How big is Helium Atom? | Scale of the Universe

A helium atom has two electrons around a nucleus of two protons and, almost always, two neutrons. Measured the same way as the explorer's hydrogen atom, as twice the distance at which its electrons are most likely found, it is about 0.06 nanometers across, or 62 picometers. A picometer is a trillionth of a meter.

Size

An atom has no rigid shape or size. Its electrons form a cloud around the nucleus with no sharp outer boundary, so there are several ways to define an atom's size, and each gives a different number. The figure here comes from calculations of where the electrons are most likely to be.

By that measure, helium's electrons are most likely found about 31 picometers from the nucleus, so the atom is about 62 picometers across. That makes helium smaller than hydrogen, even though it has twice as many electrons. Its nucleus has two protons instead of one, and their stronger pull draws the electrons in closer.

Chemists also measure how closely two separate atoms can approach before they push each other apart. That measure, the van der Waals radius, makes a helium atom about 280 picometers across.

Size comparisons

A hydrogen atom, 106 picometers across by the same measure, is about 1.7 times as wide as a helium atom.
A helium atom is about 18,000 times as wide as its nucleus, which is about 3.4 femtometers across. A femtometer is a millionth of a nanometer.
If a helium atom were enlarged to the length of an American football field, 110 meters (120 yards) including the end zones, its nucleus would be about 6 millimeters (0.2 inches) across.
About 1.3–1.6 million helium atoms side by side would span the width of a human hair.
About 60 gamma-ray wavelengths of 1 picometer, laid end to end, would span a helium atom.

Discovered in the Sun

Helium was found in the Sun before it was found on Earth. Each element gives off light in a pattern of sharp colored lines, called its spectrum, which works like a fingerprint. In 1868, the astronomers Pierre Janssen and Joseph Norman Lockyer each saw a yellow line in the Sun's spectrum that matched no known element. Lockyer named the new element after Helios, the Greek word for the Sun.

In 1895, the chemist William Ramsay found helium on Earth, in gas released when a uranium mineral was dissolved in acid. The Swedish chemists Per Teodor Cleve and Nils Abraham Langlet, working in Uppsala, found it independently the same year.

Where helium comes from

Helium is the second most abundant element in the universe, after hydrogen, and makes up about a quarter of the Sun's mass. On Earth, it forms when uranium and thorium decay. These elements shoot out alpha particles, each of which is a helium nucleus. Once it picks up two electrons, it is a helium atom.

Some of this helium stays trapped underground, and it is extracted from natural gas, which can contain up to 7% helium. Helium that escapes into the air is light enough to leave Earth altogether and drift into space. Liquid helium boils at −269 degrees Celsius (−452 degrees Fahrenheit), the lowest boiling point of any substance, so it is used to cool the magnets in most hospital MRI scanners.