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
Prime element if, the principal ideal generated by is a nonzero prime ideal. (Note that in an integral domain, the ideal is a prime ideal, but is an exception in the definition of 'prime element'.) Interest in prime elements comes from the Fundamental theorem of arithmetic, which asserts that each nonzero integer can be written in essentially only one way as 1 or −1 multiplied by a product of positive prime numbers. This led to the study of unique factorization domains, which generalize what was just illustrated in the integers. Being prime is relative to which ring an element is considered to