Formulas of Compounds

Formulas of Compounds

I.  Formula writing for ionic compounds

A. Systematic Chemical Names

IUPAC- International Union of Pure and Applied Chemistry set the ground rules for naming

1.  Inorganic compounds- those not containing carbon

a.  Positive portion comes first.

b.  Negative portion comes last.

There are additional rules if the compound is binary, ternary or higher or takes the form of acids, bases or salts.

B Writing formulas

1.  Now we can use names and formulas of cations and anions to write formulas of compounds.

2.  In these formulas, the sum of the total cation and anion charges must be zero.

3.  We use subscripts to balance the charges.

4.  In most cases, we write the cation first and then the anion. (Positive first).

5.  Rules for writing formulas:

a.  By convention, the simplest whole number ratio of the elements is usually used for writing formulas (empirical formula).

b.  Only a single formula can be written for any compound.

C Examples:

1.  Ba+2 + F-1 à BaF2

2.  Al+3 + Br-1 à AlBr3

3.  Li+1 + SO4-2 à Li2SO4

4.  NH4+ + SO3-2 à (NH4)2SO3

5.  Sr+2 + PO4-3 à Sr3(PO4)2

The use of parenthesis in formula writing is also very important, it is used when a polyatomic ion is present in the formula. The subscript tells the reader how many molecules of the polyatomic ion are present. The rule is adhered to without fail unless there is only one ion present in which case the parenthesis are not used. Ex (NH4)2SO4 the 2 means there are 2 ammonium ions present.

Solve the following

F- / S2- / N3- / P3- / I- / Se2- / Cl-
Li+ / LiF
Ca2+
Sr2+
Cs1+
Fe3+
Cr+
Cu2+
IONS / Fluoride / Iodide / Nitride / Oxide / Sulfide / Bromide
IONS / F-
Aluminum / Al3+ / AlF3
Gallium
Potassium
Strontium
Barium
Lithium
Calcium
Magnesium
Sodium
Beryllium

Bonding and Naming

Combine the following ions to make balanced compounds. The name on the negative will end in “ide.”

Ion(+) Ion(-) Compound Name

Mg2+ Cl- MgCl2 Magnesium chloride

Li+ P3-

Be2+ N3-

K+ Se2-

Ca2+ I-

Al3+ S2-

H+ O2-