Which element is first in the periodic table?

Which element is first in the periodic table? Hydrogen. Hydrogen occupies atomic number 1 because every hydrogen atom has one proton in its nucleus.

Hydrogen is the lightest element and, under ordinary conditions, is a colorless, odorless gas that usually exists as H₂, a molecule made of two hydrogen atoms. Its single electron gives it a simple structure, but its chemistry does not fit perfectly into just one family. It is often shown above the alkali metals because it has one outer electron, while its nonmetallic behavior also makes comparisons with the halogens useful.

The first periodic tables were arranged largely by atomic mass and chemical patterns. Modern tables are ordered by atomic number, a sequence based on proton count. That rule makes hydrogen the unambiguous first element, followed by helium at 2 and lithium at 3.

Hydrogen is the most abundant element in the universe and powers stars through nuclear fusion. On Earth, it is found mainly bonded to other elements, especially in water and hydrocarbons, rather than as a large reservoir of free H₂ gas.