In the 1920s, Werner Heisenberg put forth his uncertainty principle,
which states that, at any one time, it is impossible to calculate both
the momentum and the location of an electron in an atom; it is only
possible to calculate the probability of finding an electron
within a given space. This meant that electrons, instead of traveling
in defined orbits or hard, spherical “shells,” as Bohr proposed,
travel in diffuse clouds around the nucleus.
When we say “orbital,” the image below is what we picture in our
minds.
To describe the location of electrons, we use quantum numbers.
Quantum numbers are basically used to describe certain aspects of the
locations of electrons. For example, the quantum numbers n,
l, and ml
describe the position of the electron with respect to the nucleus, the
shape of the orbital, and its special orientation, while the quantum
number ms describes the
direction of the electron’s spin within a given orbital.
Below are the four quantum numbers, showing how they are depicted and what aspects of electrons they describe.