Period 1 element lithium and fluorine (appearing twice), or left floating above the other elements and not assigned to any group in the periodic table. Helium is almost always placed above neon (which is in the p-block) in the periodic table as a noble gas, although it is occasionally placed above beryllium due to their similar electron configuration. Hydrogen (H) is the chemical element with atomic number 1. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H. With an atomic mass of 1.00794 amu, hydrogen is the lightest element. Hydrogen is
Periodic table rather than in rows, and those columns numbered I to VIII corresponding with the element's oxidation state. He also gave detailed predictions for the properties of elements he had earlier noted were missing, but should exist. These gaps were subsequently filled as chemists discovered additional naturally occurring elements. It is often stated that the last naturally occurring element to be discovered was francium (referred to by Mendeleev as "eka-caesium") in 1939. Plutonium, produced synthetically in 1940, was identified in trace quantities as a naturally occurring element in 1971. The popular periodic table layout, also known as the common or standard