Hydrogen is the first element in the periodic table because it is the simplest atom of them all: it contains only one proton and one electron. On the other hand, pure hydrogen is a transparent gas. The largest planets, such as Jupiter, are huge balls of hydrogen mixed with other gases, such as helium and methane. On Earth, hydrogen is commonly found in water, H2O. Although it is scarce in Earth's atmosphere, it is the most common element in the Universe. Stars, such as the Sun, contain large amounts of hydrogen. At the center of a star, atoms of this element fuse, releasing heat and light. New stars form inside nebulae. These are clouds of hydrogen gas that slowly collapse in on themselves. Hydrogen gas, H2, is the lightest element of all, and much lighter than air. That is why balloons filled with hydrogen can fly higher than those filled with air. Supercooled liquid hydrogen is used as rocket fuel. Hydrogen atoms fuse together to produce a lot of energy in hydrogen bomb explosions. Pure hydrogen is also a clean energy source used in some vehicles.
Summary of properties (H)
| Atomic weight | [1.00784, 1.00811] |
| Discoverer (year) | Cavendish, Henry (1776) |
| Natural form | gas (H2) |
| Electron configuration | 1s1 |
| Melting point (ºC) | -259 |
| Boiling point (ºC) | -253 |
| Abundance in earth's crust (ppm) | 1400 |
| Isotope (abundance) | 1H (99.9885), 2H (0.0115) |
| Density g/cm3 | 0.09 |
| Van der Waals radius (pm) | 110 |
| Covalent radius (pm) | 32 |
| Electronegativity (Pauling) | 2.2 |
| Vaporization enthalpy (kJ/mol) | 0.90 |
| Enthalpy of fusion (kJ/mol) | 0.12 |
| Specific heat capacity (J/g·K) at 25ºC and 1 atm | 14.3 |
| Thermal conductivity (W/cm·K) at 25ºC and 1 atm | 0.002 (F2) |
| Oxidation state | +1, -1 |
| Electron affinity (eV) | |
| 1st Ionization potential (eV) | 13.5984 |
