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Why is methane's molecular formula conventionally “CH4”, while water is “H20” (among others)?

If I have a formula containing 4 hydrogen atoms and 1 carbon

Should it be written $\small\ce{CH4}$ or $\small\ce{H4C}$?

In general, how do I know the order to put elements in, in a chemical formula?


1 Answer 1


There's something called Hill System. This is the system of writing chemical formulas. In this system the carbon atoms are first, then hydrogen atoms and then other in alphabetical order. When the formula contains no carbon or hydrogen atoms, then all the elements are sorted alphabetically.

source: http://en.wikipedia.org/wiki/Hill_system

So it should be: $\ce{CH4}$

However, the Hill system is not the only method of ordering elements in a chemical formula. We can instead choose to list the elements based on their electronegativity. ManishEarth explains this in his answer to this question.

  • $\begingroup$ Where can i see the entire hill system, I would like to memorize it :) $\endgroup$ Sep 26, 2012 at 22:35
  • $\begingroup$ Here's something you should check: en.wikipedia.org/wiki/Hill_system $\endgroup$
    – bluepawel
    Sep 26, 2012 at 22:37
  • $\begingroup$ So its alphabetical? $\endgroup$ Sep 26, 2012 at 22:42
  • $\begingroup$ like Clorine before sulpher? $\endgroup$ Sep 26, 2012 at 22:42
  • $\begingroup$ Yes, but there are some exceptions $\endgroup$
    – bluepawel
    Sep 26, 2012 at 22:43

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