As Zarkonnen said, the Atomium was built for the World's Fair of 1958. But the most important reason was that Belgium needed something to represent them at this fair. Someone soon came up with the idea of building the Atomium.
Why build the Atomium? Why build a giant structure of nine atoms, to what purpose?
At the time that the Atomium was constructed, Belgium was divided into nine provinces. Therefore the Atomium has more significance than a mere structure of nine atoms. But it represented our nine (small as they may have been) provinces.
Later a tenth province was added (as the province Brabant split in two) , but the atomium can still be found in our capital Brussels, where it once stood symbol for our nation.
Nowadays it is a restaurant, well... part of it anyway.