ElI5 How did chemists figure out the number of atoms and structure in a chemical compound?


ElI5 How did chemists figure out the number of atoms and structure in a chemical compound?

In: Chemistry

Mostly by weighting reactants and reaction products. Let’s say you burn hydrogen (H2) in oxygen (O2) two get water: 2 H2 + O2 = 2 H2O (definitely not the reaction they used, but it’s simpler). You carefully weigh all three of them (hydrogen and oxygen before the reaction, water after), and notice that you need to burn 4 grams of hydrogen with 32 grams of oxygen to get 36 grams of water. If you already know the equation for this particular reaction (the one I’ve written a couple sentences back), then it’s quite straightforward to deduce that oxygen atomic mass is 16 times that of hydrogen.

Now these are 2 different questions and will be answered thus in 2 parts.

Even before coming to the number of atoms, we first need to know WHAT atoms are in the compound. This was achieved by decomposing a compound into it’s constituent elements by extreme procedures such as – heating, burning, electrolysis etc. Once you know the constituent elements, finding the number is child’s play. You know the molar mass of the elements so find out the number of moles of elements found
eg. electrolysing 18 grams of water will give you 16 grams of oxygen and 2 grams of hydrogen. Since molar mass of oxygen atoms is 16 and hydrogen atoms is 1, we can
say there are 2 atoms of H for 1 atom of O, thus we reach H20

About the structure? I am sorry but I don’t think that’s ELI5-able. There is an entire branch of study for this known as ‘spectroscopy’. I had it as an elective in college. We had some 14-15 forms of spectroscopy in there and it was quite convoluted. It’s more technology than science. Let’s just say we use machines to observe them and draw conclusions from the findings. There is no unified spectroscopic method and which method to use depends totally on the properties of the compound – polar / non-polar , transparent / opaque , solid / liq / gas at room temperature , conductive/non-conductive etc.