Transcript Chapter 8 – Basic Concepts of Chemical Bonding Abegim Undieh

Chapter 8 – Basic Concepts of
Chemical Bonding
Abegim Undieh
8.1 – Chemical Bonds, Lewis Symbols,
and the Octet Rule
• When atoms or ions are strongly attached to
one another, there is a chemical bond
between them
• There are THREE types of chemical bonds:
ionic, covalent, and metallic
Lewis Symbols
• The electrons involved in chemical bonding
are called valence electrons
• Valence electrons are in the outer shell of an
atom
• Valence electrons are symbolized using Lewis
Symbols
• The Lewis Symbol of an element shows the
chemical symbol for the element and a dot for
each valence electron
Picture of a Lewis Symbol
• Lewis Dot Symbol for Se:
http://www.cbu.edu/~mcondren/Se-Lewis-dot-structure.jpg
The Octet Rule
• All noble gases (besides He) have 8 valence
electrons
• Noble gases have the most stable electron
arrangements
• The Octet Rule: Atoms tend to gain, lose, or
share electrons until they are surrounded by
eight valence electrons
8.2 – Ionic Bonding
• The bonding of a metal and a non-metal
through the electrostatic attraction of
opposite charges is called ionic bonding
• In an ionic bond, the metal transfers electrons
to the nonmetal
• This occurs when there is a metal of low
ionization energy and a nonmetal with a high
electron affinity
Formula showing a sample Ionic Bond
Na + Cl  NaCl
Na + Cl  Na+ + [Cl]-
http://upload.wikimedia.org/wikipedia/comm
ons/7/75/Ionic_bonding_animation.gif
Ionic Bonding (cont.)
• Ionic bonding is a very exothermic process
• The measure of the stability of an ionic bond
is called the lattice energy
• Lattice energy – the energy required to
completely separate a mole of a solid ionic
compound into gaseous ions
Ionic Bonding (cont.)
• Formula for lattice energy:
E = kQ1Q2
D
Lattice energy increases as the charges on the ions
increase and as their radii decrease
Ionic radii do not vary much over a wide range, so
the magnitude of lattice energy depends mainly
on the ionic charges
8.3 – Sizes of Ions
• Ionic size determines many properties of ionic
solids, such as lattice energy, the way it packs
in a solid, and the properties of the ions in
solution
• The size of an ion depends on
– its nuclear charge
– the number of electrons it possesses
– the orbitals in which the valence electrons reside
Sizes of Ions (cont)
• In positive ions, electrons from the outermost
orbital have been removed
• This decreases the total electron-electron
repulsion, allowing the atom to pull closer
together
• Therefore, cations are smaller than their parent
atoms, and conversely..
• Anions are larger than their parent atoms
• For ions of the same charge, size increases as you
go down a group in the periodic table
Sizes of Ions (cont)
• an isoelectronic series is a series of ions that have
the same number of electrons
• Example:
O2-, F-, Na+, Mg2+, Al3+ all have 10 electrons
arranged in a 1s22s22p6 configuration like [Ne]
• In an isoelectronic series, the radius of the ion
decreases with increasing nuclear charge
Ionic Radii
http://www.chem.umass.edu/~botch/Chem111F04/Chapters/Ch8/IonicRadii.jpg
8.4 – Covalent Bonding
• In covalent bonding, two nonmetals share
electrons
• The atoms are held together by the
electrostatic attraction between the two
nuclei
Multiple Bonds
• The sharing of a pair of electrons produces
one covalent bond, called a single bond
• In some cases, molecules form multiple bonds
in order to achieve an octet
Multiple Bonds
• Two shared electrons produce a double bond
• Three shared electrons produce a triple bond
the triple bond in N2 as shown by a Lewis structure
• As a rule, the distance between bonded atoms
decreases as the number of shared electron
pairs increases
8.5 – Bond Polarity and
Electronegativity
8.6 – Drawing Lewis Structures
8.7 – Resonance Structures
• In some molecules, the arrangement of atoms
cannot fully be described by a single Lewis
structure
• This requires showing multiple equivalent
Lewis structures, called resonance structures
• The actual structure of the molecule is a blend
of its resonance structures
Resonance Structures (cont.)
• Example:
– In ozone, both of these resonance structures are
equally as correct, and the molecule does not oscillate
between the two:
http://www.mikeblaber.org/oldwine/chm1045/notes/Bonding/Resonan/IMG00007.GIF
Remember to put brackets ‘[ ]’ around each resonance
structure that you draw, with the double arrow in-between
Resonance in Benzene
• Benzene, C6H6, has two equivalent Lewis
structures, each showing three C-C single
bonds and three C=C double bonds
http://wpcontent.answers.com/wikipedia/commons/thumb/7/79/Benz3.svg/300
px-Benz3.svg.png
Resonance in Benzene
• Benzene is often represented as such:
•
OR
• Millions of organic compounds have
structures based on Benzene
8.8 – Exceptions to the Octet Rule
• Molecules with an odd number of electrons
• Molecules in which an atom has less than an
octet
• Molecules in which an atom has more than an
octet
Odd Number of Electrons
• In some cases, complete pairing of electrons is
impossible and an octet around each atom
cannot be achieved
• Examples: ClO2, NO, NO2
ClO2 Lewis Diagram
Less than an Octet
• There are also cases in which an atom has less
than 8 electrons
• This most frequently occurs in compounds
containing boron or beryllium
• Example:
http://www.up.ac.za/academic/chem/mol_geom/bf32.gif
BF3 Lewis structure, showing that B only has 6 electrons surrounding it.
More than an Octet
• Many molecules/ions have more than an octet
of electrons
• This only occurs for elements in period 3 and
beyond
Diagram for PCl5, showing an expanded octet around P.
http://www.wwnorton.com/college/chemistry/gilbert/concepts/chapter7/pcl5.gif
More than an Octet
• The larger the central atom, the larger the
number of surrounding atoms it can hold
• Expanded shells occur most often when the
central atom is bonded to the smallest and
most electronegative atoms, such as F, Cl, O
8.9 – Strengths of Covalent Bonds
• Bond enthalpy – the enthalpy change for the
breaking of a particular bond into a mole of
gaseous substance
• Bond enthalpy is always a positive quantity
• The strengths of covalent bonds increase with
the number of shared electron pairs
Strengths of Covalent Bonds
• The bond length between two bonded atoms
is the distance between the two nuclei
• The average bond length between two atoms
decreases as the number of bonds between
them increases