Hydrogen (H)
Element 1 · Reactive nonmetal · Gas at 25 °C · Group 1, Period 1
Hydrogen is the most abundant element in the universe — the fuel of stars — and one of the two elements in every molecule of water.
On the periodic table, hydrogen sits in group 1 — the alkali metal group — in period 1. It is one of 7 elements classified as a reactive nonmetal, and at room temperature (25 °C) it is a gas — one of 11 elements that are gases at room temperature.
The reactive nonmetals are a diverse family that includes hydrogen, carbon, nitrogen, oxygen, phosphorus, sulfur, and selenium. They gain or share electrons in chemical reactions, forming the covalent bonds that hold molecules together. Most of the atoms in living things — including virtually all of you — belong to this category.
Properties of hydrogen
| Atomic number (Z) | 1 |
|---|---|
| Symbol | H |
| Atomic mass | 1.008 |
| Category | Reactive nonmetal |
| Group | 1 |
| Period | 1 |
| Block | s-block |
| Metal / nonmetal | Nonmetal |
| State at 25 °C | Gas |
| Electron configuration | 1s1 |
| Electronegativity (Pauling) | 2.2 |
| Melting point | -259.14 °C (14.01 K / -434.45 °F) |
| Boiling point | -252.87 °C (20.28 K / -423.17 °F) |
| Density | 0.00009 g/cm³ |
Position in the periodic table
Hydrogen shares group 1 — the alkali metal group with Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs), Francium (Fr). Elements in the same group have the same number of outer electrons, so they tend to behave alike chemically — comparing them is one of the fastest ways to see periodic trends in action.
Physical properties
Hydrogen melts at -259.14 °C (14.01 K / -434.45 °F) and boils at -252.87 °C (20.28 K / -423.17 °F), giving it a liquid range of about 6.3 °C at standard pressure.
As a gas at standard conditions, its density is 0.00009 g/cm³ — measured at STP like the other gaseous elements here.
See also: the interactive periodic table, how to read the periodic table, periodic trends.