Russian chemist Dmitri Mendeleev ( 1834-1907) is credited with creating the periodic table of elements.
He was smart enough to realize that his table was incomplete so he left blanks where he predicted that yet to be discovered elements would fit.
What I find amazing about Mendeleev’s table is how it is based upon the 19th century thinking that electrons were like little planets spinning around the nucleus. The image was of electron “shells” layering upon each other as the complexity of the elements increased. Each row of the periodic table represents a shell while each column represents a series of elements that are chemically similar as in #29 copper (Cu), #47 silver (Ag), and #79 gold (Au). I’ve always felt sorry for #7 nitrogen (N). She sits right next to her sister atom #8 oxygen (O) on the periodic table and she is like the plain Jane with her seven electrons. Oxygen’s extra electron makes her the more “attractive” of the two and gives her a host of chemical talents that nitrogen could never hope to duplicate such as being the essential element for life and the ability to combust. Fortunately, nitrogen has her important roles and one of them is to keep her more popular sister from damaging your HVAC system. Certain metals find oxygen irresistible. Iron will rust and copper, the same material in the cooling tubes of your HVAC, will form the familiar greenish patina that can flake off and damage your HVAC system. This is why a nitrogen purge is needed when a new HVAC system is being installed or as part of a refrigerant recharge. Nitrogen just loves to carry that oxygen along with her into the atmosphere where she outnumbers her wayward sister by a four to one ratio.