For each of the following, predict the chemical formula of the binary ionic comp

Question

For each of the following, predict the chemical formula of the binary ionic compound they form. Show steps using the "criss-cross" method and check to make sure the charges balance.

    a.   barium and sulfur

    b.   potassium and iodine

    c.    calcium and chlorine

    d.   lithium and oxygen

    e.   zinc with a +2 charge and oxygen

    f.    silver with a +1 charge and chlorine

Example of the "criss-cross" method :

What is the chemical formula for the ionic compound formed from iron with a charge of +3 and sulfur?

1.   There are 2 elements named, iron and sulfur so we know this is a binary ionic compound.

2.   The charge for iron is given as +3 so we know that the cation is Fe+3

3.   Sulfur is in group 16 so we remember it forms an anion with a charge of -2, or S-2

4.   Using "criss-cross" we look at the superscript for iron. The superscript (charge) is +3 so we know the subscript for sulfur will be 3.

5.   Using "criss-cross" we look at the superscript for sulfur. The superscript (charge) on sulfur is -2, so we know the subscript for iron will be 2.

6.   The chemical formula for iron (III) sulfide is Fe2S3. Two irons and three sulfur atoms per formula unit. The ratio is 2 iron atoms to 3 sulfur atoms.

1.   We can check this by seeing if the charges balance:

1.   2 iron ions with a charge of +3 each: 2 x (+3) = +6

2.   3 sulfur ions with a charge of -2 each: 3 x (-2) = -6

3.   The charges balance, or add up to zero: (+6) + (-6) = 0

Explanation / Answer

(a)

Ba2+ S2- = BaS Barium sulphide

(b)

K+ I- = KI potassium iodide

(c)

Ca2+ Cl- = CaCl2 Calcium chloride

(d)

Li+ O2- = Li2O   Lithium oxide

(e)

Zn2+ O2- = ZnO   Zinc oxide

(f)

Ag+ Cl- = AgCl Silver chloride

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