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A p orbital is the type of atomic orbital in which an electron is located that is located in the second lowest sublevel of an electron shell or principal energy level in a given atom. In other words, the p orbitals represent the second energy sublevels within each shell or main energy level.
According to quantum theory, the electrons in an atom cannot be located anywhere, but are confined to regions of space called atomic orbitals. These orbitals are divided into different energy levels and sublevels, and the p orbitals represent one of these sublevels. In fact, they represent the second sublevel in increasing order of energy. Other types of atomic orbitals are s, d, and f orbitals.
As will be seen later, the p in p orbital comes from the word polar, and refers to the fact that the different p orbitals point towards the opposite poles of the Cartesian coordinate axes.
p orbital quantum numbers
The atomic orbitals are defined by the first three quantum numbers that are:
- n – Principal quantum number or energy level (it can assume any natural number from 1 onwards, that is, 1, 2, 3, …)
- l – Secondary quantum number or angular momentum (can only assume values from 0 to n-1)
- m l – Magnetic quantum number (can only assume integer values between –l and l)
What defines the type of orbital is the secondary quantum number, l, or angular momentum number. The following table shows the values of l for the different types of orbitals
| Angular momentum ( l) | Orbital Type Sublevel |
| 0 | orbital s |
| 1 | p-orbital |
| 2 | d-orbital |
| 3 | orbital f |
As can be seen, the p orbitals are characterized by having an angular momentum equal to 1. But, for each energy level, the angular momentum can only take values ranging from 0 to n-1 . For this reason, in the n=1 level there cannot be p orbitals, since, in this case, l can only be zero.
In a few words, from level 2 onwards we can find p orbitals, and these are differentiated by putting the value of n before them (2p, 3p, 4p, etc.)
Shape of the p orbital
Atomic orbitals describe a region in space where you are likely to find an electron revolving around the nucleus. Depending on the type of orbital in question, said region will have a particular shape. In the case of s orbitals, for example, it is a sphere, but in the case of p orbitals, these are shaped like two lobes or flattened spheres with the nucleus between the two. You can imagine a p orbital as taking two party balloons of the same size and tying them together at the bottom ends.
Although this is the common way atomic orbitals are represented, it’s important to understand that they don’t actually have as sharp boundaries as the image suggests. In fact, these figures correspond to the area where the electron is most likely to be found, but the electron can be found outside this region of space and still be in a p orbital.
A more realistic way of looking at p orbitals is as a density of points that are more concentrated near the nucleus and further apart as we move further away from it. This can be better understood by looking at the following figure.
This figure gives a more realistic image of what a p orbital is. As can be seen, the density of the points is greater in a region that has a similar shape to the one shown in the first figure, but there is no defined boundary that encloses a p orbital, nor any other atomic orbital, yes let’s go.
A key point regarding p orbitals is that the plane that separates the two lobes is what is known as a nodal plane. This means that it is the only region of space in which a p-electron can never be (the probability of finding a p-electron in the nodal plane is 0).
Orientation of p orbitals
Atomic orbitals can have different orientations depending on their magnetic quantum number , ml . Since there are 3 possible values of magnetic moment for p orbitals (-1, 0, +1), then there are 3 different p orbitals pointing in three different directions.
These orientations coincide with the directions of the three Cartesian coordinate axes, x, y, and z , so these p orbitals are called np x , np y , and np z , respectively (here n refers to the energy level). and are shown in the following figure.
Examples of p orbitals
- 4p x represents a p orbital in the fourth energy level (n=4) oriented along the x axis.
- 2p z represents a p orbital in the second energy level (n=2) oriented along the z axis.
- 3p y represents a p orbital in the third energy level (n=3) oriented along the y-axis.
References
Barradas S., Francisco (2016). Orbitals in chemistry education: an analysis through its graphic representation. Doctoral Thesis. Retrieved from https://eprints.ucm.es/id/eprint/40501/1/T38136.pdf
Quantum numbers (January 14, 2020). Recovered from https://cursoparalaunam.com/los-numeros-cuanticos
Merriam-Webster. (nd). orbital p. In Merriam-Webster.com dictionary . Retrieved from https://www.merriam-webster.com/dictionary/p%20orbital
Atomic orbitals and electron configuration. (October 29, 2020). Retrieved from https://espanol.libretexts.org/@go/page/2310
