Naming Compounds And Determining Formulas Page 2
ADVERTISEMENT
1
2
3Formula from name (a little more difficult and what you almost always have to do)
This is a matter of reversing the above process, but some memorization is required. For instance, you must
2+
2+
remember that Fe
is ferrous (ferrum is Latin for iron, hence the symbol) or iron(II), but Cu
is cupric
(cuprum is Latin for copper) or copper(II). The common names such as ferric are seldom used. You will be
much more likely to see Fe(III) and hear “iron-three.”
For ionic compounds of representative elements (the “A” groups) the charge on the ions is determined by
the family in the periodic table.
+
+
+
All group 1A metals (alkali metals) form cations with a +1 charge, Li
, Na
, K
, etc.
2+
2+
2+
All group 2A metals (alkaline earth metals) form cations with a +2 charge, Be
, Mg
, Ba
, etc.
3+
3+
Group 3A metals, when they form ions, form cations with a +3 charge, Al
, Ga
.
Group 4A is full of enigmas, but +4 and –4 are in play. Fortunately, group 4A elements don’t usually form
ionic compounds.
Group 5A is variable, but, as anions, the ionic charge is –3.
Group 6A, as anions, -2.
Group 7A (the halogens), as anions, -1.
Forget about group 8A, the noble gases. They aren’t important, chemically speaking.
In short, a cation’s charge = (group number). An anion’s charge = 8 – (group number).
+
-
So, sodium chloride is composed of Na
(1A) and Cl
(7A). Since the total of the ionic charges must be zero
in a neutral compound, the ratio must be 1:1. Formula: NaCl.
2+
3-
Now a tougher one: magnesium nitride is composed of Mg
(2A) and N
(5A). There must be 3
magnesium (+2) ions for every two nitride (-3) ions in order to have a net charge of zero.
3x(+2) + 2x(-3) = 0
Hence, the ration of 3:2 and the formula is Mg
N
.
3
2
This works for all binary ionic compounds composed of representative elements, but when the cation is a
transition metal, you are told the positive charge, so you don’t even have to refer to the periodic table.
3+
2-
Iron (III) oxide is composed of Fe
(given) and O
(6A). There must be 2 iron (+3) ions for every 3 oxide
(-2) ions. The ratio is 2:3. The formula is Fe
O
.
2
3
For binary molecular formulas, the same rules can be applied if the Latin prefixes aren’t used, but the
prefixes are usually given. If no prefix is given, you may assume it’s one.
Boron trifluoride is BF
. Obvious.
3
. Even if you didn’t know this one, figuring it out would be trivial.
Carbon dioxide is CO
2
Dinitrogen pentoxide is N
O
.
2
5
Silicon dioxide is, well, you must get the point by now.
Some common names, such as nitrous oxide (laughing gas), will catch you unawares, but the formula is
O, actually makes sense. It’s the oxide of nitrogen with the smallest
nearly always given. The formula, N
2
oxidation number (analogous to ionic charge). That’s the pattern with all the “ous” endings.
ADVERTISEMENT
0 votes
Related Articles
Related forms
Related Categories
Parent category: Education