II. CHEMICAL BONDS
In their chemical interactions the atoms of different
elements tend to achieve a stable rare gas configuration
1s2 or ns2np6. Interactions that occur between atoms are
called chemical bonds.
1. Strong chemical bonds:
a) ionic bond (between metals and nonmetals);
b) covalent bond (between nonmetals);
c) metallic bond (between metallic atoms).
2. Weak chemical bonds:
a) van der Waals forces (dipole-dipole attraction)
b) hydrogen bonding
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II. CHEMICAL BONDS
1. Strong chemical bonds
a) Ionic bond = a type of chemical bond based on the
electrostatic attraction forces between ions having opposite
charges.
Ionic
bond
forms
between
electropositive
and
electronegative elements, e.g. between metal and non-metal
ions.
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II. CHEMICAL BONDS
The metal, with a few electrons on the last shell, donates one
or more electrons to get a stable electron configuration and
forms positively charged ions (cations). These electrons are
accepted by the non-metal to form a negatively charged ion
(anion) also with a stable electron configuration. The
electrostatic attraction forces between the anions and cations
causes them to come together and form a bond.
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II. CHEMICAL BONDS
Example: the formation of ionic bond between Na and Cl
Ionic bond formation in NaCl
Na: 1s22s22p63s1 Na loses one e- from its outer shell Na+: 1s22s22p6
Cl: 1s22s22p63s23p5 Cl gains one e- Cl-: 1s22s22p63s23p6
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II. CHEMICAL BONDS
When sodium and chlorine react, the outer
electron of the sodium atom is transferred to the
chlorine atom to produce sodium ion Na+ and chlorine
ion Cl- , which are held together by the electrostatic
force of their opposite charges. NaCl is an ionic
compound.
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II. CHEMICAL BONDS
NaCl formation may be illustrated showing the outer
electrons only (Lewis symbol):
In a similar way, a calcium atom may lose two electrons
to two chlorine atoms forming a calcium ion Ca2+ and two
chloride ions Cl-, that is calcium chloride CaCl2 :
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II. CHEMICAL BONDS
In sodium chloride, the ionic bonds are not only
between a pair of sodium ion Na+ an chlorine ion Cl-, but
also between all the ions. These electrostatic interactions
have as a result the formation of NaCl crystal.
We write the formula of sodium chloride as NaCl, but
this is the empirical formula. The sodium chloride crystal
contains huge and equal numbers of Na+ and Cl- ions pocket
together in a way that maximizes the electrostatic forces of
the oppositely charged ions.
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Sodium chloride crystal
8
Lithium bromide crystal
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b) Covalent bonds = is a type of chemical bond formed by
sharing pairs of electrons between atoms.
When two electronegative atoms react together,
ionic bonds are not formed because both atoms have a
tendency to gain electrons. In such cases, an stable
electronic configuration may be obtained only by sharing
electrons. First, consider how chlorine atoms Cl react to
form chlorine molecules Cl2 :
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II. CHEMICAL BONDS
Each chlorine atom shares one of its electrons with the
other atom. The electron is shared equally between both
atoms, and each atom in the molecule has in its outer shell 8
electrons – a stable electronic configuration corresponding to
that of Ar.
The sharing of a single pair of electrons results in a
single covalent bond, often represented by a dash sign, so
chlorine molecule may be written as follow:
Cl — Cl
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II. CHEMICAL BONDS
If two pairs of electrons are shared we have a double
covalent bond
Ex: the oxygen molecule O2, each oxygen atom shares two
electrons O ═ O
If three pairs of electrons are shared we have a triple
covalent bond
Ex: the nitrogen molecule N2, each nitrogen atom shares
three electrons. N ≡ N
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II. CHEMICAL BONDS
In a similar way a molecule of carbon tetrachloride CCl4
is made up of carbon and four chloride atoms. The
carbon atom shares all its four electrons and the chlorine
atoms share one electron each. The carbon atom forms 4
covalent bonds with 4 chlorine atoms. In this way, both
the carbon and all four chlorine atoms attain a stable
electronic structure.
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Coordinate bond
A molecule of ammonia NH3 is made up of one nitrogen and
three hydrogen atoms:
The nitrogen atom forms three bonds and the hydrogen
atoms one bond each. In this case, one pair of electrons is
not involved in bond formation and this is called a lone pair
of electrons.
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It is possible to have a shared electron pair in which
the pair of electrons comes just from one atom and not from
both. Such bond is called coordinate covalent bond.
Even though the ammonia molecule has a stable
configuration, it can react with hydrogen H+ by donating the
lone pair of electrons, forming the ammonium ion NH4+:
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Partial ionic character of covalent bonds
In the chlorine molecule Cl – Cl the pair of electrons
of the covalent bond is shared equally between both
chlorine atom. Because there is not a charge separation
between the chlorine atoms, Cl2 molecule is nonpolar.
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On the contrary, in HCl molecule, there is a shift of
electrons
toward
the
chlorine
atom
which
is
more
electronegative than hydrogen. Such molecule, in which a
charge separation exists is called a polar molecule or
dipole molecule.
+
e-
+
H
d
+
Cl
The polar molecule of hydrochloric acid
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The separation between the positive and negative
charges is given by the dipole moment μ. The dipole
moment is the product between the magnitude of the charges
(δ) and the distance separating them (d):
μ=δ·d
The symbol δ suggests small magnitude of charge,
less than the charge of an electron (1.602 · 10-19 C).
The unit for the dipole moment is Debye (D):
1D = 3.34 · 10-30 C m
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Dipole moment values for some molecules:
Carbon dioxide CO2
μ=0D
Carbon monoxide CO
μ = 0.112 D
Water H2O
μ = 1.85 D
Hydrochloric acid HCl
μ = 1.03 D
dist. between H and Cl atoms is d = 136 pm (136 10-12 m)
We can calculate the charge δ for HCl molecule:

1.03  3.34  10 30
 20
 

2
.
53

10
C
12
d
136  10
The charge δ for HCl molecule represents about 16%
of the electron charge (1.602 10-19 C). We can say that the
covalent H – Cl bond has about 16% ionic character.
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c) Metallic bond
The metallic bond represents the electromagnetic
attraction forces between delocalized electrons and the metal
nuclei. The metallic bond is a strong chemical bond, as
indicated by the high melting and boiling points of metals.
A metal can be regarded as a lattice of positive metal
“ions” in a “sea” of delocalised electrons.
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Metal atoms contain few electrons in their outer shells.
Metals cannot form ionic or covalent bonds.
Sodium has the electronic structure 1s22s22p63s1.
When sodium atoms come together, the electron from the 3s
atomic orbital of one sodium atom shares space with the
corresponding electron of a neighbouring atom to form a
molecular orbital. All the 3s orbitals of all the atoms overlap
to give a vast number of molecular orbitals which extend over
the whole piece of metal. There is a huge numbers of
molecular orbitals because any orbital can only hold two
electrons.
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Crystal structure of sodium
The electrons can move freely within these molecular orbitals and so each
electron becomes detached from its parent atom. The electrons are called
delocalized electrons. The “free“ electrons of the metal are responsible for
the characteristic metallic properties: ability to conduct electricity and heat,
malleability (ability to be flattened into sheets), ductility (ability to be drawn
into wires) and lustrous appearance.
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