Argon is an inert gas in group VIII A of the periodic table with atomic number 18, an atomic weight of 39.948, and a density of 1.40 Mg/m 3. In the solid state it forms an fcc crystal with a melting ...

For example, the mass number of argon atoms and calcium atoms can both be 40. The symbol for an atom can be written to show its mass number at the top, and its atomic number at the bottom.

For elements in the first three periods of the periodic table (hydrogen - argon), the orbitals can simply be filled up by increasing energy energy level. A difficulty arises when we come to potassium.