How are electrons arranged around the nucleus?
Electrons are arranged into very specific rings known as orbitals. Electron orbitals, sometimes referred to as electron shells, contain a fixed maximum number of electrons (see the above image). The innermost orbital can hold a maximum of two electrons. It can have either one or two electrons depending of whether it's hydrogen (H) or helium (He).
The octet rule, or 2-8-8 rule, dictates that the orbitals can hold a maximum of 2 electrons in the 1st orbital, then a maximum of eight in the following orbitals. For our purposes, we will use this rule; however, if you proceed deeper into chemistry you will find that the electron orbitals of larger elements get considerably more complicated!
What is the significance of these electron shells? In the next two lessons we will learn how the arrangement of electrons in the outermost orbitals plays a key role in chemical bonding and the formation of molecules. In this lesson, we simply want to memorize the basic rules for the placement of electrons in the various orbitals. (See the multimedia enhancement for a visual interactive slide show on how electrons are arranged in atoms.)