We understand that the electronic configuration, particularly the outermost electrons, determines the chemistry of a particular element. We will not attempt, even briefly, to describe the mathematics that led to the conclusion that the electrons associated with a particular element are distributed into specific (quantized) orbitals, and that the shells have particular shapes. From our vantage point, we simply want to apply the results and make certain predictions about how specific elements combine (i.e., how many bonds an element forms with another element) and the shape of the molecule that emerges.

First, each atomic orbital can house at most two electrons. These electrons are paired in the ground state of an element.

Second, each orbital has a particular shape: s for spherical and p for "dumbbell" shaped. These shapes represent the regions of highest probability of finding the electron in space. The following is a summary of the orbitals from lowest energy to highest energy (for the elements of most interest to us).

Figure 1.

The dumbbell-shaped orbitals are typically depicted as oriented in perpendicular directions along the x, y, and z axis. Consequently, these p orbitals bear a subscript x, y, and z, and we will ultimately see why this directionality is important to us. A 2p_x and 2p_y orbital are identical; they are just oriented in different directions.

What does the picture in Figure 1 allow us to do? We can now depict the atomic configuration for any element. For example, nitrogen (atomic number 7) has the atomic configuration shown in Figure 2. The seven electons are "loaded" into atomic orbitals, first filling the 1s, then the 2s and finally, partially filling the 2p orbitals. A question arises here as to how we fill the three 2p orbitals. According to Hund's’rule, we maximize the number of parallel spins or in other words, we fill the 2p_x with one electron, the 2p_y with a second, and the 2p_z with a third. It does not matter which of the three 2p orbitals we load first, second or third; all three are identical and equal in energy.

Figure 2. Atomic configuration of nitrogen.

Finally, a shorthand method has been developed to specify atomic configuration. For example, nitrogen would be described as 1s²2s²2p_x¹2p _y¹2p _z¹ or better still, since all the 2p orbitals are equal in energy, 1s²2s²2p³.

PROBLEMS:

1. What is the configuration of fluorine (atomic number 9) and phosphorus (atomic number 15)?
2. What is the atomic configuration of manganese (atomic number 25)?. Hint: the following picutre diagrams the order in which atomic orbitals are "filled".

1. fluorine 1s²2s²2p⁵, phosphorus 1s²2s²2p⁶3s²3p³

2. manganese 1s²2s²2p⁶3s²3p⁶4s²3d⁵