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What is an element? The term element has many applications, but this article will concern itself with a chemical element. In chemistry, an element is pure chemical substance consisting of one type of atom that is distinguished by its atomic number. The atomic number is the number of protons in an element’s nucleus.
As of March 2010 there are 118 elements and only 94 of them are naturally occurring on Earth and have been seen in the spectra of other celestial bodies. Of these, only 80 elements have stable isotopes. These are atomic numbers 1-82 except technetium(#43) and promethium(#61). The lightest elements are hydrogen and helium. Both are believed to be products of the original nucleosynthesis from the Big Bang.
Of the 118 known elements, 24 have been derived artificially. All of the elements that are derived through artificial means are radioactive and each has a very short half life. It is possible that these elements were present at the formation of Earth, but they are extremely likely to have already decayed. Technetium was originally thought to be non-naturally occurring. The element was synthesized in 1937, but trace amounts were later found in nature. This pattern of artificially producing an element then later finding it in nature has been repeated with several other trace elements.
All elements have an atomic number and atomic mass. Atomic number is equal to the number of protons in the nucleus of that element. Helium has 2 protons in every nucleus. That will never change, but the number of neutrons can be altered to create isotopes of helium. Atomic mass is the number of nucleons(protons and neutrons) in the element’s nucleus. Using that definition, you can see that every isotope of an element will have a different atomic mass. This is usually shown with the atomic mass on the left of the element, so a helium isotope with the atomic mass of 24 would appear as 24He.
As with many terms in chemistry, the answer to ‘what is an element’ has a simple answer that can be expounded on until it becomes confusing. With a little study, will come a great understanding of the subject.
We have written many articles about elements for Universe Today. Here’s an article about the newest element, Copernicium, and here’s an article about the Element 115.
If you’d like more info on the Element, check out NASA’s Discussion on the Elemental Composition of the Universe, and here’s a link to the Periodic Table of Elements Page.
We’ve also recorded an entire episode of Astronomy Cast all about the Composition of the Atom. Listen here, Episode 164: Inside the Atom.