The nickel atom has two electron configurations, 3d8 4s2 and [Ar 3d9 4s1, which are very close in energy – the symbol refers to the argon-like core structure. There is some disagreement on which configuration has the lowest energy. Chemistry textbooks quote the electron configuration of nickel as [Ar 4s2 3d8, which can also be written 3d8 4s2. This configuration agrees with the Madelung energy ordering rule, which predicts that 4s is filled before 3d. It is supported by the experimental fact that the lowest energy state of the nickel atom is a 3d8 4s2 energy level, specifically the 3d8(3F) 4s2 3F, "J" = 4 level.