The most abundant element in the universe is hydrogen. Hydrogen accounts for nearly three-quarters of all matter, while helium makes up nearly one-quarter. Oxygen is the third most-abundant element. The sum of all the other elements adds up to about one percent of the total mass!
Element Abundance in the Universe
Here is the estimated abundance of elements in the Milky Way galaxy, which you can take as representative of the composition of the universe:
| Atomic Number | Element | Mass Percent |
| 1 | Hydrogen (H) | 73.9 |
| 2 | Helium (He) | 24.0 |
| 8 | Oxygen (O) | 1.0 |
| 6 | Carbon (C) | 0.5 |
| 10 | Neon (Ne) | 0.1 |
| 26 | Iron (Fe) | 0.1 |
| 7 | Nitrogen (N) | 0.1 |
| 14 | Silicon (Si) | 0.065 |
| 12 | Magnesium (Mg) | 0.058 |
| 16 | Sulfur | 0.044 |
| – | All others combined | ~0.05 |
Scientists use spectroscopic data to measure the abundance of elements in the universe. Our understanding of the composition of the universe is always changing, plus new tools change the way we measure it. But, the universe isn’t exactly the same everywhere and element abundances are estimates. Basically, references agree on the order of elements in terms of abundance, but disagree (sometimes widely) on the actual numbers. You should know hydrogen is most abundant, followed by helium, and then oxygen, carbon, neon, and iron.
Why Is Hydrogen the Most Abundant Element?
The reason hydrogen is the most abundant element in the universe goes back to the Big Bang. The Big Bang quickly led to the formation of protons, neutrons, and electrons. Because hydrogen is the simplest element, it formed most readily. Technically, even a lone proton classifies as an atom of hydrogen. A neutral atom also has an electron. Most hydrogen atoms don’t have any neutrons, although the less-common isotope deuterium has one neutron and the rarer isotope tritium has two neutrons.
How Are the Elements Formed?
Initially, the universe was richer in hydrogen than it is today. Around one quarter of the helium in the universe formed during the Big Bang, but another 3% formed from hydrogen during fusion in stars.
Oxygen forms from fusion in stars just before they go supernova. As stars age and die the percentage of oxygen in the universe rises. Carbon forms mainly in red giants. Neon, like oxygen, forms in pre-supernova stars. Nitrogen comes from stars like the Sun from the fusion process involving carbon and oxygen. Magnesium forms via fusion when massive stars explode. Silicon, iron, and sulfur come from exploding massive stars and white dwarf. Heavier elements form from merging neutron stars and fusion in dying lower mass stars. Technetium and elements heavier than uranium are mainly synthesized in accelerators and nuclear reactors. While it’s possible they might form naturally, they decay so quickly they aren’t present in detectable amounts.
Matter Versus Dark Matter
The elements are examples of ordinary or baryonic matter. Baryonic matter makes up planets, stars, interstellar clouds, and intergalactic gases. Scientists believe only about 4.6% of the universe consists of ordinary matter and energy, while 68% is dark energy and 27% is dark matter. But, we have been unable to directly observe dark matter and dark energy, so their nature isn’t well-understood or characterized.
References
- Anders, E; Ebihara, M (1982). “Solar-system abundances of the elements”. Geochimica et Cosmochimica Acta. 46 (11): 2363. doi:10.1016/0016-7037(82)90208-3
- Cameron, A.G.W. (1973). “Abundances of the elements in the solar system”. Space Science Reviews. 15 (1): 121. doi:10.1007/BF0017244
- Croswell, Ken (February 1996). Alchemy of the Heavens. Anchor. ISBN 0-385-47214-5.
- Suess, Hans; Urey, Harold (1956). “Abundances of the Elements”. Reviews of Modern Physics. 28 (1): 53. doi:10.1103/RevModPhys.28.53