Period 1: hydrogen and helium

The first shell: a single 1s subshell

The first shell is the one closest to the nucleus, and it contains only one subshell, 1s, with no p, d or f orbitals. That single orbital holds at most two electrons. Hydrogen is 1s¹ and helium is 1s², so the shell is already full at element 2, and lithium has to start the second shell and a new row.

Helium’s full shell is why it sits above neon in group 18, even though it has two outer electrons rather than eight. Hydrogen, with its lone electron, is usually placed at the top of group 1.

Hydrogen versus helium in numbers

With just two members, the trend across the row is a single step, but a large one. Helium’s two protons pull on two electrons in the same small shell, so its first ionization energy, 24.587 eV, is the highest of any element; hydrogen’s is 13.598 eV.

Both are colorless gases with tiny densities, 0.0000899 g/cm³ for hydrogen and 0.000179 g/cm³ for helium. Hydrogen boils at 20.27 K, and helium at only 4.22 K, the lowest boiling point on the table. Hydrogen has an electronegativity of 2.2 and bonds with most other elements, while helium forms no stable compounds under ordinary conditions.

Where you meet period 1

Hydrogen is the most abundant element in the universe, and stars like the Sun shine by fusing it into helium. On Earth it is locked up mostly in water and in the molecules of living things, and industry uses huge amounts of it to make ammonia and refine fuels.

Helium was detected in sunlight in 1868, 27 years before it was found on Earth, and its name comes from the Greek word for the Sun. It lifts balloons and airships, and liquid helium keeps the superconducting magnets of MRI scanners cold.