UNIT 1.3 ATOMIC STRUCTURE
Chemistry is the study of matter and interactions. Chemistry overlaps with many other sciences.
Below is the contents for this sub unit. This will allow you to jump to any section you desire. If you have taken Chemistry 1, this should all be review but it is always good to brush up on the basics (significant figures especially).
DEFINITE PROPORTIONS
Each compound is unique and has a definite proportion of elements. This means a compound, like water, will always be made out of the same number of hydrogen (2) and oxygen (1) atoms. This makes H2O.
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If you change the ratio to hydrogen 2 and oxygen 2, you get H2O2. This is no longer water but hydrogen peroxide. The properties change (more reactive and less stable).
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But each compound is made out of whole number ratio of elements.
EMPIRICAL VS MOLECULAR FORMULA
Molecules can have 2 different types of formulas, empirical and molecular. Let's take sugar.
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A simple sugar is C6H12O6. This is it's molecular formula.
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An empirical formula is the simplest whole number. So we can divide everything by 6 to get CH2O6. That would be the empirical formula.
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Let's see a few more examples:
Name Molecular Formula Empirical Formula
Methane CH4 CH4
Ethane C2H6 CH3
Propane C3H8 C3H8
Butane C4H10 C2H5
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Ethene C2H4 CH2
Butene C4H8 CH2
So each compound has a unique molecular formula but some can have the same empirical formula.
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WHY does this matter?
The instrument we use to help analyze samples can output the relative amounts of each sample, giving the empirical formula. So that is how we are able to help identify compounds. More on this later.