Download Chapter #7 - TeacherWeb

Chapter #7
Chemical Bonds.
Chemical Bond
• An attractive force that holds two atoms
together in a complex unit.
• Electrons combine to form chemical
bonds.
Ionic Bond
• Bond formed between ions
• Transfer of electrons from one atom to the
other.
Covalent Bond
• Formed by sharing of one or more
electrons between 2 atoms.
• Molecular compounds are formed.
Covalent/Ionic Bonds
• Only valence electrons participate in
bonding.
• They try and follow the octet rule( eight
electrons)
Valence electrons and Lewis
Symbols
• Valence electrons-Electrons in the
outermost electron shell of a
representative element or a noble gas
element.
• Found in s or p orbitals or both.
Valence Electrons
• How many valence electrons are in:
• Mg, Cl, Se, Ca , S , Br
Lewis Symbol
• Chemical symbol of an element
surrounded by dots equal to the number of
valence electrons.
• Also called as electron dot structures.
Draw Lewis symbols for:
• Li, Be, B, C, N, O, F, Ne
Lewis symbols prove
• Representative elements in same group
have same number of valence electrons.
• Number of electrons in Lewis symbol=
group number.
• Maximum number of valence electron is 8.
• He has 2 valence electrons.
The Octet rule
•
•
•
•
Noble gases are stable.
They are the most unreactive elements.
They all are gases.
Octet Rule: Atoms of an element lose, gain
or share electrons to produce a noble gas
configuration for each atoms involved.
The Ionic Bond Model
• Ion: Atom or group of atoms that is
electrically charged by loss or gain of
electrons.
• Cation: Positively charged ion.
• Anion: Negatively charged ion.
Ions
• What kind of ions are formed by Na, Mg,
Al, P, S, Cl.
Ionic bond
• A chemical bond resulting from attraction
of positive and negative ions for each
other.
Sign and Magnitude of Ionic
Charge
• Metal atoms containing 1,2,or 3 valence
electorns lose electrons to acquire a noble
gas configuration.
• Noble gas involved is the one preceeding
the metal
• Group 1= +1 ions
• Group 2 = + 2 ions
• Group 3 =+ 3 ions
Sign and Magnitude of Ionic
Charge
• Nonmetal atoms containing 5,6,or 7
valence electrons gain electrons to acquire
a noble gas configuration.
• Noble gas involved is the one following the
nonmetal in the periodic table.
• Group7=-1 ions.
• Group 6=-2 ions.
• Group 5=-3 ions.
Sign and Magnitude of Ionic
Charge
• Elements in group IV A form covalent
bonds.
• They have +4 or -4 charges.
Isoelectronic Species
• Ions or an atom and ions having the same
number and configuration of electrons.
Ionic Compound Formation
• Lewis structure: Grouping of Lewis
symbols that show either the transfer or
sharing of electrons in chemical bonds.
• Ionic compounds contain both metals and
nonmetals.
• Ex: NaCl
• Covalent compounds contain only
nonmetals. Ex: H2O
Formulas For Ionic Compounds
• Total charge on an ionic compound is
zero.
• Symbol for positive ion is written first.
• Charges are not shown in the formula.
• Numbers in the formula (subscript) give
combining ratio for the ions.
Formulas For Ionic Compounds
Write formulas for:
Na, P
Al, P
Ba, Cl
Ba, Sr
Ba, N
Ionic compounds
•
•
•
•
•
Contain metal and nonmetal.
Metal forms cation.
Nonmetal forms anions.
Compound is neutral.
Metals in group 1=+1, group 2=+2 , group
3 =+ 3 ions.
• Metals in group 7=-1 , group 6=-2, group
5=-3.
Structure of Ionic Compounds
• Each ion is surrounded by opposite
charges.
• Formula Unit: Smallest whole number
repeating ratio of ions present in an ionic
compound that results in charge neutrality.
• Repeated regular patterns.
• Ex: Crystalline solids.
Polyatomic Ions.
• Monoatomic ion- Ion formed from a single
atom through loss or gain of electrons.
• Polyatomic ion- Ion formed by group of
atoms through loss or gain of electrons.
• Do practice Ex: 7.7 Page # 212.
The Covalent Bond Model
• Ionic bond is formed between metalsnonmetals.
• Ionic bond-transfer of electrons.
• Covalent bond = sharing of electrons.
• Covalent compounds are called molecular
compounds.
• Ionic compound in water conducts
electricity, but a covalent compund does
not.
The Covalent Bond Model
• Covalent bond-Chemical bond resulting
form two nuclei attracting same shared
electrons.
• Ex: H-H
Lewis Structures for molecular
compounds
•
•
•
•
Formation of H-Cl
Formation of Cl-Cl
Formation of Br-Cl
Bonding electrons: Pairs of valence
electrons that are shared between atoms
in a covalent bond.
• Nonbonding electrons: pairs of valence
electrons about an atom not involved in
electron sharing, also called as lone pairs.
Lewis Structures for molecular
compounds
• Number of covalent bond an electron
forms= number of electrons it needs to
achieve noble gas configuration.
• Structures of water, ammonia, methane.
Lewis Structures for molecular
compounds
•
•
•
•
•
•
•
Write Lewis structures for :
P and H
O and Cl
S and F
N and F
C and Cl
S and Br
Single, Double, Triple covalent
bonds
• Single= two atoms share 1 pair of valence
electron Ex: H-H
• Double=two atoms share 2 pairs of
valence electrons Ex: O= O
• Triple=two atoms share 3 pairs of valence
electrons Ex: N-N bonds.
Valence electron count and number
of covalent bonds formed.
• O bonds: Two single or 1 double bond.
• N bonds: 3 single, one single and one
double, one triple bond.
• C bonds: 4 single, 2 single and 1 double, 2
double, 1 single and 1 triple bond.
Coordinate covalent bonds
• One atom supplies 2 electrons and other
none.
• Both electrons of shared electron pair
come from one of the two atoms involved
in covalent bond.
• Ammonia + hydrogen
• HOCl
• CO
Resonance structures
• Bond strength: Measure of energy it takes
to break a covalent bond.
• Bond length: Distance between nuclei of
covalently bonded atoms.
• Resonance structures: Two or more Lewis
structures for a molecule or polyatomic ion
that have same structures but different
arrangement of electrons.
• Ex: SO2
Drawing Lewis Structures
1. Determine total number of valence
electrons
2. Write symbols of atoms arranged in a way
they are bonded to each other. Place a
single covalent bond.
3. Add nonbonding electron pairs to
complete the octet.
4. Complete octet around central atom.
Drawing Lewis Structures
5. If not enough electrons on central atom to
complete octet, form multiple bonds.
6. Count total number of valence electrons
and check if number =step 1 number.
Drawing Lewis Structures
•
•
•
•
•
•
•
Draw Lewis structures for :
H2SO4
HClO3
Na2SO3
PO33O3
SO3
Molecular Geometry
• A description of three dimensional arrangement
of atoms within a molecule.
• Determines physical and chemical properties.
• VSEPR: Valence shell electron pair Repulsion
Theory: The electrons try and place themselves
in such a way that they minimize the repulsion.
• Predicting geometry of a molecule from Lewis
structure
Molecular Geometry
• 2 e pairs-linear-180
• 3 e pairs- trigonal planar-120
• 4 e pairs- tetrahedral- 109.5
VSEPR Electron group: Group of valence
electron present in a localized region
about an atom in a molecule.
Count double, triple bonds as single bonds.
Do practice 7.12, P.234
Electronegativity
• Measure of the relative attraction that an
atom has for the shared electrons in a
bond.
• Higher the electronegativity of an element,
greater is the electron attracting ability of
an element.
Electronegativity
• For representative elements
electronegativity value increases from left
to right.
• For period 2 left to right increase by 0.5
• H=2.1
• For period 3 elements increase left to right
by 0.3 units, last two is 0.4,0.5.
• Increase from bottom to top within a
group.
Bond polarity
• Nonpolar covalent bond: equal sharing of
electrons.
• Polar covalent bond: Unequal sharing of
electrons.
• Unequal sharing causes partial + and _
charges.
• Ex: H-Cl
Bond polarity
• Bond polarity: Measure of degree of
inequality in the sharing of electrons in a
chemical bond.
• Percent ionic character of a bond: A
measure of how actual charge separation
in a bond due to electronegativity
difference of the bonded atoms compares
to the complete charge separation
association with ions.
Bond polarity
1. Similar eneg non polar bonds.
2. Eneg diff > 0.5 but < 2.0-polar covalent
bonds.
3. Eneg diff=or > 2.0-ionic bonds.
Rank following in order of increasing polarity.
C-Cl, Ca-F, C-O, B-H,P-Br
N-S, H-H, Na-F,K-Cl,F-Cl
Molecular polarity
• Measure of degree of inequality in the
attraction of bonding electrons to various
locations eithin a molecule.
• Polar molecule: unsymmetrical distribution
of electronic charge.
• Nonpolar molecule: symmetrical
distribution of electronic charge.
• Ex: CO2, H2O,HCN.
• Practice Ex: 7.14, page #242.