The atomic mass of the element neon is 20.18 amu, or atomic mass units. This number, which appears on the periodic table underneath the chemical symbol for neon, Ne, represents the weighted average of the isotopic masses of neon's isotopes based on their abundance in nature.
Three stable isotopes of neon exist in nature: Ne-20, Ne-21 and Ne-22. Their respective isotopic masses are 19.99 amu, 20.99 amu and 21.99 amu, rounded to two decimal places. There are also 16 known radioactive isotopes of neon, all with short half-lives of 1 minute or less except for Ne-24, which has a half-life of 3.38 minutes.
Neon has an atomic number of 10, meaning that one atom of neon contains 10 protons and 10 electrons. Each neon atom also contains 10 neutrons. It has a melting point of -248.6 degrees Celsius and a boiling point of -246.1 degrees Celsius. At 293 degrees Kelvin, neon has a density of 0.90 grams per cubic centimeter.
Neon is one of the six noble gases listed under group 18 of the periodic table. The other five noble gases are helium, argon, krypton, xenon and radon. These elements do not typically form compounds with other elements because their atoms are already stable, possessing full outer electron shells.