chapter 8 honors chemistry (partial) covalent bonding 1
TRANSCRIPT
Chapter 8 Honors Chemistry(partial)
Covalent Bonding
1
Electronegativity
• In a covalent bond, we have seen that electron pairs are shared between two nonmetals
• Rarely are these electrons shared equally as one of the atoms has a stronger “desire” to have those electrons
• How can we measure which atoms wants the electrons more?
• Electronegativity (EN) !!!!!• It is a measure of an atoms ability to attract a
pair of electrons in a molecule
2
F is the most electronegative
element and is given a value of
4.0 and all elements E.N. values
are in comparison to this
Left to right across a period =
in EN
Down a group in EN or stays
about the same3
ElectronegativityElectronegativity
The higher the EN value, the more the
atom will attract shared electrons to it
Depending upon how great the difference
in electronegativity is between the atoms
the bond can have highly positive and
negative regions
This is called a polar bond4
• The only bond that is purely 100% covalent where the electrons are equally shared is one in which the EN = 0
• This only occurs when the electrons are shared by identical atoms, like H2, or any of the diatomic molecules
5
ElectronegativityElectronegativity
Polarity
• A bond is considered to be non-polar covalent if the EN is 0 – 0.4
• A bond is considered to be moderately polar-covalent if the EN is 0.5 – 1.0
• A bond is considered to be very polar-covalent if the EN is 1.0 – 1.7
• Any bond with EN that is ≥ 1.7 is considered to be ionic
6
Electronegativities of the Elements
Using the chart of Electronegativities (Pg. 177),
determine the type of bond formed between the
following pairs of atoms:
C and O
Fe and O
N and Br
C and H
Na and F
Cl and Cl
ElectronegativityElectronegativity
C and O EN = 1.0 polar
Fe and O EN = 1.7 ionic
N and Br EN = 0.2 nonpolar
C and H EN = 0.4 nonpolar
Na and F EN = 3.1 ionic
Cl and Cl ΔEN = 0 nonpolar
9
• In a molecule of H2O, a pair of electrons are shared between each O and H
• The EN of O = 3.5 and H = 2.1• EN = 1.4 – therefore is a polar covalent bond• This means that O attracts the electrons
towards it and so will become slightly negative while the electrons move away from each H atom and they become a bit positive
10
ElectronegativityElectronegativity
• This means the electrons are not shared evenly and that one area is slightly positive, the other negative.
• This is called a polar molecule• Indicated using small delta (δ).
11
Dipole Moments
• A molecule with a center of negative charge and a
center of positive charge is a DIPOLE
• (two poles),
• or has a dipole moment.
• Center of charge doesn’t have to be on an atom.
• Will line up in the presence of an electric field.
How It is drawn
H - F+ -
H - F+ -
H - F+ -
H - F
+-H - F+
-
H - F
+-
H - F +-
H - F+-
H - F
+-
H - F
+-
H - F+ -
H - F
+-H - F+
-
H - F
+-
H - F +-
H - F+-
H - F
+-
H - F
+-
+-
H - F+ -
H - F+ -
H - F+ - H - F
+ -
H - F+ -
H - F+ -
H - F+ -
H - F+ -
- +
Which Molecules Have Dipole Moments?
• Any two atom molecule with a polar bond.
H2O or FBr
• With three or more atoms there are two considerations.
1. There must be a polar bond.
2. Geometry can’t cancel it out (more about geometry later)
CH4
CO2
SO2
Ionic vs. Molecular Compounds
There are two types of forces involved in chemistry
Intermolecular forces are those between molecules
and are responsible for holding these molecules
together (inter = between)
Intramolecular forces are those between atoms
inside the actual molecule and are responsible for
holding the molecule together (intra = within)
These two forces explain many of the properties of
ionic and covalent compounds
19
• Ionic compounds are formed of positive and negative ions and these forces are very strong
• Each ion is held in place by at least 6 other ions and so both the inter and the intra molecular forces are strong
20
Ionic vs. Molecular CompoundsIonic vs. Molecular Compounds
• Covalent compounds have strong intramolecular forces holding the atoms together to form a molecule, but rather weak intermolecular forces holding the adjacent molecules together
• Because the intermolecular forces are weak, covalent compounds have low boiling and melting points (little energy is needed to move molecules apart from a solid to liquid to gas)
• Many are gases at room temp
21
• Solid ionic compounds do not conduct electricity
as the ions are held tightly, but when in the liquid
state (called molten) the ions are free to move and
so can conduct electricity
• Ionic compounds dissolve easily in water as water
is a polar molecule and water molecules surround
the ions and pull them apart into the solution
(process called solvation)22
Ionic vs. Molecular CompoundsIonic vs. Molecular Compounds
Metallic Bonding
• How are metal atoms held in place?
• Most metals have 1, 2 or 3 valence electrons
• The metal atoms are relatively close to each other and their valence
energy levels overlap
• This allows the valence electrons to move freely from one metal
atom to those it overlaps with
• These electrons are not bonded to one particular metal atom and are
called delocalized electrons
• This is often referred to as the “Electron Sea Model” of metallic
bonding
2323
Ionic Bond, A Sea of Electrons
2424
Metallic BondingMetallic Bonding
Sea of Electrons
• Metals conduct electricity.
• Electrons are free to move through the
solid.
2525
Hydrogen Bonding• This is a type of bonding involving
hydrogen and either F, O or N• When hydrogen bonds with either of these
elements there is a large ΔEN• This results in a very polar molecule with
large dipoles• This produces relatively high inter
molecular forces to adjacent molecules they are held together “tightly”
• This accounts for the relatively high boiling and melting point of H2O compared to other covalent compounds
2626
Hydrogen Bond
11.2
The The hydrogen bondhydrogen bond is a special dipole-dipole is a special dipole-dipole interaction between the hydrogen atom in a polar interaction between the hydrogen atom in a polar N-H, O-H, or F-H bond and an electronegative O, N-H, O-H, or F-H bond and an electronegative O, N, or F atom.N, or F atom.
2727