Essentially, the way the periodic table is arranged embodies an order by which you "add" electrons to get from one element to the next. Electrons increase in elements as protons do, which is from left to right and from top to bottom on the periodic table. Therefore, the element with the fewest electrons would be in the top left-hand corner of the table and the element with the most electrons would be in the bottom right hand corner. We categorize electrons according to what orbital level in which they reside. The four orbitals are s, p, d, and f. The diagonal rule is a guideline explaining the order in which electrons fill the orbital levels (from lowest energy to highest energy). The 1s2
orbital is always filled first, and it can contain 2 electrons. Then the 2s2
level is filled, which can also hold 2 electrons. After that, electrons begin to fill the 2p6
orbital, and it continues. The diagonal rule provides a rule stating the exact order in which these orbitals are filled, and looks like this:
As you can see, the red arrows indicate the filling of orbital levels. Starting at the top, the first red arrow crosses the 1s2
orbital. If you follow these arrows down the list, you can easily determine the order that electrons fill the orbital levels. If you look at Dr. Lavelle's periodic table, it depicts the electron configurations for each element. Go through them, and see if they make sense.
In terms of orbital drawing, you will have to practice and memorize the different shapes (for example, s orbitals are spherical, whereas p orbitals are dumbbell-shaped).