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It has a constant composition and a unique set of properties. Its composition is represented by its formula, which lists the symbols of the elements it contains, with each symbol followed by a subscript that tells how many atoms of that element are contained in the simplest unit of the compound.
Some compounds exist as molecules. These molecular compounds usually contain only nonmetals. Table given below lists several molecular compounds, their common names, and their melting points.
Melting point, �C
The formula of an ionic compound is neutral. The ratio of the cations and anions it contains is such that there is no excess charge. Thus, the combination of a sodium ion, Na+, with a sulfate ion, SO42-, to form sodium sulfate must be in a 2:1 ratio so that the resulting compound is neutral, Na2SO4. The combination of an aluminum ion, Al3+, with a chloride ion, Cl-, to form aluminum chloride must be in a 1:3 ratio, giving the formula AlCl3, which is neutral. The combination of an ammonium ion, NH4+, with a phosphate ion, PO43-, to form ammonium phosphate must be in a 3:1 ratio, giving the neutral formula (NH4)3PO4. Notice in this last formula that the ammonium ion is enclosed in parentheses and followed by the subscript 3. This notation means that the whole ion is taken three times. When a polyatomic ion is taken more than once in a formula, it is enclosed in parentheses and the number of ions contained in the formula is indicated by a subscript following the parentheses. Monatomic ions and polyatomic ions taken only once (for example, the sulfate ion in sodium sulfate) are not enclosed in parentheses.
Notice two things about the examples shown below: In both magnesium nitrate and aluminum sulfate, the polyatomic ion was enclosed in parentheses because it was taken more than once; the monatomic ion was not enclosed in parentheses regardless of its subscript. (2) If the charge on the two ions differs in magnitude, the number of times the cation is taken equals the magnitude of the charge on the anion. Similarly, the number of times the anion is taken equals the magnitude of the charge on the cation. For example,