How To Determine A Chemical Formula (Page 2 of 2) in pdf

IONIC RULES

Write the positive ion first, then the negative ion:

Positive ions (formed by a metal, or ammonium ion):

Ammonium (NH

) ion is a special case. It has a +1 charge.

Group I metals (Li, Na, K, etc.) have a +1 charge; so does silver (Ag).

Group II metals (Mg, Ca, Sr, etc.) have a +2 charge; so does zinc (Zn).

Aluminum has a +3 charge.

All other metals have different charges in different compounds.

Roman numerals tell you the charge on the metal:

I is +1, II is +2, III is +3, IV is +4, etc.

Examples: iron(II) is Fe

; iron(III) is Fe

; copper(II) is Cu

; etc.

Negative ions:

If the negative ion is formed from a single nonmetallic element (ends in “-ide”):

Fluoride, chloride, bromide, iodide have -1 charge

Oxide, sulfide, selenide, telluride have -2 charge

Nitride and phosphide have -3 charge

Carbide has -4 charge

Look up the chart of polyatomic ions for all other cases.

Write down both ions (positive ion always comes first).

Determine how many times each ion is present by either of these two methods:

1. “Bucket” method: Imagine a bucket of the positive ions and a bucket of the

negative ions; your task is to select the smallest combination that produces

a neutral molecule (same number of positive and negative charges). The

total number of positive charges and the total number of negative charges will

each be equal to the “least common multiple” of the positive and negative charges.

Example: If ions are +1 and -2, formula is X

Y, because this is the smallest possible

electrically neutral combination. There are a total of two positive charges and two

negative charges (two is the “least common multiple” of 1 and 2).

Example: If ions are +3 and -2, formula is X

, because this is the smallest possible

electrically neutral combination. There are a total of six positive charges and six

negative charges (six is the “least common multiple” of 3 and 2).

2. “Criss-cross” method:

The charge on the positive ion becomes a subscript on the negative ion;

the charge on the negative ion (ignoring the minus sign) becomes a subscript on

the positive ion.

Now, if the subscripts are the same, or if one is a multiple of the other, you must

“reduce to lowest terms”. Example: Ca

becomes CaO.

IMPORTANT: If a polyatomic ion occurs more than once, you must put parentheses around it!

How To Determine A Chemical Formula Page 2

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