unit 7 (last one!!!!) chapters 8, 9.1-9.3. chemical bonding and molecular geometry lewis symbols and...
TRANSCRIPT
Unit 7 (last one!!!!)
Chapters 8, 9.1-9.3
Chemical Bonding and Molecular Geometry
Lewis Symbols and the Octet Rule
Ionic Bonding
Covalent Bonding
Molecular Geometry
Introduction
Salt vs. Sugar
solutions conduct solutions don’t electricity conduct electricity
electrolyte non-electrolyte
ionic molecular
• Sugar and salt differ in the type of attractive forces between the atoms/ions in the compound.• Chemical bond: strong attractive force that exists between atoms (or ions) in a compound
Chemical Bonds
• Three basic types of bonds– Ionic
• Electrostatic attraction between ions (cations-anions)
– Covalent• Sharing of electrons
– Metallic• Metal atoms bonded to
several other atoms
Chemical Bonds
• Covalent Bonds: the attractive force between atoms in a molecule that results from sharing of one or more pairs of electrons– non-metals
• H2O : O
H H
• Cl2 : Cl Cl
H-O and Cl-Cl bonds result
from sharing of electrons
Remember?
• Electron configuration for sodium:
[Ne]3s1
• The 3s electron of a sodium atom is a valence electron.– electrons residing in the incomplete outer
shell of an atom• involved in chemical bonding and ion formation
Lewis Symbols
• Since valence electrons are involved in the formation of chemical bonds, it’s important to keep track of them.– electron-dot symbols or Lewis symbols
• simple way to depict valence electrons and track them during the formation of chemical bonds
To write Lewis Symbols
• You must be able to determine the number of valence electrons for the main group elements.
• For main group elements, the number of valence electrons for an element = group number of the element – N (group 5A) has 5 valence electrons– Br (group 7A) has 7 valence electrons
Lewis Symbols
• Lewis symbol has two components:– chemical symbol for the element– Plus a dot for each valence electron
• dots are placed on all 4 sides of the chemical symbol
– all four sides of the symbol are equivalent
• up to 2 dots (electrons) per side (start with single dot per side of square, then double up)
Lewis Symbol for Oxygen
Chemical symbol: O
Group number: 6A
# of valence electrons: 6
O
Lewis Symbols
Draw the Lewis symbol for silicon.
Chemical symbol: Si
Group number: 4A
# of valence electrons: 4
Si
Lewis Symbols
Draw the Lewis symbol for argon.
Chemical symbol: Ar
Group number: 8A
# of valence electrons: 8
Ar
Octet Rule
• The noble gases are particularly stable because their outer shell is full of electrons.– With the exception of He, all noble gases have 8
valence electrons.
ns2np6
• Octet Rule: Atoms tend to gain, lose, or share electrons until they are surrounded by 8 valence electrons
Octet Rule
• The octet rule can be used to predict the charge of ions formed by main group elements as well as the structure of molecular compounds.
• BUT: There are many exceptions to the octet rule.
Covalent Bonding
• Molecular compounds are held together by covalent bonds that result from the sharing of electrons.
• Simplest example of a covalent bond is
H H
Indicates 2 shared electrons = 1 bond
Covalent Bonding• When 2 H atoms approach each other, electrostatic
interactions occur between their respective electrons and their nuclei.
– The two nuclei repel each other– The two electrons repel each other– The nuclei and the electrons attract each other.
Covalent Bonding
• The attractions between the nuclei and the electrons cause the electron density to concentrate between the two nuclei.
• The atoms in H2 are held
together by the electrostatic attraction of the two nuclei for the concentration of negative charge between them.
• The shared pair of electrons between the two nuclei acts as “glue”.
Covalent Bonding
• Lewis structures (also called electron-dot structures) can be used to represent the covalent bonds that are present in a molecule.– Symbol for each atom– Bond between atoms depicted using a solid
line– Unshared electron pairs are shown around
the appropriate atom
Lewis Structures
Lewis structures are representations of molecules showing all valence electrons, bonding and nonbonding.
NOTE: Octet of electrons around each Cl atom!
Covalent Bonding
• The formation of H2:
H + H H H or H H
• The Lewis structure for HF:
H + F H F = H F
Covalent Bonding
• The bond between H and F in HF is called a single bond:– sharing of one pair of electrons
• In some molecules, atoms attain an octet of electrons by sharing more than one pair of electrons.– Double bond– Triple bond
Covalent Bonding
• Double bond: two electron pairs are shared between atoms– depicted using two lines to represent the two
shared electron pairs
O C O or O C O
Carbon dioxide
Covalent Bonding
• Triple bond: three electron pairs are shared between atoms– depicted using three lines to represent the 3
pairs of shared electrons
N + N N N or N N
Nitrogen (N2)
Covalent Bonding
• In some molecular compounds, the bonding electrons are shared equally between the atoms in the molecule:
H2 F2 N2
• Nonpolar covalent bond: bonding electrons are shared equally
Covalent Bonding• In many molecular compounds, however, one
atom attracts the bonding electrons more strongly than the other.
+ - Fluorine attracts electrons more strongly than hydrogen.H – F
+ -
The fluorine end of the molecule has greater electron density than the hydrogen end.
The H – F bond is a polar covalent bond.
Covalent Bonding
• Polar covalent bond: – a chemical bond in which the electrons are not
shared equally• one atom attracts the bonding electrons more
strongly
• The polarity of a covalent bond can be determined using the difference in electronegativity between the two atoms.
Covalent Bonding
• Electronegativity:– The ability of an atom in a molecule to attract
electrons to itself • Range: 0.7 (Cs) - 4.0 (F)
• As electronegativity increases, the attraction that an atom has for electrons increases.
Covalent Bonding• Trends to know:
– Electronegativity increases:• From left to right across
a row• From bottom to top of a
column– The four most
electronegative elements are:
• F (4.0)• O (3.5)• N (3.0)• Cl (3.0)
Covalent Bonding• Chemicals bonds exist along a continuum:
• The greater the difference in electronegativity between two atoms, the more polar their bond.
F – F en = 0 nonpolar covalent (en<0.5)
H – F en = 1.9 polar covalent (0.5<en<2.0)
Li – F en = 3.0 ionic (en>2.0)
IonicBonds
Polar CovalentBonds
Nonpolar CovalentBonds
Testing
Which of the following bonds is a polar covalent bond?
Br-Br C-H O-H Al-Cl
EN= 0 0.45 1.34 1.55
Polar covalent (0.5< en < 2.0)