Atomic Number 2 on the Periodic Table

What Element Is Atomic Number 2?

Helium is element atomic number 2 on the periodic table.
Helium is element atomic number 2 on the periodic table. Science Picture Co, Getty Images

Helium is the element that is atomic number 2 on the periodic table. Each helium atom has 2 protons in its atomic nucleus. The atomic weight of the element is 4.0026. Helium does not readily form compounds, so it is known in its pure form as a gas.

Fast Facts: Atomic Number 2

  • Element Name: Helium
  • Element Symbol: He
  • Atomic Number: 2
  • Atomic Weight: 4.002
  • Classification: Noble Gas
  • State of Matter: Gas
  • Named For: Helios, the Greek Titan of the Sun
  • Discovered By: Pierre Janssen, Norman Lockyer (1868)

Interesting Atomic Number 2 Facts

  • The element is named for the Greek god of the sun, Helios, because it was initially observed in a previously unidentified yellow spectral line during the 1868 solar eclipse. Two scientists observed the spectral line during this eclipse: Jules Janssen (France) and Norman Lockyer (Britain). The astronomers share credit for the element discovery.
  • Direct observation of the element did not occur until 1895, when Swedish chemists Per Teodor Cleve and Nils Abraham Langlet identified helium emanations from cleveite, a type of uranium ore.
  • A typical helium atom contains 2 protons, 2 neutrons, and 2 electrons. However, atomic number 2 can exist without any electrons, forming what is called an alpha particle. An alpha particle has an electrical charge of 2+ and is emitted during alpha decay.
  • The isotope containing 2 protons and 2 neutrons is called helium-4. There are nine isotopes of helium, but only helium-3 and helium-4 are stable. In the atmosphere, there is one atom of helium-3 for every million helium-4 atoms. Unlike most elements, the isotopic composition of helium greatly depends on its source. So, the average atomic weight may not really apply to a given sample. Most of the helium-3 found today was present at the time of the Earth's formation.
  • At ordinary temperature and pressure, helium is an extremely light, colorless gas.
  • Helium is one of the noble gases or inert gases, which means it has a complete electron valence shell so it's not reactive. Unlike gas of atomic number 1 (hydrogen), helium gas exists as monatomic particles. The two gases have comparable mass (H2 and He). Single helium atoms are so small they pass between many other molecules. This is why a filled helium balloon deflates over time -- the helium escapes through tiny pores in the material.
  • Atomic number 2 is the second most abundant element in the universe, after hydrogen. However, the element is rare on Earth (5.2 ppm by volume in the atmosphere) because nonreactive helium is light enough that it can escape Earth's gravity and be lost to space. Some types of natural gas, such as that from Texas and Kansas, contain helium. The primary source of the element on Earth is from liquefaction from natural gas. The largest supplier of the gas is the United States. The source of helium is a non-renewable resource, so there may come a time when we run out of a practical source for this element.
  • Atomic number 2 is used for party balloons, but it's primary use is in the cryogenic industry for cooling superconducting magnets. The principal commercial use of helium is for MRI scanners. The element is also used as a purge gas, to grow silicon wafers and other crystals, and as a protective gas for welding. Helium is used for research into superconductivity and the behavior of matter at a temperature approaching absolute zero.
  • One distinctive property of atomic number 2 is that this element cannot be frozen into a solid form unless it is pressurized. Helium remains liquid down to absolute zero under normal pressure, forming a solid at temperatures between 1 K and 1.5 K and 2.5 MPa pressure. Solid helium has been observed to possess a crystalline structure.


  • Hammond, C. R. (2004). The Elements, in Handbook of Chemistry and Physics (81st ed.). CRC press. ISBN 978-0-8493-0485-9.
  • Hampel, Clifford A. (1968). The Encyclopedia of the Chemical Elements. New York: Van Nostrand Reinhold. pp. 256–268.
  • Meija, J.; et al. (2016). "Atomic weights of the elements 2013 (IUPAC Technical Report)". Pure and Applied Chemistry. 88 (3): 265–91.
  • Shuen-Chen Hwang, Robert D. Lein, Daniel A. Morgan (2005). "Noble Gases". Kirk Othmer Encyclopedia of Chemical Technology. Wiley. pp. 343–383. 
  • Weast, Robert (1984). CRC, Handbook of Chemistry and Physics. Boca Raton, Florida: Chemical Rubber Company Publishing. pp. E110.
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Your Citation
Helmenstine, Anne Marie, Ph.D. "Atomic Number 2 on the Periodic Table." ThoughtCo, Jul. 29, 2021, Helmenstine, Anne Marie, Ph.D. (2021, July 29). Atomic Number 2 on the Periodic Table. Retrieved from Helmenstine, Anne Marie, Ph.D. "Atomic Number 2 on the Periodic Table." ThoughtCo. (accessed April 2, 2023).