What is the lightest element on the periodic table?

Hydrogen sits first in the periodic table because its atomic number is one, meaning a single proton in the nucleus. In the most common form there is no neutron at all, just that proton and one electron, which makes it the simplest and lightest atom possible. Everything else in the table is heavier because it has more protons and neutrons packed into the nucleus. Hydrogen is also overwhelmingly the most abundant element in the universe, making up around three quarters of ordinary matter, and it is the fuel of stars: fusion of hydrogen into helium in stellar cores is what makes them shine. On Earth free hydrogen gas is rare because it is light enough to escape the atmosphere and reactive enough to bind into compounds, above all water. It is placed awkwardly in the table, sometimes drawn above the alkali metals and sometimes set apart, because it shares properties with more than one group.

Hydrogen has heavier isotopes, deuterium and tritium, which carry one and two neutrons respectively.