SFC 2200
INTRODUCTORY
CHEMISTRY
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CHEMICAL BOND
Dr. Tengku Sharifah Marliza Tengku Azmi
CONTENT
Ionic, covalent and polar bonds
Lewis and molecule structure
Geometry molecule
Bonding theory
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Why form chemical bonds?
a) Atoms are trying to reach the most stable (lowest-energy) state that they can.
b) Many atoms become stable when their valence shell is filled with electrons or when they satisfy
the octet rule (by having eight valence electrons).
c) If atoms don’t have this arrangement, they’ll “want” to reach it by gaining, losing, or sharing
electrons via bonds.
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A comparison of metals and nonmetals.
TYPE OF CHEMICAL BONDS
1) Ionic bond
2) Covalent bond
3) Metallic bond
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Three models of chemical bonding.
Lewis Electron-Dot Symbols
To draw the Lewis symbol for any main-group element:
Note the A-group number, which gives the number of
valence electrons.
Place one dot at a time on each of the four sides of the
element symbol.
Keep adding dots, pairing them, until all are used up.
Example:
Nitrogen, N, is in Group 5A and therefore has 5 valence electrons.
••
••
•• • • •
•N• or • N• or • N or N•
• •• • •
Lewis electron-dot symbols for elements in Periods 2 and 3.
PRACTISE
Draw the Lewis dot structure for the following elements.
1. Mg 2. Si 3. Cl 4. F-
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Lewis Symbols and Bonding
For a metal, the total number of dots in the Lewis symbol
is the number of electrons the atom loses to form a cation.
For a nonmetal, the number of unpaired dots equals
- the number of electrons the atom gains to form an anion
- or the number it shares to form covalent bonds.
The octet rule states that when atoms bond, they lose,
gain, or share electrons to attain a filled outer level of 8
electrons (or 2, for H and Li).
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1. IONIC BOND
Ionic bonding involves a transfer of an electron, so one atom gains an electron while one atom
loses an electron. One of the resulting ions carries a negative charge (anion), and the other ion
carries a positive charge (cation). Because opposite charges attract, the atoms bond together to
form a molecule.
Ions come in two types
Cations are positive ions formed by losing electrons. E.g. sodium, atom loses an electron to
become a sodium cation, Na+
Negative ions are formed by electron gain and are called anions
Anions are named using the ending “-ide”: for example, the anion of chlorine Cl− is called
chloride.
When one atom loses an electron and another atom gains that electron, the process is
called electron transfer.
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Example :
Three ways to depict electron transfer in the formation of Li+ and F–.
Electron configurations Li 1s22s1 + F 1s22s22p5 → Li+ 1s2 + F–
1s22s22p6
Orbital diagrams
Li ↑↓ ↑ Li+ ↑↓
+ 1s 2s 2p 1s 2s 2p
F ↑↓ ↑↓ ↑↓ ↑↓ ↑ F- ↑↓ ↑↓ ↑↓ ↑↓ ↑↓
1s 2s 2p 1s 2s 2p
Lewis electron-dot symbols••
••
••
•• •• –
Li• •F Li+ + F
•• ••
Properties of Ionic Compounds
Ionic compounds tend to be hard, rigid, and brittle, with high
melting points.
Ionic compounds do not conduct electricity in the solid state.
In the solid state, the ions are fixed in place in the lattice and do not move.
Ionic compounds conduct electricity when melted or dissolved.
In the liquid state or in solution, the ions are free to move and carry a
current.
Why ionic compounds crack.
Electrical conductance and ion mobility.
Solid ionic Molten ionic Ionic compound
compound compound dissolved in water
Melting and Boiling Points of Some Ionic Compounds
Compound mp (ºC) bp (ºC)
CsBr 636 1300
NaI 661 1304
MgCl2 714 1412
KBr 734 1435
CaCl2 782 >1600
NaCl 801 1413
LiF 845 1676
KF 858 1505
MgO 2852 3600
Example of ionic bonds in everyday life
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The ability of an atom in a covalent bond to attract the
2. COVALENT BOND
shared electron pair is called its electronegativity
The most common bond in organic molecules, a covalent bond involves the sharing of electrons
between two atoms.
This bond is formed between atoms that have similar electronegativity—the affinity or desire for
electrons.
Because both atoms have similar affinity for electrons and neither has a tendency to donate them, they
share electrons in order to achieve octet configuration and become more stable.
Covalent bond may involve, 1, 2, or 3 pairs of electrons.
More bonds (more shared electrons) means stronger bonds, and closer the two atoms together.
Single Bond Shared 1 pair of electrons
Double Bond Shared 2 pairs of electrons
Triple Bond Shared 3 pairs of electrons
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Covalent
bond
Non-polar
Polar bond
bond
A nonpolar covalent bond is formed A polar covalent bond is formed when
between same atoms or atoms with very atoms of slightly different
similar electronegativity electronegativity share electrons.
***electronegativity
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= the relative ability of an atom in a molecule to attract shared electrons to itself 22
Properties of a Covalent Bond
The bond order is the number of electron pairs being
shared by a given pair of atoms.
A single bond consists of one bonding pair and has a bond order of 1.
The bond energy (BE) is the energy needed to
overcome the attraction between the nuclei and the
shared electrons. The stronger the bond the higher the
bond energy.
The bond length is the distance between the nuclei of
the bonded atoms.
Table 9.3 The Relation of Bond Order, Bond Length, and
Bond Energy
Trends in Electronegativity
The most electronegative element is fluorine.
In general electronegativity decreases down a group as
atomic size increases.
In general electronegativity increases across a period
as atomic size decreases.
Nonmetals tend to be more electronegative than metals.
ELECTRONEGATIVITY
TRENDS
IN PERIODIC TABLE
Increasing electronegativity
Electronegativity increases from left to
Decreasing electronegativity
right and generally decreases from top to
bottom
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Example of covalent bonds in everyday life
Diamond structure
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METALLIC BOND
Metallic bonding exists between metal atoms.
Metals have relatively low ionization energies (easily removed electrons) but also low electron
affinities (very little tendency to gain electrons).
So, metals will share electrons.
However, it is a different sort of bonding than covalent bonding.
Metals share valence electrons, but these are not localized between individual atoms.
Instead, they are distributed throughout the metal and are completely delocalized.
They are often described as being a "sea" of electrons which flow freely between the atoms.
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Properties of Metals
Metals are generally solids with moderate to high melting points and
much higher boiling points.
Melting points decrease down a group and increase across a period.
Metals can be shaped without breaking.
The electron sea allows the metal ions to slide past each other.
Metals are good conductors of electricity in both the solid and liquid
states.
The electron sea is mobile in both phases.
Metals are good conductors of heat.
Melting and Boiling Points of Some Metals
Element mp (ºC) bp (ºC)
Lithium (Li) 180 1347
Tin (Sn) 232 2623
Aluminum (Al) 660 2467
Barium (Ba) 727 1850
Silver (Ag) 961 2155
Copper (Cu) 1083 2570
Uranium (U) 1130 3930
Why metals dent and bend rather than crack.
Example of metallic bonds in everyday life
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LEWIS STRUCTURE
Lewis Structure: An electron-dot structure, represents valence electrons by dots.
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In the Lewis structure of a molecule or polyatomic ion, valence electrons ordinarily occur in pairs.
There are 2 kinds of electron pairs:
1. a pair of electrons shared between 2 atoms is a covalent bond- shown as a straight line between bonded atoms.
2. an unshared pair of electrons, is shown as a pair of dots on atom – often referred to as lone pair.
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Each atom is surrounded by 8 valence electrons.
A single electron pair is shared between 2 bonded atoms. This bond is called single bond.
Bonded atoms can share more than 1 electron pair.
A double bond = bonded atoms shared 2 electron pairs
A triple bond = bonded atoms shared 3 electron pair
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DRAWING LEWIS STRUCTURE
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RESONANCE FORM
Whenever it’s possible to write more than one valid electron-dot structure for a molecule, the actual electronic
structure is an average of the different possibilities, called a resonance hybrid
The different resonance forms differ only in the placement of the valence-shell electrons.
eg: Lewis structure for the nitrate ion, NO3- is shown below:
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EXCEPTIONS TO THE OCTET RULE
1) Molecules containing electron-deficient atoms (have fewer than 8 electron)
2) Molecules containing odd electron atoms
3) Molecules containing atoms with expended valence shells
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PRACTISE
Draw the Lewis dot structure for each of the following molecules.
a. H2S b. NH3 c. NH4+
https://www.youtube.com/watch?
v=j8bHuNYS32k&t=22s
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