Periodic table period
Period 1
Period 1 is a horizontal row of the periodic table with 2 elements, from Hydrogen (atomic number 1) to Helium (atomic number 2). Every element in the row has 1 occupied electron shell. With only the 1s orbital available, period 1 holds just two elements, and both are gases: hydrogen, the most abundant element in the universe, and inert helium.
At a glance
- Elements
- 2
- Atomic numbers
- 1 to 2
- Block
- s-block
- Period
- 1
- Phase at STP
- gas (2)
Shared properties
- Each member has 1 occupied electron shell
- Atomic radius generally decreases from left to right across the period
- Ionization energy and electronegativity generally increase across the period
- Character shifts from metallic on the left to nonmetallic on the right