1.1 What is Chemistry?

http://www.ck12.org Chapter 5. Electrons in Atoms

TABLE5.5: Atomic Number: 4 Element: Beryllium

n l ml ms Orbital Name

1 0 0 +^12 1s

1 0 0 -^12 1s

2 0 0 +^12 2s

2 0 0 -^12 2s

Beryllium has a configuration of 1s^2 2s^2.

Boron Through Neon - The 2p Orbitals

Now that the 1s and 2s orbitals are filled, the next lowest energy orbitals are the three 2p orbitals. For p orbitals,l=
1, which means thatmlcan have values of -1, 0, or +1. If there is only one electron in a set of p orbitals, it does not
matter which of the possible values are used formlandms. One possible example is shown in the following table:

TABLE5.6: Atomic Number: 5 Element: Boron

n l ml ms Orbital Type

1 0 0 +^12 1s

1 0 0 -^12 1s

2 0 0 +^12 2s

2 0 0 -^12 2s

2 1 -1 +^12 2p

Boron has a configuration of 1s^2 2s^2 2p^1.

Beginning with carbon, we start to see Hund’s rule come into play. The rule states that orbitals of equal energy
are each occupied by one electron before any orbital is occupied by a second electron, and all electrons in singly
occupied orbitals must have the same spin. So the sixth electron in carbon goes into another p orbital (with a different
mlvalue), and its value formsmust match as many of the other 2p electrons as possible. A possible set of quantum
numbers that satisfies these criteria is shown below:

TABLE5.7: Atomic Number: 6 Element: Carbon

n l ml ms Orbital Type

1 0 0 +^12 1s

1 0 0 -^12 1s

2 0 0 +^12 2s

2 0 0 -^12 2s

2 1 -1 +^12 2p

2 1 0 +^12 2p

Carbon has a configuration of 1s^2 2s^2 2p^2.

Nitrogen has a third 2p electron, which should go into an orbital with the third possible value forml. Again, the
msvalues should be the same for as many 2p electrons as possible, provided it does not violate the Pauli exclusion
principle. In this case, all three can have the same spin value. Nitrogen has a configuration of 1s^2 2s^2 2p^3.