Basic Concepts of Chemical Bonding

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Basic Concepts of Chemical Bonding

• By Ms. Buroker

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8.1 Chemical Bonds, Lewis Symbols, and the Octet
Rule

• The properties of substances are largely due to
  the type of bonds that hold them together.

Salt NaCl
Sugar C12H22O11
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Chemical Bonding

• The force that holds two atoms together is
  known as a chemical bond.

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Compounds are formed from chemically bound atoms
or ions
Substances become more stable through chemical
bonding, where 2 or more atoms are joined
together by a simultaneous attraction.
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Bonding involves only the valance electrons.
Na
Cl
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Lewis Symbols

• They are used to express the number of valence
  electrons surrounding an atom.

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The Octet Rule

• Atoms gain, lose, or share electrons in an
  effort to fulfill a full valence shell
  containing
• 8 electrons.

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8.2 Ionic Bonding
In ionic compounds, electrons are lost by the
metal and gained by the non-metal, thereby
forming an attraction between the positively and
negatively charged ions.
Formation of NaCl
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Ionic Bonds
Non- Metals
Metals
Ionic Bonding occurs between metals and
non-metals. This happens when metals lose an
electron(s) (positive charged ion) and non-metals
gain electron(s) (negatively charged ion). of
e- transferred of valance e-
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Energetics of Ionic Bond Formation

• The formation of ionic compounds or ionic
  bonds are largely exothermic meaning, energy is
  released.
• The loss of an electron is always endothermic.
• The gain of electrons is always exothermic.

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Crystal Lattice
We call this three dimensional arrangement of
ions a crystal lattice.
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Crystal Lattice Properties

• High Melting Points and Boiling Points
• Hard, Rigid, Brittle
• They form Electrolytes

Electrolytes are solutions that conduct
electricity.
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Lattice Energy

• Lattice energy is a measure of the energy
  required to completely separate a mole of a solid
  ionic compound into its gaseous ions.

Crystal Lattice
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Coulombs Law

• The magnitude of the lattice energy of a
  solid depends on the charges of the ions, their
  sizes, and their arrangement in the solid. The
  potential energy of two interacting charged
  particles is give by

So, the lattice energy increases as the
charges on the ions increases and the distance
between the ions decreases.
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Graphically Determine the Relationship Between
the ionic radii and Lattice Energy

• LiF, LiCl, LiI
• Li 76pm, F- 133pm, Cl- 181pm, I- 220pm
• LiF -1032kJ/mol
• LiCl -852 kJ/mol
• LiI -761 kJ/mol
• X-axis ionic radii
• Y- axis lattice energy (use as a positive value)

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Lattice Energy Continued

• The value of lattice energy is also affected
  by the charge of the ion .
• The ionic bond formed from the attraction of
  ions with larger positive or negative charges
  generally has a more negative lattice energy.

  Lattice Energy   Lattice Energy
Compound (kJ/mol) Compound (kJ/mol)
KI -632 KF -808
KBr -671 AgCl -910
RbF -774 NaF -910
NaI -682 LiF -1030
NaBr -732 SrCl2 -2142
NaCl -769 MgO -3795
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Predict .

• Which of the following compounds would you
  think has the highest lattice energy?
• NaF, CsI, or CaO?
• Answer CsI lt NaF lt CaO

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8.4 Covalent Bonding

• The vast majority of what we come into contact
  with are covalent compounds substances that are
  bonded by the sharing of electrons.
• Usually, at room temperature, these compounds
  are gases, liquids, or solids with low melting
  points.

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Covalent Compounds

• Covalent Bonds are those chemical bonds that
  result from the sharing of electrons.

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Ionic or Covalent
In pure ionic bonds- electrons are completely
given away to other atoms. In pure covalent
bonds- electrons are totally owned by both
atoms. It turns out neither is completely
true. In actuality, the truth lies somewhere in
between. What makes the difference is how
strongly the atoms of each element attract
electrons.
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Electronegativity
Electronegativity is a measure of the ability of
an atom in a chemical compound to attract
electrons from another atom in the compound.
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Electronegativity
We can use Electronegativity values to determine
whether a compound is ionically bound or
covalently bound.
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Factors Determining Electronegativity
When you subtract the electronegativity values of
two atoms bound together you use the value to
determine what kind of bond you have. Non-polar
covalent 0-0.3 Polar Covalent Bonds 0.3- 1.7
Ionic Bonds 1.7- 3.3
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Covalent Bonds
Polar- Covalent Bond a covalent bond in which
the bonded atoms have an unequal attraction for
the shared electrons. Non-polar covalent Bond a
covalent bond in which the bonding electrons are
shared equally by the bonded atoms, resulting in
a balance distribution of electrical charge.
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Molecular Formula
When we have molecules we have to write their
formula, or what theyre made of. Molecular
Formulas show the type and numbers of atoms
combined in the covalently bonded compound.
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Covalent Bond Formation
Everything in nature favors stability covalent
bonds form from a need for stability. There is a
perfect distance apart the atoms need to be.
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Structural Properties
The strength of covalently bound compounds or
molecules depends on distance separating the
atoms (size will affect this).
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Property Factors

• There are two factors we have to think about
  when dealing with bonds of any kind.
• 1.) Bond Length
• 2.) Bond Energy- the amount of energy it takes to
  break a bond.

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Electron Dot Structures and Lewis Structures
We use electron dot structures as a way to
symbolize the number of valance electrons an atom
has.
6e-
7e-
To put it simply, Lewis Structures are our way of
explaining how atoms in a covalent bond share
their valance electrons. Its our best guess at
whats going on in the covalent bonding process.
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Lewis Structures

• Lewis structures use dots and/or dashes to
  represent bonding pairs of electrons and what we
  call lone pairs (electrons not directly involved
  in bonding.

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8.6 Drawing Lewis Structures

• Step 1 Sum the valence electrons from all the
  atoms
• Step 2 Draw the skeletal structure for the
  compound and bond all the terminal atoms to the
  central atom
• Step 3 Complete the octet for each atom
• Step 4 Place any left over electrons on the
  central atom
• Step 5 If there are not enough electrons to give
  the central atom an octet, try multiple bonds.

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Exceptions to the Octet Rule

• Be 4e-
• B 6e-
• Elements Beyond Period 2 10-12e-

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Formal Charge

• When there is more than one way for electrons
  to align themselves in a covalent bond, we use
  formal charge to determine the best way.
• Formal charge is the charge an atom would have
  in a molecule if all the atoms had the same
  electronegativity.

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To Calculate Formal Charge

• 1.) All of the unshared (nonbonding) electrons
  are assigned to the atom on which they are found.
• 2.) Half of the bonding electron are assigned to
  each atom in the bond.
• 3.) The formal charge equals the number of
  valence electrons in the isolated atom, minus the
  number of electrons assigned to the atom in the
  Lewis structure.

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Resonance Structures

• Used to describe equally equivalent Lewis
  Structures.

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Resonance in Benzene
Resonance is very Important in Organic chem.
The family of Aromatic molecules Is based on
the Benzene molecule. C6H6
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8.9 Strengths of Covalent Bonds

• In covalent bonds, stability and strength are
  directly related.
• When speaking about bond strength, we use the
  term bond enthalpy the enthalpy change, DH,
  which takes place when bonds are broken (1mol in
  gaseous state).

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Bond Enthalpies and Enthalpies of Reaction

• We can use known bond enthalpies to calculate
  the enthalpies of reaction, DHrxn.
• DHrxn S(bond enthalpies of bonds broken)
• S(bond enthalpies of bonds formed)