Hydrogen vs Helium
Hydrogen is a non-metal (diatomic non-metal); Helium is a non-metal (noble gas).
The differences in plain English
- They share period 1, the same row, so their outer electrons are in the same shell but they differ in how many there are: Hydrogen has 1, Helium has 2.
- Both are gass at room temperature.
- Helium is denser, about 2.0 times more than Hydrogen (0.1786 against 0.08988 g/L).
- Hydrogen melts at -259 °C, Helium at -272 °C.
- An atom of Helium is about 4.0 times heavier than an atom of Hydrogen (4.00 u against 1.01 u).
Side by side
| Hydrogen | Helium | |
|---|---|---|
| Atomic number | 1 | 2 |
| Symbol | H | He |
| Family | Diatomic non-metal | Noble gas |
| Group / period | 1 / 1 | 18 / 1 |
| State at room temperature | Gas | Gas |
| Atomic mass | 1.008 u | 4.003 u |
| Density | 0.08988 g/L | 0.1786 g/L |
| Melting point | -259 °C | -272 °C |
| Boiling point | -253 °C | -269 °C |
| Electronegativity | 2.2 | – |
| First ionisation energy | 1312 kJ/mol | 2372.3 kJ/mol |
| Electron configuration | 1s1 | 1s2 |
| Discovered by | Henry Cavendish | Pierre Janssen |
Hydrogen
Hydrogen is atomic number 1 for a simple reason: its atom has a single proton and a single electron, and nothing else. It is a colourless, odourless gas at room temperature, and it is so light that any hydrogen released into the open air drifts upward and eventually leaks away into space.
- Around three quarters of the ordinary matter in the universe, by mass, is hydrogen.
- Hydrogen has three natural isotopes. Ordinary hydrogen has no neutron, deuterium has one, and tritium has two and is radioactive.
Helium
Helium is the second lightest element and the least reactive of them all. It is a colourless gas with no smell or taste, and it refuses to form stable compounds with anything under normal conditions. Every helium atom has two protons and two electrons, and those two electrons completely fill its only shell. That full shell is the reason helium is so aloof.
- Helium was identified in sunlight in 1868, decades before it was found on Earth.
- Liquid helium cooled below about minus 271 degrees Celsius becomes a superfluid with zero viscosity that can climb the walls of an open container.
Colour the bubbles by density or melting point and the difference jumps out.
More comparisons
Data from Periodic-Table-JSON (CC BY-SA); temperatures rounded to the nearest degree. Written by Anthony.