Elements and Compounds

What is the difference between a formula unit and a molecular formula?

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January 29, 2009 6:17AM

Molecular formulas refer to covalently bonded substances (molecules). The molecular formula shows the exact number of atoms of each element present in the smallest unit of the substance. For example, benzene is a molecule composed of six carbon and six hydrogen atoms and has a formula C6H6.

A formula unit refers to ionic compounds and network solids (both are crystals). Because in ionic crystals each ion is electrostatically bonded to every oppositely charged nearest neighbor, the ions in the whole crystal can be thought of as part of one single bonded entity. It is the same with network solids except that the bond is covalent. So rather than using a formula that shows all of the elements present in any one particular crystal, the formula unit shows the elements present in the smallest whole number ratio. In a common example, sodium chloride, the formula unit is NaCl since sodium and chloride are present in the crystal in a 1:1 ratio. The NaCl crystal itself, however, would have >>quintillians each of Na and Cl if it was big enough to see with the unaided eye. It is important to not confuse formula units with emprical formulas. Emprical formulas are simply the lowest whole number ratio of elements in a compound. Formula units are always empirical, whereas molecular formulas are usually not. For example, the empirical formula of benzene, C6H6, is CH, since there is a 1 to 1 ratio of C to H. Note that several molecules can have the same empirical formula. For example acetylene, with molecular formula C2H2, also has the same empirical formula as benzene, even though the two molecules are very different! Water, H2O, is an example of a molecular formula that is also empirical.

A formula unit is the symbolic representation of an ionic compound or network solid, and a molecular formula is the symbolic representation of a molecular compound.

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