The magnetic alignment of spins is determined by the parameter known as exchange energy which is given by

E_{ex}= - 2JS_{i}.S_{j} where J is the exchange integral and S_{i} and S_{j} are the spins.

If J is +ve, for minimum energy S_{i} and S_{j} should be parallel and hence ferromagnetic

If J is -ve, for minimum energy S_{i} and S_{j} should be antiparallel and hence antiferromagnetic.

The exchange energy term explains the ferro and antiferromagnetic nature of certain elements in the periodic table.

**Bethe Slater Curve**

The Bether Slater Curve tells us why certain elements are antiferromagnetic and ferromagnetic according to the exchange integral term.

In the figure shown above, the x-axis gives the ratio of interatomic distance (r_{ab}) to the radius of 3d shell of electron (r_{d}). The elements with positve J value shows ferromagnetic nature such as Fe, Co and Ni and elements with -ve J shows antiferromagnetic nature.