy scale are the five 3d orbitals. The 4s orbital energy level is slightly below the energy level of the 3d electrons. The electrons fill in the order 4s then 3d then 4p. This order continues through the elements. According to the Aufbau principle the order of filling and therefore the order of energy levels for electrons is as follows: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s and so on.
Place the electrons for the element of concern into the orbitals beginning with the lowest energy first. Two electrons may occupy each orbital. The s orbital holds two electrons, the p orbitals hold six electrons, the d orbitals hold 10 electrons and the f orbitals hold 14 electrons. At energy levels that include more than one orbital, follow Hund's rule, which states that each orbital fills with one electron until all the orbitals have one electron and then the next electron doubles up in the first orbital and continues in this manner until two electrons occupy each orbital. Once you fill that energy level, place the next electron into the next lowest energy orbital.