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How many unpaired electrons are present in phosphorous? 

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The ground state electron configuration of a phosphorous atom has three unpaired electrons. They're in the 3p sublevel. The electron configuration is:

`1s^2 2s^2 2p^6 3s^2 3p^3` , for a total of 15 electrons. According to Hund's Rule, when a sublevel isn't full each orbital will contain one electron before any contain two. In other words, electrons don't pair up until they have to.

The image below is the orbital diagram for phosphorous. It illustrates Hund's Rule. Sulfur, for example, which has 16 electrons, will have another electron in the first 3p orbital and therefore only have two unpaired electrons. This is a periodic trend. Elements in the same group have the same number of unpaired electrons.

The opposite directions of the arrows in the orbital diagram represent the Pauli Exclusion principle, which states that a maximum of two electrons can occupy one orbital and they must have opposite spin.

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