2.6—Chemical Formulas - AP Chemistry - Chemistry - 2011-09-09

2.6—Chemical Formulas

Chemical formulas are used to express the composition of molecules and ionic compounds in terms of chemical symbols.

molecular formula shows the exact number of atoms of each element in the smallest unit of a substance.

An allotrope is one of two or more distinct forms of an element. (Fore example oxygen (O2) and ozone (O3) are allotropes of oxygen).

The structural formula shows how atoms are bonded to one another in a molecule.

The empirical formula tells us which elements are present and the simplest whole-number ratio of their atoms, but not necessarily the actual number of atoms in a given molecule.

Empirical formulas are the simplest chemical formulas.

Molecular formulas are the true formulas of molecules.

In the charges on the cation and anion are numerically different, we apply the following rule to make the formula electrically neutral: The subscript of the cation is numerically equal to the charge on the anion, and the subscript of the anion is numerically equal to the charge on the cation.

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