Lecture 4. Chemical Bond

Lecture 4
Chemical bond
Prepared by PhD Valentina Yuz’kova

What is a Chemical Bond?
Bonding is the force of attraction that holds atoms
together in an element (N2) or compound (CO2 or
NaCl).
Bonding Forces
 Electron – electron
repulsive forces
 Nucleus – nucleus
repulsive forces
 Electron – necleus
attractive forces
Types of Bonding
Ionic Bonding
• electrons are transferred between valence shells of atoms,
• ionic compounds are made of ions, NOT MOLECULES
• electronegativity difference > 2.0,

• always formed between metals and non-metals
• NON-DIRECTIONAL bonding via Coulomb (charge)
interaction
Ionic Bond
Li + F Li+ F -
1s22s1 1s22s22p5 1s2 1s22s22p6
[He] [Ne]
Li Li+
1s 2s 2p 1s 2s 2p
+ F + F-
1s 2s 2p 1s 2s 2p
Covalent Bonding
molecules
• Pairs of e- are shared between non-

metal atoms,
• electronegativity difference < 2.0 ,
• forms molecules or polyatomic ions
Types of Covalent Bonds
NON-Polar bonds
Between identical atoms
•Electrons shared evenly in the bond Diatomic molecules
•E-neg difference is zero
Polar bonds
•Electrons unevenly shared,
•E-neg difference greater than zero but less than 2.0
- water is a polar molecule because oxygen is more electronegative than

hydrogen, and therefore electrons are pulled closer to oxygen.
Non-Polar and Polar Covalent Bonds
Electronegativities (EN)
The ability of an atom in a molecule to attract shared electrons to itself
Linus Pauling
1901 - 1994 10
Classification of Bonds
Difference in EN Bond Type
0 Covalent
>2 Ionic
0 < and <2 Polar Covalent
Increasing difference in electronegativity
Covalent Polar Covalent Ionic
share e- partial transfer of e- transfer e-
11
Metalic Bond, A Sea of Electrons
Three types of chemical bonding
13
Comparison of Types of Bonding
Ionic Covalent Metallic
Shared electrons Cations in a sea
Formation Anion & cation
Transferred of mobile valence
electrons electrons
Metal + nonmetal Two nonmetals Metals only
Source
Relatively high Relatively low Generally high
Melting point
Dissolve best in water Dissolve best in non- Generally do not
Solubility and polar solutions polar solvents dissolve
Water solutions Solutions conduct Conduct

Conductivity conduct electricity poorly or not electricity well
electricity at all
Other Strong crystal

lattice
Weak crystal
structure
Metallic
properties; luster,
properties malleability etc.
Place these molecules in order
of increasing bond polarity
which is least and which is most?
• HCl
• CH4
• NH3 a.k.a.
“ionic character”
• N2
• HF
Covalent, Ionic, metallic bonding?
• NO2 • NH4+
• CO
• Hg • KCl • Co
• H2S • HF
• CuSO4 • NaNO3
Drawing ionic compounds using
Lewis Dot Structures
• Symbol represents the KERNEL of the
atom (nucleus and inner e-)
• dots represent valence e-
• Step 1 after checking that it is IONIC
– Determine which atom will be the +ion
– Determine which atom will be the – ion
• Step 2
– Write the symbol for the + ion first.
• NO DOTS
– Draw the e- dot diagram for the – ion
• COMPLETE outer shell
• Step 3
– Enclose both in brackets and show each charge
-
[Na]+ [ Cl ]
This is the finished Lewis Dot Structure
Drawing molecules using
Lewis Dot Structures
Always remember atoms are trying to
complete their outer shell!
The number of electrons the atoms needs is
the total number of bonds they can make.
Ex. … H? O? F? N? Cl? C?
Lewis Dot Structure: NF3
Molecular
Dot Structures: NF3
formula
count
valence e-
Atom
placement
Add in
valence e-
done
Sometimes . . .
• You only have two atoms, so there is no central
atom, but follow the same rules.
• Check & Share to make sure all the atoms are
“happy”.
Cl2 Br2 H2 O2 N2 HCl

• DOUBLE bond
– atoms that share two e- pairs (4 e-)
O O
• TRIPLE bond
– atoms that share three e- pairs (6 e-)
N N
Bond Strength
The GREATER the number of bonds

(bond order) the HIGHER the bond
strength and the SHORTER the bond.
Coordinate Covalent Bonds
• Coordinate covalent bonds occur when one atom
donates both of the electrons that are shared
between two atoms
• Coordinate covalent
bonds are also called
Dative Bonds
25
Draw the Lewis Dot Diagram for
polyatomic ions
• Count all valence e- needed for covalent
bonding
• Add or subtract other electrons based on the
charge
REMEMBER!
A positive charge means it LOST
electrons!!!!!
Ammonium NH4+
Sulfate SO42-
Resonance
Delocalized Electron-Pair Bonding
O O
O3 can be drawn in 2 ways -
O O O O
Neither structure is acurate. Reality is hybrid of the two.
B B
O O O
O O O O O O
A C A C
Resonance structures have the same relative atom placement but a

difference in the locations of bonding and nonbonding electron pairs.
is used to indicate that resonance occurs.
Polar and non-polar MOLECULES
• Sometimes the bonds within a molecule are
polar and yet the molecule is non-polar because
its shape is symmetrical. H
• Polar molecules (dipoles) H C H
Not equal on all sides: H
–Polar bond between 2 atoms makes a polar
molecule
–asymmetrical shape of molecule
+ H Cl -
VSEPR Theory
Valence Shell Electron Pair Repulsion Theory
• Electron pairs orient themselves in order to
minimize repulsive forces.
Types of e- Pairs
Bonding pairs - form bonds
Lone pairs repel
Lone pairs - nonbonding e-
more strongly
than bonding
pairs!!!
4 Shapes of molecules
1. Linear (straight line)
Space filling model Ball and stick model
2. Bent
Space filling Ball and stick

model model
3.Trigonal pyramid
Space filling model

Ball and stick model
4.Tetrahedral
Ball and stick model Space filling model

Intermolecular attractions
Attractions between molecules
1)van der Waals forces –
• Weak attractive forces between non-polar
molecules
• Non-polar molecules can exist in liquid and solid
phases because van der Waals forces keep the
molecules attracted to each other
• Exist between CO2, CH4, CCl4, CF4, diatomics and
monoatomics molecules.
• increase with molecular mass.
• increase with closer distance between molecules
2) Hydrogen “Bonding”
• Strong polar
attraction
– Like magnets
• Occurs ONLY
between H of one
molecule and N, O,
F of another
H “bond”
Strong intermolecular forces cause high b.p., m.p. and
slow evaporation (low vapor pressure) of a substance.
Hydrogen “Bonding”
Which substance has the highest
boiling point?
• HF
• NH3
• H2O
• WHY?

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Lecture 4

Prepared by PhD Valentina Yuz’kova

• electronegativity difference > 2.0,

• Pairs of e- are shared between non-

- water is a polar molecule because oxygen is more electronegative than

Increasing difference in electronegativity

Covalent Polar Covalent Ionic

share e- partial transfer of e- transfer e-

Water solutions Solutions conduct Conduct

Other Strong crystal

Cl2 Br2 H2 O2 N2 HCl

The GREATER the number of bonds

Neither structure is acurate. Reality is hybrid of the two.

Resonance structures have the same relative atom placement but a

Space filling model Ball and stick model

Space filling Ball and stick

Space filling model

Ball and stick model Space filling model

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