Here we’ll show you how to write a formula for a binary ionic compound in which
the metal ion has only one possible charge.
Binary Compounds
-Compounds containing only 2
elements
Remember, binary compounds are compounds containing only 2 elements
Binary Compounds
-Compounds containing only 2
elements
Binary Ionic Compounds
-Contain a metal and a non-metal
A Binary Ionic compound is made up of a metal and a non-metal
Metals
Non -Metals
On the periodic table, the elements to the left of the staircase are metals and those
to the right are non-metals.
Metals with Only One
Common Charge
Here, we’ll write formulas only for compounds of metals with a single charge. These are shaded
in green here. We’ll write formulas for compounds formed by the other metals in a later video.
Write the formula for the compound
strontium fluoride.
We’re asked to write the formula for the compound strontium fluoride.
Write the formula for the compound strontium
fluoride.
We find strontium on the periodic table. It is a metal and its ion is Sr with a
positive 2 charge.
Write the formula for the compound strontium
fluoride.
Remember, names for non-metals in binary compounds end in I-D-E, but the names of the elements
themselves don’t. So we find an element that sounds like fluoride but doesn’t end in –ide.
Write the formula for the compound strontium
fluoride.
We see it is fluorine, a non-metal with an ion charge of negative one.
Write the formula for the compound strontium
fluoride.
Because strontium is a metal and fluorine is a non-metal, this is an ionic
compound.
Write the formula for the compound strontium
fluoride.
2+
Sr
+2
Formula
–
F
SrF2
The strontium ion in the compound is Sr 2+
–1
Write the formula for the compound strontium
fluoride.
2+
Sr
+2
Formula
–
F
–
F
–1
SrF2
The ion that fluorine forms is called the fluoride ion. It has a negative 1 charge.
Write the formula for the compound strontium
fluoride.
2+
Sr
Total
Positive
Charge
–
F
+2
Formula
–1
Total
Negative
Charge
SrF2
If we have one Sr2+ ion and one F minus ion, the total positive charge is +2 and the
total negative charge is negative 1.
Write the formula for the compound strontium
fluoride.
2+
Sr
Total
Positive
Charge
–
F
+2
Formula
–1
Total
Negative
Charge
These must
balance and add
up to zero
SrF2
In order for the formula to be correct, the total positive charge and the total
negative charge must balance, or add up to zero.
Write the formula for the compound strontium
fluoride.
2+
Sr
Total
Positive
Charge
–
F
+2
Formula
And these do not add up to zero.
SrF2
–1
These DO NOT
add up to zero
Total
Negative
Charge
Write the formula for the compound strontium
fluoride.
2+
Sr
Total
Positive
Charge
+2
Formula
–
F
–
F
SrF2
So what we do here is add another F minus ion.
–1
Add another F–
Total
Negative
Charge
Write the formula for the compound strontium
fluoride.
2+
Sr
Total
Positive
Charge
+2
Formula
–
F
–
F
–2
Total
Negative
Charge
SrF2
The total negative charge is now negative 1 plus negative 1, which is negative 2.
Write the formula for the compound strontium
fluoride.
2+
Sr
Total
Positive
Charge
+2
Formula
–
F
–
F
–2
Total
Negative
Charge
SrF2
NOW these add
up to zero
The positive and negative charges now add up to zero, so they are balanced.
Write the formula for the compound strontium
fluoride.
2+
Sr
+2
–
F
–
F
–2
SrF2
Because we have one strontium ion and two fluoride ions (click), the formula is
SrF2.
Write the formula for the compound strontium
fluoride.
2+
Sr
+2
Formula:
–
F
–
F
–2
SrF2
We have now answered the question. The final formula for strontium fluoride is SrF2. Notice that no charges
are written in the final formula for a compound. If you have any charges in your final formulas, make sure you