Transcript ionic bond - Mrs Molchany`s Webpage

Bonding
Ch 8
Objectives
SWBAT identify the bond type of a
molecule by using electronegativity
differences.
SWBAT identify the polarity in small
molecules.
Types of Bonding
Ionic
Covalent
Metallic
Octet Rule
Atoms tend to gain, lose or share
electrons until they are
surrounded by 8 valence
electrons.
Ionic bonding
Transfer of electrons from the metal to
the non-metal.
Formation of cations and anions
Na + Cl
+
Na
+
Cl
Ionic Bonding
When an extremely electronegative atom,
like fluorine, bonds with an electropositive
atom, like sodium, the resulting bond is
ionic due to the huge difference in
electronegativity (difference > ~1.7).
Ionic Bonding
The electronegative atom's pull on the
bonding electrons is so strong that it pulls
the bonding electron off the electropositive
atom resulting in two oppositely charged
ions which are held together by
electrostatic attraction (an ionic bond).
Energetics of Ionic Bond Formation
If you look in Appendix C (∆Hf values) you
will find that the heat of formation for ionic
compounds is exothermic.
The removal (or loss) of electrons is
always an endothermic process.
When a non-metal gains an electron the
process is generally exothermic.
Lattice Energy
The principal reason that ionic compounds
are stable is the attraction between ions of
unlike charge.
Lattice energy – is the energy required to
completely separate a mole of a solid ionic
compound into its gaseous ions.
ΔH°f [NaCl(s)] = ΔH°f [Na(g)] + ΔH°f
[Cl(g)]
+ I1(Na) + E(Cl) - ΔHlattice
Lattice Energy Problems
Look at the example on B&L page 267
We will go through the example as a
group.
Try problems # 19 and 20 at the end of the
chapter.
New Book P. 308
Try 27, 28
Covalent Bonding
Bonding between non-metals consists of
two electrons shared between two atoms.
In covalent bonding, the two electrons
shared by the atoms are attracted to the
nucleus of both atoms. Neither atom
completely loses or gains electrons as in
ionic bonding.
http://www.elmhurst.edu/~chm/vchembook/152Apolar.html
Types of Covalent Bonding
Polar Covalent Bonding
results when two different non-metals
unequally share electrons between them.
Non-Polar Covalent Bonding
results when two identical non-metals
equally share electrons between them.
Non-polar covalent bonds
Identical non-metallic atoms have identical
electronegativity
For example in an H-H bond both H atoms have the
same electronegativity so the bond is non-polar
Different atoms can have the same electronegativity,
such as, N-Cl.
Both N and Cl have an electronegativity of 3.0 so the
bonding electrons will be shared equally between the
two atoms resulting in a non-polar covalent bond.
Polar Covalent Bonds
When atoms of similar, but different,
electronegativities (a difference < ~1.7) bond,
the more electronegative atom has a greater
share of the bonding electrons than the less
electronegative atom.
The more electronegative atom has a partial
negative charge, and the less electronegative
atom has a partial positive charge. The resulting
covalent bond is called a polar covalent bond.
Show the Class
Show how to draw a partial positive and
partial negative charge symbol.
Multiple Bonds
The length of the bond decreases with
increasing numbers of bonds.
Comparison of Bonding Types
Compare Ionic, Polar and
Non-Polar Bonding
Electronegativity
Electronegativity is the relative tendency of a
bonded atom to attract electrons to itself.
An atom with extremely low electronegativity,
like a Group I metal, is said to be
electropositive since its tendency is to lose
rather than to gain, or attract, electrons.
Non-metals are more electronegative than
metals.
http://www.ausetute.com.au/bondpola.html
Electronegativity Values
and Bond Types
Electronegativity values are useful in
determining if a bond is to be classified as
non-polar covalent, polar covalent or ionic.
What you should do is look only at the two
atoms in a given bond. Calculate the
difference between their electronegativity
values. Only the absolute difference is
important.
http://dbhs.wvusd.k12.ca.us/webdocs/Bonding/Electroneg-Bond-Polarity.html
Electronegativity
and Associated Bond Type
< 0.5
0.5 – 1.9
2.0 or greater
non-polar covalent
polar covalent **
ionic
Note: To find the electronegativity
values of many common
elements, look at the chart printed on the
bottom of your orbital diagram.
** there are some exceptions
Calculating
Electronegativity Values
Calculate the electronegativity and determine
the bond type of an O-H bond
O has an electronegativity of 3.5
H has an electronegativity of 2.1
The difference in electronegativity is:
3.5 - 2.1 = 1.4
1.4 is less than 1.7, so the resulting bond is polar
covalent.
AP Students Use This Slide
Here are the rules:
1. If the electronegativity difference (usually called
ΔEN) is less than 0.5, then the bond is nonpolar
covalent.
2. If the ΔEN is between 0.5 and 1.6, the bond is
considered polar covalent
3. If the ΔEN is greater than 2.0, then the bond is
ionic. That, of course, leaves us with a problem.
What about the gap between 1.6 and 2.0? So, rule
#4 is:
4. If the ΔEN is between 1.6 and 2.0 and if a metal is
involved, then the bond is considered ionic. If only
nonmetals are involved, the bond is considered
polar covalent.
http://dbhs.wvusd.k12.ca.us/webdocs/Bonding/Electroneg-Bond-Polarity.html
AP Example
Sodium bromide
(formula = NaBr; ENNa = 0.9, ENBr = 2.8) has a
DEN = 1.9
Hydrogen fluoride
(formula = HF; ENH = 2.1, ENF = 4.0)
has the same DEN.
We use rule #4 to decide that NaBr has ionic
bonds and that HF has a polar covalent bond in
each HF molecule.
Electronegativity Trend
Electronegativity decreases down a Group in the
Periodic Table as the atomic radius and number
of inner electron shells both increase.
Electronegativity increases across a Period of
the Periodic Table, in general, due to increasing
nuclear charge and decreasing atomic radius.
For the commonly encountered atoms in high
school science, the order in decreasing
electronegativity is:
F > O > N ~ Cl > Br > C ~ S ~ I > P ~ H > Si
Metallic Bonding
Metallic bonding is characterized by a
“sea of electrons.”
Dipole
Polarity results from the uneven partial
charge distribution between various atoms
in a compound.
Atoms, such as nitrogen, oxygen, and
halogens, that are more electronegative
have a tendency to have partial negative
charges.
Atoms, such as carbon and hydrogen,
have a tendency to be more neutral or
have partial positive charges.
Dipole
Electrons in a polar covalent bond are
unequally shared between the two bonded
atoms, which results in partial positive and
negative charges. The separation of the
partial charges creates a dipole.
The word dipole means two poles: the
separated partial positive and negative
charges. A polar molecule results when a
molecule contains polar bonds in an
unsymmetrical arrangement.
Dipole
Non-polar molecules are of two types.
Molecules whose atoms have equal or
nearly equal electronegativities have zero
or very small dipole moments.
A second type of nonpolar molecule has
polar bonds, but the molecular geometry is
symmetrical allowing the bond dipoles to
cancel each other.
Dipole Moment
The dipole moment is a measure of the
unevenness, or lack of symmetry, of the
charge distribution in a molecule.
The mathematical definition of the dipole
moment involves adding up the size of
each charge in the molecule multiplied by
the average distance that charge is from
an arbitrary origin.
www.chem.unsw.edu.au/.../dipolemoments.html
Dipole Moment
symbol for dipole moment
→
Formal Charge
This is a method for determining the most
probable structure of a molecule.
It is ideal to have a formal charge of 0 for
as many of the atoms as possible.
Formal charge = [# of valence electrons] –
[electrons in lone pairs + 1/2 the number of
bonding electrons]
Formal Charge
See page 280 in the “old” B & L text
http://www.chemprofessor.com/bonding_files/image033.jpg
Try this website
http://www.mhhe.com/physsci/chemistry/c
arey/student/olc/ch01lewis.html