Common Cations and Anions

+1   A cation (pronounced cat-eye-on) is an atom or molecule that has gained a positive charge.  This often happens in water, because of the unique nature of the water molecule which, like a magnet, has positive and negative ends (it is "polar").

WATER

 

This is what happens to table salt, NaCl, in water:

"The importance of this polar molecule is that the water molecules can reduce the attraction between the positive cation and negative anion by orientating with their negative poles towards the cation and their positive poles towards the anion, as shown above. It is this special property that allows the two (or more) charged particles of a molecule formed by electrostatic bonding to dissociate in aqueous solution."

The ocean is full of cations (and anions). There are cations in fresh water, too, especially in "hard" water.

Alkali Metals have just one electron in their outer shell, so they tend to "ionize" very quickly by shedding that once electron, thereby gaining a charge of +1.  Alkaline Earth Metals tend to shed two electrons and have a charge of =+2.  

- 1 An anion (pronounced ann-ion) is an atom or molecule that has a negative charge. Cations  are abundant in seawater and hard water. The Halogens are missing one electron in their last shell, so they very quickly absorb one electron and have a charge of -1. Most of the common anions are not single atoms, however, but groups of atoms, called molecules.  Example include sulfates and carbonates.

Note that there is a naming convention.  The "-ide" ending refers to a simple anion. The anion of chlorine is chloride.  The anion for fluorine is fluoride. Sulfur becomes sulfide.  Anions that contain oxygen often end in "-ate".   Carbonate, sulfate and phosphate are examples. This will help you remember which is which. 

It is worth memorizing the more common anions because they are so common and have such a great impact upon our lives.  The cations that are most common react with the anions that are most common, especially when the water they are in dries up.  The products of these reactions are called "salts".  The name for the compound is the name of the cation followed by the name of the anion, for example, sodium chloride.   1

The total charge of the salt should equal zero.  For example:

Three sodium cations (Na+) are needed to make a neutral salt with one phosphate anion (PO4-3)  and the formula for the salt is then Na3PO4

Ions from water molecules - a special case.  Water, H2O, can be thought of like this:

H-O-H            or            H+  and   OH-

Sometimes water has more H+ ions than OH- ions. This water is said to be "acidic".  In fact, acidity on the pH scale is a measurement of the number of H+ in water. 

On the other hand, water can have more OH- ions, or "hydroxides ions".  The water is then said to be basic, alkaline, or caustic.  The pH scale, above 7, measures the number of hydroxide ions.

Both hydrogen and hydroxide ions are very reactive.  That is why acids and bases burn you!

Common Cations (positive charge) Common Anions (negative charge)
hydrogen  H+ hydroxide  OH-
sodium  Na+ chloride Cl-
potassium  K+ sulfide  S-2
calcium  Ca+2 bicarbonate  HCO3-
magnesium  Mg+2 carbonate  CO3-2
ferrous (iron) Fe+2 sulfate  SO4-2
ferric (iron) Fe+3 phosphate PO4-3

Questions:

Name the following cations and anions:

1. Fe+2 ferrous (iron)

2. CO3-2  Carbonate

3. Fe+3 ferric (iron)

4. HCO3- Bicarbonate

5. Cl- Chloride

6. SO4-2 Sulfate

7. S-2  sulfide

8. OH- hydroxide

9. PO4-3 phosphate

10. Water that has a lot of extra hydroxide ions (OH-) is said to be what?  basic, alkaline or caustic (either one is correct).

See if you can name these salts.

1. NaCl   Sodium Chloride (table salt)

2. Ca2CO3   Calcium carbonate

3. Na2CO3  Sodium carbonate

4. NaOH   Sodium hydroxide

5. MgCO3  Magnesium carbonate

6. CaCl2   Calcium chloride

7. FeCO3   Ferrous carbonate

8. NaHCO3   Sodium bicarbonate (baking soda)

9. HCl  hydrogen chloride

10. KOH  Potassium hydroxide

11. Which two salts listed above are likely to be strongly basic because they will release hydroxide ions into water?  sodium hydroxide and potassium hydroxide.   Both are commonly called "lye".

12. Which salt listed above will release hydrogen ions into a water solution to make a strong acid?  HCl, hydrogen chloride, which is called hydrochloric acid when mixed with water, or muriatic acid by non-chemists.