Periodic Table


Atomic number: 1
Atomic weight: 1.00794
Symbol: H
Group number: 1 (17)
Electronic configuration: 1s1


Hydrogen is the lightest element and is by far the most abundant element in the universe, making up about about 90% of the universe by weight. Hydrogen as water (H2O) is absolutely essential to life and is present in all organic compounds. Hydrogen gas was used in lighter-than-air balloons for transport but is far too dangerous because of the fire risk (Hindenberg).

General information

Discoveror: Henry Cavendish
Date discovered: 1766
Discovered at: London, England
Meaning of name: From the Greek words "hydro" and "genes" meaning "water" and "generator"

Physical data

Standard state: gas (the lightest known gas)
Colour: colourless
Density of solid at ambient temperature/kg m-3: no data
Molar volume/cm3: 11.42

Radii /pm

Atomic: 60
Covalent (single bond):
Pauling radius for the ion [H]-: 208

Valence shell orbital radius maxima (Rmax)
orbital s p d f
radius52.9no datano datano data


Both values are quoted on the Pauling scale.

Pauling: 2.20
Allred Rochow: 2.20

Crystal Structure

structure: hcp (hexagonal close-packed)

The following CrystalMaker image represents the solid state structure. For most elements, the most stable allotrope is illustrated. Try WebElements version 2 for interactive virtual reality and CHIME images.


Temperatures (/K)

melting point: 14.01
boiling point: 20.28

Enthalpies /kJ mol-1

fusion: 0.117
vaporization: 0.91

single bond enthalpies:
H-F H-Cl H-Br H-I H-H
565 428 362 295 435.990

Ionization enthalpies /kJ mol-1

Number Enthalpy


This section gives some data for naturally occurring isotopes.
Nominal mass Accurate mass % natural abundance
1H1.007825035 (12)99.985 (1)
2H2.014101779 (24)0.015 (1)

Further Information

Copyright 1997 Mark Winter
Department of Chemistry at the University of Sheffield, Sheffield S3 7HF, England.

The current version of this document is at http://www.shef.ac.uk/~chem/web-elements-I/H.html