Hydrogen (H). Diagram showing the nuclear composition and electron configuration of an atom of hydrogen-1 (atomic number: 1), the most common isotope of the element hydrogen. Its nucleus consists of a single proton (red) and no neutrons. A single electron (white) binds to the nucleus and occupies the first of successive electron shells. The stability of an element's outer electrons determines its chemical and physical properties. Hydrogen is a non-metal in group 1, period 1, and the s-block of the periodic table. At room temperature and pressure it forms a colourless gas (H2) that liquefies at minus 253 degrees Celsius.