Gadolinium is an element, number 64 on the periodic table.
According to Spock's answer in his memory test taken in 2285, the electron configuration of gadolinium was 5s22s22p63d104s24p64d104f75s25p64d16s2. (Star Trek IV: The Voyage Home)
For the majority of stars, the rare element europium was enriched relative to gadolinium and samarium. (TNG: "Manhunt")