Transcript More on Bonding - Hemet High School

Water
Chapter 6.2
Covalent Bonding and
Molecular Compounds
Fructose
Carbon Dioxide
Ammonia
Why Do Atoms Bond?
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To get eight valence electrons
To become more stable
In ionic bonds, metals lose electrons
and non-metals gain electrons.
What happens when both
elements need electrons?
Molecules and Molecular Compounds
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Compounds that are NOT held together
by an electrical attraction, but instead
by a sharing of electrons.
Atoms held together by sharing electrons
and filling the outer energy levels are
joined by a covalent bond.
NONMETALS ONLY!! - No metals
Molecules and Molecular Compounds
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A molecule is a neutral group of atoms
joined together covalent bonds. A
compound composed of molecules is
called a molecular compound.
The chemical formula for a molecule is
called the molecular formula.
A chemical formula tells you how
many atoms of each element one
molecule of a compound contains.
Learning Check
Indicate whether a bond between the following
would be 1) Ionic
2) covalent
Ionic
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Covalent
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Covalent
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Ionic
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Covalent
A.
sodium and oxygen
B.
nitrogen and oxygen
C.
phosphorus and chlorine
D.
calcium and sulfur
E.
chlorine and bromine
Monatomic (One Atom)
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Noble gases are monatomic.
They exist as single atoms and do
not combine with any other
elements.
Ex: He, Ne, Ar, Kr, Xe, Rn
7 Diatomic Molecules
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Some elements will covalently bond to
themselves to form a molecule
composed of TWO atoms.
Some elements occur as “diatomic”
molecules in nature because they are
more stable than individual atoms
The 7 diatomic elements are all gases:
H2, O2, N2, Cl2, Br2, I2, F2
Strength of Covalent Bonds
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Distance between two bonding nuclei at
the position of maximum attracting is
bond length
Bond length is determined by the size of
the atoms and how many electron pairs
are shared
Bond energy is the energy required to
break a chemical bond and form neutral
isolated atoms.
Octet Rule in Covalent Bonds
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Remember that all compounds want to
attain the electron configuration of
noble gases.
Hydrogen only needs 2, the rest need 8.
Regarding covalent bonds, electrons are
shared between the atoms so that they
attain the electron configuration of noble
gases.
Exceptions to the Octet Rule
A small group of molecules has an odd
number of valence electrons and cannot
form an octet around each atom
-Ex: NO2
2)
O
N
O
1)
Fewer than eight electrons: BORON is
stable with 6!
H-B-H
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Ex: BH3
H
Exceptions to the Octet Rule
Some central atoms have more than
eight valence electrons
3)
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Referred to as an “expanded” octet
Explained by d-orbitals
PCl5 (10 e-)
SF6 (12 e-)
Drawing Valence Electrons
“Electron-dot notation”:
Electrons are
represented as dots located around the
symbol of the element. You must put one
electron on each side before you double up.
X
Examples: Nitrogen =
N
Carbon =
Hydrogen =
H
C
In-Class Examples
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Chlorine
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Neon
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Magnesium
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Sulfur
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Silicon
Homework
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Page 209 #10-12, 15, 19
6.2 Notes Continued
To draw Lewis structures for covalent bonds, use the
NASB method:
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N (Needed): Find the number of electrons needed
to form full octets for all elements. For most
nonmetals, they need 8. Hydrogen needs only 2.
A (Available): Find the number of electrons
available by adding up all of the valence electrons
for all elements involved.
S (Shared): Subtract the two numbers. S= N-A
B (Bond): A bond is formed with 2 electrons, so
divide by 2 to find how many bonds to draw
between the elements.
Draw the molecule. Put first atom in the center.
H’s are always outside. Draw in the bonds, then
fill in the rest of the electrons.
Check to ensure all atoms have a full octet.
Draw the Lewis-dot-structure for the
following molecules
1. HF
2. CCl2H2
Draw the Lewis-dot-structure for the
following molecules
1. H2O
2. CO2
Types of Bonds
one _________
pair
• Each bond involves the sharing of _____
of
electrons.
2 e-’s
Single Bonds= __
4 e-’s
Double Bonds= __
6 e-’s
Triple Bonds=__
Resonance Structures
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Occurs when more than one valid
Lewis Structure can be written for a
molecule or ion
Differ only in the position of electron
pairs, never the atoms positions
Actual molecule behaves as if it has
one structure
Example: O3
Homework
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6.2 page 209 #16-18, 21, 23