Period 1
Period 1
The shortest period: only hydrogen and helium, which fill the single 1s orbital.
Element count
2
First element
Z = 1
Last element
Z = 2
Period 1 contains only two elements because the K shell admits a single s subshell (2 electrons maximum). It illustrates the fundamental contrast between a highly reactive non-metal (hydrogen) and a chemically inert noble gas (helium) — the founding observation that inspired the structure of the periodic table.
Filling subshells
1s