COMMENT follow the Madelung rule. Although scan- differentiates it from calcium is a 3d one, even physicists and philosophers still need to step dium’s extra electron lies in its 3d orbital, though it is not the final electron to enter the in to comprehend the gestalt of the periodic experiments show that, when it is ionized, it atom as it builds up. table and its underlying physical explana- loses an electron from 4s first. This doesn’t In other words, the simple approach to tion. Experiments might shed new light, make sense in energetic terms — textbooks using the aufbau principle and the Made- too, such as the 2017 finding that helium say that 4s should have lower energy than 3d. lung rule remains valid for the periodic table can form the compound Na2He at very high Again, this problem has largely been swept viewed as a whole. It only breaks down when pressures11. The greatest icon in chemistry under the rug by researchers and educators. considering one specific atom and its occu- deserves such attention. ■ Schwarz used precise experimental pation of orbitals and ionization energies. spectral data to argue that scandium’s 3d The challenge of trying to derive the Eric Scerri is a historian and philosopher orbitals are, in fact, occupied before its 4s Madelung rule is back on. of chemistry at the University of California, orbital. Most people, other than atomic Los Angeles, California, USA. spectroscopists, had not realized this THEORIES STILL NEEDED e-mail:
[email protected] before. Chemistry educators still described This knowledge about electron orbitals does 1.