# Helium

Figure ${\displaystyle 1}$. The structure of a Helium atom showing the nucleus and electron shell.

Helium is a chemical element with an atomic number of 2 and He for the symbol. It was the first element discovered in space (actually found inside the Sun), which is why Helium is named after the Greek sun god Helios.

## Physical Properties

Helium is an inert noble gas. Only at 2,5 million Pa of pressure, helium's melting point is -272.2 °C, so it is impossible to have solid helium at normal pressure. Helium is the second lest massive element, making Helium gas rise upwards quickly, which is why Helium is rare in the Earth's atmosphere.

State (at room temperature) Gas -273.15 °C -268.928 °C 0.0001785 g/cm3

## Atomic Properties

Despite Helium atoms having two electrons, Helium is a noble gas, placing on the right hand edge of the Periodic Table. This is because a Helium electron shell can only hold 2 electrons, making it inert. Nine isotopes are known but only two of them are stable. Helium is the smallest element with an atomic radius of less than 20 picometers (a trillionth of a meter). Helium atoms being the smaller rather than Hydrogen atoms is due to a Helium atom's nucleons pulling on its only electron shell. However, the reason why heavier elements are larger than both Hydrogen and Helium is because there are more electron shells (Figure ${\displaystyle 1}$).

Due to Helium being a noble gas, Helium has no compounds.

Atomic Number (number of Protons and Electrons) 2 4 20 picometers Stable 9 (only 2 are stable) 2

## Uses

Helium is mostly found in helium balloons, which rise into the air due to Helium having a low mass.

## Alpha Particles

Helium nuclei that are by-products of radioactive decay are called alpha particles. They are formed by radioactivity within the Earth but get trapped within rocks before escaping past the atmosphere.