Electron spin is a property of electrons that arises from their intrinsic angular momentum. It plays a crucial role in determining the magnetic properties of atoms. Each electron can be thought of as spinning around its own axis, similar to how the Earth spins on its axis. However, unlike the macroscopic world, electron spin is quantized and can only have two possible values: up spin (represented by the symbol ↑) and down spin (represented by the symbol ↓).

Spin has important consequences for the behavior of electrons in magnetic fields. When an atom possesses one or more unpaired electrons with either up or down spin, it exhibits magnetic properties. Let's take the example of oxygen, which has eight electrons. Its electron configuration is 1s² 2s² 2p⁴, meaning it has two electrons in the 1s orbital, two in the 2s orbital, and four in the 2p orbital. The 2p orbital has two unpaired electrons, each with opposite spin. This unpaired electron configuration gives oxygen its magnetic properties, making it paramagnetic. In contrast, if all the electrons are paired (like in helium), the atom is considered diamagnetic and does not exhibit a net magnetic moment.