All practice pages/Interatomic theory/Elements
Element 2, a noble gas
The second lightest element and the most aloof: its only shell is full, it forms no compounds, and it stays liquid down to absolute zero unless it is squeezed.
Period 1, group 18. The other noble gases are tinted, and every cell opens its own article.
Helium has 2 electrons, one for every proton, so the atom carries no charge. Below are the same 2 electrons written two ways.
1s²
The simple form counts electrons shell by shell. The orbital form says which subshell each one sits in: s holds 2, p holds 6, d holds 10 and f holds 14.
Helium sits in period 1 and has 1 shell. The period number counts the shells.
The outer shell holds 2 electrons. Those are the ones helium can lose, gain or share, so they are the ones that decide what it bonds with. For a main-group element the group number gives that count: group 18 means 2.
Two electrons fill its only shell, so helium does not gain, lose or share electrons. It forms no stable compounds and exists as single atoms.
The second most abundant element in the universe, made in the Big Bang and in stars. It is rare on Earth because it is light enough to escape the atmosphere; air holds about 5 parts per million. Almost all the helium we use comes from natural gas wells, where it collects after being made by the radioactive decay of uranium and thorium underground.
Pierre Janssen and Norman Lockyer saw an unexplained yellow line in the sun's spectrum in 1868, and Lockyer and Edward Frankland named the unknown element after the Greek helios, sun. William Ramsay found it on Earth in 1895, released from a uranium mineral called cleveite.
Cooling the superconducting magnets of MRI scanners takes the largest share, as liquid helium at −269 °C. It also fills party and weather balloons, shields welding and chip manufacturing from air, is breathed with oxygen by deep-sea divers, and finds leaks in vacuum equipment.
Not toxic, but breathing it straight from a tank pushes oxygen out of the lungs and can cause fainting or worse. Pressurized cylinders are dangerous if damaged.