Chromium has an electron configuration of 3s1 3d5, that is to say the electron that would fill the 3s orbital is moved to the 3d orbital.
Molybdenum, 1 row down, also has a 4s1 4d5 configuration, doing the same thing.
Tungsten, 1 row down from Molybdenum, does not move the electron around and just has a 5s2 5d4 arrangement.
Why?