Hydrogen at a glance
- Atomic number
- 1
- Symbol
- H
- Atomic mass
- 1.008 u
- Category
- Reactive nonmetal
- Group / period
- 1 / 1
- Electron configuration
- 1s 1
- State at 25 °C
- Gas
- Melting point
- -259.1 °C
- Boiling point
- -252.9 °C
- Density
- 0.0899 g/L
- Electronegativity
- 2.2
- 1st ionization energy
- 1312 kJ/mol
- Oxidation states
- +1, −1
- Discovered
- 1766
Atomic structure
A neutral hydrogen atom has 1 proton and 1 electron, and its most common isotope carries about 0 neutrons. One atom weighs about 1.67 × 10⁻²⁷ kg, so a mole of hydrogen (6.022 × 10²³ atoms) weighs 1.008 g. The electrons fill 1 shell (K 1), which gives the configuration 1s 1. With 1 electron in its outer shell, it tends to lose electrons and form positive ions. It has 2 stable isotopes: H-1 and H-2. No other element has a stable isotope with mass number 1, so H-1 has no stable isobars; its neighbours on that line decay into it or away from it.
Physical properties
At room temperature (25 °C) hydrogen is a gas. It melts at -259.1 °C and boils at -252.9 °C, so it stays liquid across a 6.27-degree range. To turn liquid it has to be cooled below -252.9 °C, colder than liquid hydrogen (−253 °C). Only 1 element has a lower melting point. As a gas it weighs 0.0899 g per litre, 13 times lighter than air. Its atoms have a radius of about 25 pm (Helium next door is 31 pm). In the solid, its atoms pack into a hexagonal close-packed lattice.
Chemical behaviour
Its electronegativity is 2.2 on the Pauling scale. Pulling off its outermost electron takes 1312 kJ/mol. A bond between hydrogen and chlorine would be polar covalent, with an electronegativity difference of 0.96. It releases 73 kJ/mol when it gains an extra electron. Its common oxidation states are +1, −1, so in compounds you will usually meet it as H⁺ and H⁻. Group 1 also holds Lithium, Sodium, Potassium, Rubidium, Caesium and Francium, which share its outer-shell pattern. Its Lewis symbol is H surrounded by 1 dot. Handle with care: as a pure element it is flammable or reacts with water.
Where it comes from
It makes up 1400 mg/kg of Earth's crust, the 10th most abundant element there. In the human body it accounts for about 9.5 % of body mass, making it one of the main building blocks of life. It was first identified in 1766, making it the 19th element to be discovered (counting the 11 known since antiquity together). When excited it glows at its own set of colours; its brightest visible lines are at 656.3 nm, 486.1 nm and 434.0 nm.