COMPOUNDS AND BONDING
FORMATION OF COMPOUNDS
Elements react to form compounds in order to attain full outer shells and thereby achieve a stable, lower energy
state. A full outer shell can be obtained by gaining electrons, or by losing electrons to expose the next full shell
below. In so doing, atoms become positively or negatively charged ions. An ion is a charged particle.
Noble gases are unreactive because they already have full outer shells and are stable:
p=10
n=10
p=2
n=2
Helium
p=18
n=18
Neon
Argon
Loss of electrons:
Elements in Groups 1 to III obtain full outer shells by losing their valence electrons and exposing the next full
shell below. Oxidation is the loss of electrons.
Example: Na  Na+ + e-
+1
p=11
n=12
p=11
n=12
Na
Na+
The charge on the Na+ ion is
determined as follows:
Charge on 11 protons = +11
Charge on 10 electrons = -10
Total charge = +1
+ e-
Gain of electrons:
Elements in Groups VI and VII obtain a full outer shell by gaining electrons. Reduction is the gain of electrons.
Example: Cl + e-  ClIn the case of negative ions, the suffix –ide is used in their naming. Chlorine
therefore becomes chloride.
-1
p=17
n=18
Cl
+ e-
p=17
n=18
Cl-
The charge on the Cl- ion is
determined as follows:
Charge on 17 protons = +17
Charge on 18 electrons = -18
Total charge = -1
Ions:
Not every element forms ions during reactions. Out of the first twenty elements in the Periodic Table, only
seven easily form ions.
The following table shows the ions and their names:
I
0
H+
hydrogen
II
III
IV
Li+
lithium
Be2+
beryllium
None
None
None
O2oxide
Ffluoride
None
Na+
sodium
Mg2+
Al3+
aluminium
None
None
S2sulphide
Clchloride
None
None
magnesium
K+
potassium
V
VI
VII
Ca2+
calcium
Hydrogen and metals form positive ions that have the same names as their atoms while non-metals form
negative ions with their names ending in the suffix –ide.
Transition elements:
Some transition elements have only one ion while others have more than one ion.
Examples:
Ion
Name
Ion
Name
Ion
Name
Ag+
Zn2+
Mn2+
silver ion
zinc ion
manganese ion
Cu+
Cu2+
Fe2+
Fe3+
copper(I) ion
copper(II) ion
iron(II) ion
iron(III) ion
Hg+
Hg2+
mercury(I) ion
mercury(II) ion
Polyatomic (compound) ions:
A polyatomic ion is a group of atoms that carries an electric charge and reacts as an entity in a chemical
reaction.
Examples:
Ion
Name
Ion
Name
Ion
Name
SO42SO32HSO4HSO3-
sulphate
sulphite
hydrogen sulphate
hydrogen sulphite
NO3NO2OHNH4+
nitrate
nitrite
hydroxide
ammonium
CO32HCO3ClO3PO43-
carbonate
hydrogen carbonate
chlorate
phosphate
IONIC BOND BONDING
An ionic bond is a bond that is formed by electrostatic attraction between positive and negative ions in a crystal
lattice.
1.
Formation of sodium chloride (NaCl):
Refer to the Bohr diagrams above to see how Na+ and Cl- ions are formed.
Sodium loses an electron to form a sodium ion:
Na  Na+ + eChlorine gains an electron to form a chloride ion:
Cl + e-  ClThe positive sodium ion attracts the negative chloride ion to form an ionic bond:
+1
attraction
Na+ + Cl-  Na+Cl-.
-1
p=17
n=18
p=11
n=12
Na+
ClThe ions do not remain in pairs. Instead, they cluster together so that each ion is surrounded by six ions
of opposite charge in a crystal lattice.
A crystal lattice is a pattern of points that describes the arrangement of particles (atoms, molecules or
ions) in a crystalline compound.
A crystal lattice consists of millions of alternate positive and negative ions. The formula of an ionic bond
therefore indicates the ratio in which the ions in the crystal lattice are bonded.
An ionic crystal is neutral overall.
2.
Formation of magnesium chloride (MgCl2)
Magnesium loses two electrons to form a magnesium ion:
Mg  Mg+ + 2e-
+2
p=12
n=13
p=12
n=13
The charge on the Mg2+ ion is
determined as follows:
+ 2e Charge on 12 protons = +12
Charge on 10 electrons = -10
Total charge = +2
Mg2+
Mg
Two chlorine atoms each gain an electron each to form two chloride ions:
2Cl + 2e-  2Cl-
-1
p=17
n=18
Cl
+ e-
p=17
n=18
Cl-
The charge on the Cl- ion is
determined as follows:
Charge on 17 protons = +17
Charge on 18 electrons = -18
Total charge = -1
Mg2+ + 2Cl-  Mg2+Cl-2.
One positive magnesium ion attracts two negative chloride ions:
-1
attraction
-1
+2
attraction
p=17
n=18
p=12
n=13
p=17
n=18
Mg2+
Cl3.
Cl-
Formation of magnesium oxide (MgO)
Magnesium loses two electrons to form a magnesium ion:
Mg  Mg+ + 2e-.
+2
p=12
n=13
p=12
n=13
+ 2e-
The charge on the Mg2+ ion is
determined as follows:
Charge on 12 protons = +12
Charge on 10 electrons = -10
Total charge = +2
Mg2+
Mg
Oxygen gains two electrons to form the oxide ion: O + 2e -  O2-.
-2
p=8
n=8
p=8
n=8
+ 2e-
The charge on the O2- ion is
determined as follows:
Charge on 8 protons = +8
Charge on 10 electrons = -10
Total charge = -2
O2-
O
The positive magnesium ion attracts the negative oxide ion to form an ionic bond:
-2
+2
attraction
p=12
n=13
Mg2+
p=8
n=8
O2-
Mg2+ + O2-  Mg2+O2-.
When representing the bonding process, we only use Lewis diagrams:
Eg: Show the bonding of sodium and chlorine to produce sodium chloride (NaCl):
eNa
-
+
+
→
Cl
Na
Cl
Eg: Magnesium and fluorine to produce magnesium fluoride (MgF2):
eMg
+
xx
x
F
xx
x
x
+
x
F
xx
xx
x
x
+
2
Mg
→
x
xx
-
F
x
x
xx
e-
2
COVALENT BONDING
A covalent bond is a bond that is formed by the overlapping of shells and the sharing of electrons.
A molecule is a group of atoms which are held together by covalent bonds.
Covalent bonds are formed during the reaction between non-metals when no electrons are gained or lost. Nonmetals have to share electrons in order to gain a full outer shell of electrons.
Again, we use Lewis diagrams to show the bonding process:
Molecules of the same element:
The three major gases of the atmosphere (H2, O2 and N2) and the halogens (F2, Cl2, Br2 and I2) exist as diatomic
molecules (two-atom molecules).
1.
The hydrogen molecule, H2.
The hydrogen atom has one electron in the K shell and requires an additional electron to fill the shell. This
additional electron is obtained by the over-lapping of shells with another hydrogen atom and the sharing
of an electron pair so that each hydrogen atom has two electrons in their outer shell.
H
+
x H
x
H
H
2.
The oxygen molecule, O2
The oxygen molecule has six electrons in the L shell and requires an additional two electrons to fill the
shell. The two additional electrons are obtained by the overlapping of the L shells with another oxygen
atom and the sharing of an electron pair so that each oxygen atom has eight electrons in their outer shell.
x xx
x Ox x
+
O
O
x xx
x Ox
x
Molecules of covalent compounds:
In a molecular compound, atoms of different elements share electrons with each other. The compounds are
called covalent compounds because of the covalent bonds between their atoms.
1.
Water, H2O
O
+
x
H
+
H
x
x H
x H
O
2.
Ammonia, NH3
H
x
N
+
+
x H
x H
+
x H
N
x
x
H
3.
Methane, CH4
C
+
Hx
+
Hx
+
H x
+
Hx
H
H
x
x
C
x
H
x
H
H