 In part one of this lecture we're going to move on a little further from Bohr's model to give you a basic feel for where current atomic theory sits. The Bohr model of the atom with its fixed energy levels arranges the electrons according to a rule which says that the maximum number of electrons in each level is found by 2n squared, where n is the number of the level. So for n equals 1, the lowest energy level, the maximum number of electrons is 2 times 1 squared, which is 2. And for n equals 2, the second energy level, the maximum number is 2 times 2 times 2, which is 8, and so on. This allows us to draw diagrams like these, where we show the electrons arranged in concentric levels around the nucleus. Carbon is element 6 on the periodic table, so it has 6 electrons to be arranged, 2 fit in the lowest level, and the remaining 4 go in level number 2. So in its simplest form, the electron configuration for carbon is 2, 4. Phosphorus is element 15 on the periodic table, so it has 15 electrons to be arranged, 2 in the lowest level, a maximum of 8 in the second level, and that leaves 5 to go in the third level. So the simple electron configuration for phosphorus is 2, 8, 5. We can work backwards with this and draw diagrams for atoms from their electron configurations. For instance, nitrogen has 7 electrons, and its electron configuration is 2, 5. So we can draw 2 electrons in the lowest energy level, and 5 in level 2. Chlorine is element number 17, so its configuration is 2, 8, 7, 2 in level 1, 8 in level 2, and the remaining 7 in level 3.