Second and third shell atoms with the same number of outer electrons are placed in the same vertical column in the drawing at the beginning of the chapter.

Lithium and sodium have one outer electron each around a filled inner core. In lithium this core consists of two electrons in the first shell, whereas in sodium the core is made up of ten electrons in the first and second shells. Lithium has one electron outside a "helium" core, and its electronic structure can be written Li 2,1.

Sodium has one electron outside a filled "neon" core, and is represented as Na 2,8,1. Carbon has four electrons outside a helium core, and silicon has four electrons outside a neon core.

Fluorine and chlorine have seven electrons each surrounding a two-electron or ten electron core. At the end of the rows, neon has a filled eight-electron second shell and argon has a filled eight-electron third shell.The argon electrons serve as a core for the heavier fourth-shell elements.