Chemistry Reference
In-Depth Information
chemical reaction. In doing this, the atoms become
more stable by getting full outer energy levels. For
example, consider what happens when sodium and
chlorine react together and combine to make sodium
chloride (Figure 3.10).
sodium + chlorine → sodium chloride
the outer electron of each sodium atom is transferred to
the outer energy level of a chlorine atom (Figure 3.11).
In this way both the atoms obtain full outer energy
levels and become 'like' the nearest noble gas. The
sodium atom has become a sodium ion and the
process is known as
ionisation
. This sodium ion has
an electron configuration like neon.
→
→
+
+
sodium atom
Na(
g
)
sodium ion
Na
+
(
g
)
electron
e
−
To lose electrons in this way is called
oxidation
.
The chlorine atom has become a chloride ion with an
electron configuration like argon.
+
+
→
→
chlorine atom
Cl(
g
)
electron
e
−
chloride ion
Cl
−
(
g
)
To gain electrons in this way is called
reduction
.
In the chemical process producing sodium chloride
both oxidation and reduction have taken place and
so this is known as a
redox reaction
. A further
discussion of oxidation and reduction in terms of
electron transfer takes place in Chapter 5 (p. 73).
Only the outer electrons are important in bonding,
so we can simplify the diagrams by missing out the
inner energy levels (Figure 3.12, p. 40).
The charges on the sodium and chloride ions are
equal but opposite. They balance each other and
the resulting formula for sodium chloride is NaCl.
These oppositely charged ions attract each other and
are pulled, or
bonded
, to one another by strong
electrostatic forces. This type of bonding is called
ionic bonding
. The alternative name,
electrovalent
bonding
, is derived from the fact that there are
electrical charges on the atoms involved in the bonding.
Figure 3.10
The properties of salt are very different from those of the
sodium and chlorine it was made from. To get your salt you would not
eat sodium or inhale chlorine!
Sodium has just one electron in its outer energy level
(
11
Na 2,8,1). Chlorine has seven electrons in its outer
energy level (
17
Cl 2,8,7). When these two elements react,
sodium atom
(Na)
chlorine atom
(Cl)
sodium ion
(Na
)
chloride ion
(Cl
)
11 protons
11
10 electrons
10
Overall charge
1
17 protons
17
18 electrons
18
Overall charge
1
