Building Up Periods 1 and

You also saw that the first electron in He has the same set as the electron in H, but the second He electron has opposing spin (exclusion principle)  

Here are two common ways to designate the orbital and its electrons  

The electron configuration. This shorthand notation consists of the principal energy level (n value), the letter designation of the sublevel (/ value), and the number of electrons ( ) in the sublevel, written as a superscript nl. The electron configuration of H is (spoken one-ess-one ) that of He is (spoken one-ess-two, not one-ess-squared ). This notation does not indicate electron spin but assumes you know that the two H electrons have paired (opposite) spins. 

The orbital diagram. An orbital diagram consists of a box (or circle, or 

The exclusion principle tells us that an orbital can hold only two electrons, so the l5 orbital in He is filled, and the n = 1 level is also filled. The n = 2 level is filled next, beginning with the Is orbital, the next lowest in energy. As we said earlier, the first two electrons in Li fill the H orbital, and the last added Li electron has quantum numbers n = 2, I = 0, irii = 0, = +5. The electron configuration 

To save space on a page, orbital diagrams are often written horizontally, as shown at left. Note that the energy of the sublevels irwreases from left to right. The orbital diagram at right shows the sublevels vertically. 

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