Ions and salts
Common salt is the best known example of the large, important group
of compounds known collectively as "salts". Sodium chloride
displays many of the properties associated with salts and, in many ways,
is a typical salt. Like all salts, sodium chloride is composed of innumerable
individual ions forming a giant ionic crystal lattice, commonly called
a salt crystal. The topic, NaCl, sodium chloride or common salt should
be read in conjunction with this topic.
Ions
Ions are formed when atoms or molecules gain or
lose electrons. Only the outermost electrons of an atom can be removed
or added to during chemical reactions, which limits the types of ions
that can be formed. Most common ions are limited to charges between
plus or minus 3.
When an atom or molecule loses electrons, it becomes positively charged.
The size of the charge is dependent on the number of electrons lost.
The sodium atom loses one electron to form a 1+ ion, while aluminium
can loses 3 electrons to form a 3+ ion.
Negative ions are formed when atoms gain electrons. The size of the
charge is again dependant on the number of electrons gained. The chlorine
atom gains one electron to form the chloride ion, Cl- ion, while oxygen
gains two electrons to form the oxide ion, O2-.
Below is a table of common ions, listed by charge.
+1
|
+2
|
+3
|
-1
|
-2
|
-3
|
|
magnesium
Mg2+
|
aluminium
Al3+
|
hydride
H-
|
oxide
O2-
|
nitride
N3-
|
sodium
Na+
|
calcium
Ca2+
|
iron(III)
Fe3+
|
fluoride
F-
|
sulfide
S2-
|
phosphide
P3-
|
potassium
K+
|
copper(II)
Cu2+
|
|
chloride
Cl-
|
sulfate
SO42-
|
phosphate
PO43-
|
copper(I)
Cu+
|
cobalt(II)
Co2+
|
|
bromide
Br-
|
sulfite
SO32-
|
|
|
|
|
iodide
I-
|
|
|
Note: when
a metal atom can form more than one positive ion, the charge on the
ion is given by Roman Numerals in brackets. The negative ions of non-metals
lose the ending of the atom's name and replace it with the letters -ide.
Polyatomic ions containing oxygen end in -ite and
-ate.
Polyatomic
ions
Molecules can also form ions, usually by gaining
electrons and forming negative ions. Electrically charged molecules
are known as polyatomic ions, literally "many atomed ions".
Although larger than most monatomic ions (those composed of only one
ion), polyatomic ions behave in the same way as other ions in forming
salts with positive ions.
Salts
Salts are formed when positive and negative ions come together
in giant ionic crystal lattices in ratios that give the overall crystal
a zero charge. In other words, the total charge of all the positive
ions equals the total charge of the negative ions. See the topic Making
salts - the rules of electrovalency for more information on how
salts form crystals and Ionic
crystals - salts for the properties of giant ionic crystals.
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