Positive and Negative Ions

A simple understanding of positive and negative ions will help you to predict the chemical formulae of most acids, bases and salts. Some atoms e.g. Sodium atoms, Potassium atoms, Fluorine atoms can easily turn into ions by losing or gaining electrons. The most reactive elements do this very readily. Elements like Neon, Argon, Krypton, and Xenon are very unreactive: it is virtually impossible for their atoms to lose or gain electrons. This means that it is not possible to find Neon ions or Argon ions etc.

All atoms are composed of and equal number of protons (in the nucleus) and electrons (orbiting around the nucleus). Apart from Hydrogen atoms, all atoms also contain Neutrons in their nuclei. Neutrons have no electric charge. Protons have a positive electric charge:

P
+

and electrons have a negative electric charge:

e
-

This means that if an atom has equal numbers of protons and electrons it will have no overall charge. When an atom loses an electron it must have more protons than electrons so it will have an overall positive charge: these are all positive ions. You will find these elements in the first column of the periodic table, this is Group I.

H
+
Li
+
Na
+
K
+

The elements in Group II also make positive ions, but instead of losing a single electron, their atoms lose 2 electrons when they turn into ions: you will find these metals in the second column of your periodic table so they are Group II.

Be
2+
Mg
2+
Ca
2+

I like Aluminium which is much more fun because it s atoms always lose 3 electrons when they turn into ions: please don't bother to look for an element with 4+ because you will be wasting your time. Carbon and Silicon are found in Group IV, they form covalent compounds, not electrovalent ones.

Al
3+

Those are the easy ones. It is much more difficult to work out what happens to metals like Copper and Iron etc. which are transition metals. When I did my GCSE (millions of years ago) I just remembered the more important ones. You will notice that apart from Hydrogen, these positive ions are all metallic ions. You can also see that some of the transition metals have more than one valence state.

Fe
2+
Fe
3+
Cu
+
Cu
2+
Ag
+

Negative ions come from non-metals, (or from non-metals combined with a metal). You will find the non-metals on the right hand side of your periodic table.First have a look at Group VIII, these are the Noble Gases; they do not make ions, they are inert or unreactive. This means that it is possible to have pure Neon (Argon, Krypton etc.) but it is not possible to make chemicals by reacting these elements with other elements. (Nobel chemistry prize winners and "A" Level chemistry students please note that this page is intended for GCSE students.)

So now have a look at Group VII. Here we find some more easy ones. The elements in Group VII all form ions with a single negative charge. This is because they contain one more electron than protons. Just to make you life a little more interesting (or difficult) chemists change the name of the substance to indicate whether it is an atom or ion they are talking about so Fluorine atoms can be turned into Fluoride ions by gaining one electron. Fluorine --- Fluoride, Chlorine --- Chloride, Bromine --- Bromide, Iodine --- Iodide, Sulphur --- Sulphide.

F
-
Cl
-
Br
-
I
-

Sulphides are rather nice chemicals; I like them because it is easy to make Hydrogen Sulphide gas from them; this is the smell of rotten eggs!

S
2-

It is also possible to find ions with 2 or 3 negative charges. Again you will just have to memorise the important ones: here are the ions Phosphate, Sulphite, Sulphate, Nitrite, Nitrate, and Carbonate. The endings ".....ite" a ".....ate" mean that they all contain Oxygen. Where there are two possibilities the ending "....ate" means that there is more Oxygen and the "....ite" means there is less Oxygen.

PO
3-
4
SO
2-
3
SO
2-
4
NO
-
2
NO
-
3
CO
2-
3

Sometimes negative ions do contain a metal as well as a non-metal. e.g. Aluminate, Chromate, Permanganate. So why not look up in your chemistry test book to find out what charges they have. Notice that these names all end with "ate".

What do you think the difference is between a chloride and a chlorate? Easy peasy, one contains some Oxygen!

  • Here are some formulae (you should be able to work out their names):
  • H
    +
    3
    		PO
    3-
    4
  • Li
    +
    		F
    -
  • Na
    +
    		Cl
    -
  • Be
    2+
    		SO
    2-
    4
  • Li
    +
    		F
    -
  • Mg
    2+
    		Br
    -
    2
  • K
    +
    		NO
    -
    3
  • Ca
    2+
    		PO
    3-
    4
  • K
    +
    		I
    -
  • Al
    3+
    3
    		( SO
    2-
    4
    		)
    3
  • Cu
    2+
    		SO
    2-
    4
  • Ag
    +
    		NO
    -
    3
  • H
    +
    		OH
    -

The positive ion is always written first: the number of positive and negative ions should balance.

Here are some names (see if you can work out their formulae):

  • Sodium Sulphate
  • Sodium Chloride
  • Calcium Carbonate
  • Aluminium Sulphate
  • Lithium Chloride
  • Sulphuric acid

Acids are slightly different from the salts because they contain Hydrogen ions instead of metal ions. Alkalis are also different from the salts because they contain Hydroxyl ions.