Pirate Chemistry
Forming Ionic Compounds
We now know how many reactive or valence (outer) electrons each element has. Because of
that, we can figure out what charge ions like to go to. Armed with this information, we should
be able to figure out the formulae of any ionic compound. Let’s look at one we already did:
Sodium atom: Na Chlorine atom: Cl
When these two ions combine, the sodium gives up its valence electron to the chlorine to become a +1 charge while the chlorine takes the electron to become a –1 charge:
Sodium ion: Na+1 Chloride ion: Cl-1
The +1 charge of the Na cancels out the –1 charge of the Cl and the two become a new compound:
Na+1Cl-1
The charges are still there internally but since they cancel each other out we typically don’t include them in the overall formula so we just end up with:
NaCl
We see that any ionic compound will be made of two parts; a positive ion and a negative ion.
Remember that metals tend to go to positive ions and non-metals tend to go to negative ions.
These ions have special names:
Cation: an ion with a positive charge
Anion: an ion with a negative charge
In the above example, sodium is the cation because it is positive and chlorine is the anion because it is negative. Let’s examine the chart below for more examples:
Metal
Non-metal
Cation
Anion
Combination
Formula
Na
Cl
Na+1
Cl-1
Na+1Cl-1
NaCl
K
I
K+1
I-1
K+1I-1
KI
Ca
O
Ca+2
O-2
Ca+2O-2
CaO
Mg
S
Mg+2
S-2
Mg+2S-2
MgS
Al
N
Al+3
N-3
Al+3N-3
AlN
But what happens if the charges aren’t equal?
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Pirate Chemistry
Combining Ions of Different Charges
What would happen if we tried to combine calcium with chloride? Remember that:
Calcium atom
Chlorine atom
20 electrons (unstable number)
2 in the first shell
8 in the second shell
8 in the third shell
2 in the fourth (valence) shell
Calcium will lose 2 electrons
in valence shell
17 electrons (unstable number)
2 in the first shell
8 in the second shell
7 in the third (valence) shell
Chloride will gain 1 electron
in valence shell
Chloride ion:
Calcium ion:
18 electrons (stable number)
2 in the first shell
8 in the second shell
8 in the third shell (octet rule satisfied)
18 electrons (stable number)
2 in the first shell
8 in the second shell
8 in the third shell (octet rule satisfied)
Chloride ion
2, 8, 8
-1
https://cornellchem.wikispaces.com/file/view/
Electron_shell_020_calcium.png/47421581
http://www.bbc.co.uk/schools/gcsebitesize/
science/images/diag_calcium_2.gif
http://upload.wikimedia.org/wikipedia/
commons/thumb/9/96/
Electron_shell_017_Chlorine.svg/558pxElectron_shell_017_Chlorine.svg.png
Calcium is giving up two electrons but chloride only needs one. If we tried to put them together
we would end up with an overall charge of +1:
Calcium ion: Ca+2 Chloride ion: Cl-1
Ca+2Cl-1
CaCl+1 ?????
All compounds need to be neutrally charged so this cannot be. Plus, where did that other electron go? Calcium has lost 2 but chloride has only gained 1. The solution is that Calcium has
the ability to satisfy two chloride ions! Instead of trying to force two electrons on one atom of
chlorine, calcium gives one electron to two different chlorines:
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Pirate Chemistry
http://www.green-planet-solar-energy.com/images/
eletron-shell-filling.gif
7
Ca
Calcium gives up one electron
to two different chlorides
Cl
Two different chlorines accept
one electron each
7
Cl
Calcium chloride is formed from
1 calcium atom
and
2 chlorine atoms
http://www.bbc.co.uk/schools/gcsebitesize/science/images/diag_calcium_chloride.gif
So the formula for calcium chloride is:
Calcium ion: Ca+2 Chloride ion: Cl-1
Ca+2Cl-1
Cl-1
CaCl2
Note that this 2 here in
subscript form indicates
that there are 2 chlorine
atoms in the formula
Calcium chloride is a white crystal. It has a variety of
uses as a dessicant, ice control on roads, and as an addiAll text copyright Chris Smith
2009.
All pictures
obtained from internet and are copyright of their owners but astive
in many
applications.
http://img.en.china.cn/0/0,0,145,15811,640,480,a308c609.jpg
sumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org, and
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Pirate Chemistry
All we need to do, then, is to make sure there are as many negative charges as positive charges
and vice versa. There are a variety of methods for doing this. The easiest way to do it is to follow these two simple steps:
1. Check to see if the charges are the same. If they are, simply put the cation first and
the anion second and you are done.
2. If the charges DON’T cancel each other out, perform the ―criss-cross‖ method. The
charge magnitude for the cation becomes the number of the anion atoms present.
The charge magnitude for the anion becomes the number of the cation atoms present.
Example 1: Strontium and Oxygen
Sr+2 O-2
These charges cancel each other out.
SrO
Final formula
Example 2: Strontium and Fluorine
Sr+2 F-1
These charges DON’T cancel out. Criss-cross the numbers down.
Sr F
Sr1F2
Note that we eliminate the + and — signs here.
Also, you don’t have to write the ―1‖. It’s implied
SrF2
Final formula
Example 3: Potassium and Oxygen
K+1 O-2
K
These charges DON’T cancel out. Criss-cross the numbers down.
O
K2O1
Note that we eliminate the + and — signs here.
Also, you don’t have to write the ―1‖. It’s implied.
K2 O
Final formula
Example 4: Aluminum and Oxygen
Al+3 O-2
Al2O3
Final formula
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Pirate Chemistry
Metal
Non-metal
Cation
Anion
Formula
Mg
Cl
Mg+2
Cl-1
MgCl2
Al
Br
Al+3
Br-1
AlBr3
Na
O
Na+1
O-2
Na2O
K
N
K+1
N-3
K3N
Al
O
Al+3
O-2
Al2O3
Be
P
Be+2
P-3
Be3P2
Naming Ionic Compounds
Naming ionic compounds is very simple. The name of the cation does not change. The name
of the anion changes to an –ide ending. Using the above examples:
Formula
Cation
Anion
MgCl2
Magnesium Chloride
AlBr3
Aluminum
Bromide
Na2O
Sodium
Oxide
K3N
Potassium
Nitride
Al2O3
Aluminum
Oxide
Be3P2
Beryllium Phosphide
http://www.homedepot.ca/wcsstore/HomeDepotCanada/
images/catalog/21296b7e-2623-41b9-ad0ca87bbfdf5dc3_4.jpg
http://image.hotrod.com/f/11785508/
hrdp_0901_24_z+rust_removal_guide+aluminum_oxide.jpg
Aluminum Oxide, Al2O3, is a very
hard, abrasive mineral. It is used on
devices like a Dremel rotary device.
When powdered, it is a major component of the ―chalk‖ used for billiards
cues.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org, and
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http://z.about.com/w/experts/Pool-Billiards-2283/2009/01/MasterChalk-jpg.jpg
Pirate Chemistry
Questions
1.
2.
3.
4.
5.
6.
What is a cation?
What is an anion?
Do metals tend to become cations or anions?
Do non-metals tend to become cations or anions?
What charge to all compounds have to be, positive, negative, or neutral?
Write the formula for the following ionic compounds:
A. Magnesium bromide
B. Aluminum fluoride
C. Potassium oxide
D. Lithium nitride
E. Sodium sulfide
F. Beryllium nitride
G. Aluminum sulfide
H. Calcium iodide
I. Cesium oxide
7. Write the name for the following ionic compounds:
A. NaBr
B. K2S
C. Ca3N2
D. BaO
E. BeCl2
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org, and
it will be.