German chemist Andreas von Antropoff had his own ideas about the periodic table. So rather than adopt Dmitri Mendeleev's popular table in the early 1900s, he made his own (seen above) and released it in 1926. There were many dissimilarities between the tables, but a notable one was the presence of an element zero. Antropoff called it neutronium. Unlike all the other elements with varying numbers of protons inside an atom's nucleus, neutronium theoretically contained only neutrons.
More than ninety years later, neutronium's status as an element has never been seriously considered by the broader scientific community, but it is commonly used to describe the substance composed almost purely of neutrons found within immensely compact neutron stars
(Image: Public Domain)