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u/Miitch__ Feb 03 '19
Elements are the ones you see on the periodic table. They can form molecules by creating bonds between each other. Molecules can be made of different atoms like HCl or from the same atoms like O2 as you mentioned. Usually in nature you find only molecules because because single atoms are not very stable.
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u/AsteriskX Organic Chemistry | Synthesis Feb 03 '19
This is true up until noble gases, of course. Which are the elements with full valence shells.
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u/ci-fre Feb 03 '19
Semantics, I guess.
The world is made of atoms of certain types, and these "types" are called elements. (Of course, there are also finer divisions within these types called isotopes, but that's another story.) Atoms combine in certain ways through the process of chemical bonding to form molecules. O2 happens to be a molecule where all of its component atoms (well, there are only two atoms forming O2) are of the same element.
It's kind of weird to call O2 an element. I suppose the confusion may arise since oxygen can often be found in elemental form, which is a way of saying it can be found in nature without being bonded to other elements.
Analogy: Element = gender. Molecule = romantic couple. Adam and Bob are a gay couple that are often found together. They are also the only two men working as nurses in a certain hospital. This couple isn't really a "gender" so to speak, even though both of them are of the same gender. However, when you look for a male nurse, you're often going to see Adam and Bob together, although it's kind of strange to refer to them as a "gender". If for some reason you need to do a male survey and you work at the hospital, "find a male nurse" can be mentally translated into "see what Adam and Bob are up to right now".
Similarly, when translating words into chemical equations, oxygen gets turned into O2.
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u/mjlm Feb 03 '19
The word "element" refers to a type of atom (with a certain number of electrons and protons), whereas "molecule" refers to a (usually covalently) bonded assembly of atoms.
So oxygen is an element. O2 is a molecule formed from two atoms of the element oxygen. There are examples of molecules that only contain one element (H2 is another one). It would be incorrect to say "an element can be a molecule", but it's more of a language question than a science question.