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# Summary of session 2

- Chemical compounds are combinations of the atoms of two or more chemical elements. The empirical formula of a compound tells us the ratio in which the atoms of its elements are combined.
- Molecular substances have structures from which discrete molecules can be picked out by using interatomic distance as a criterion; non-molecular substances do not. The formula of these discrete molecules is called the molecular formula. Most molecular compounds contain just one type of molecule, which then has the same empirical formula as the compound. Most organic compounds are molecular substances.
- Molecular substances usually have lower melting and boiling temperatures than non-molecular ones. They also tend to dissolve more easily in organic solvents such as petrol.
- The valency of an atom can be described as the number of ‘hooks’ each atom has which it can use to attach to other atoms.
- In ionic compounds valency gives an indication of the charge on cations and anions (or ionic groups).

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OpenLearn - Discovering chemistry
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