Interestingly, the Atomium is listed on a number of web sites as a sculpture, rather than a building.
Either way, in a city that seems to have taken the "Name a famous Belgian" jibe to heart by erecting statues to virtually anyone and everyone (however tenuous the connection), it is perhaps fitting that one of its most prominent landmarks is itself a memorial.
Built for Expo 58 (the 1958 Worlds Fair) it commemorates a brief time when all the world (and not just EU bureaucrats) came to Brussels. Despite the very few physical remains of that and the earlier 1935 Worlds Fair, the whole of the surrounding Hysel Park area is still haunted by the ghosts of former glories.
An extraordinary building/sculpture in a very pleasant spot, with great views from the café in the top sphere.
The Brussels tram system is in a, seemingly permanent, state of flux. In 2007, the most direct route between the central Grand Place area to Hysel Stadium (the nearest stop to the Atomium) was Tram 81*, however this has little to recommend it as the trams are small and over-crowded, and the route serves some of the rougher areas of the city.
A better bet is Tram 4*, which runs to Esplanade, a short walk across the park, using brand-new high-capacity trams. Between Van Praet and De Wand Tram 4 shares an underground track with Tram 23* which runs to Hysel Stadium and you can change at any of the intervening stations.
True tram addicts (and I'd count myself as one) might prefer to catch Tram 23* the whole way between Gare du Midi and Hysel Stadium, a fascinating mix of street running, dedicated tracks and underground (pre-metro) sections.
One and three day unlimited travel tickets are widely available from various outlets around the city.
* As far as I can make out from Sept 2011: Line 81 became Line 51, Line 4 became Line 3 and Line 23 became Line 7. Line 7, however no longer operates to Gare du Midi. Check the STIB site for current information.