chemical bonding. attractive forces that hold atoms together in compounds. the electrons involved...
TRANSCRIPT
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CHAPTER 7Chemical Bonding
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Attractive forces that hold atoms together in compounds. The electrons involved in bonding are usually those in the outermost (valence) shell.
Most elements in compounds want to gain noble gas configuration. They will do so by either losing or gaining electrons (ionic compounds) or by sharing electrons (covalent compounds)
Chemical bonds
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Chemical bonds are classified into two types:
Ionic bonding results from electrostatic attractions among ions; which are formed by the transfer of one or more electrons from one atom to another. (metals low with nonmetals high )χ χ
Covalent bonding results from sharing one or more electron pairs between two atoms.
(nonmetals only similar )χ
Ionic and Covalent bonding
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Comparison of Ionic & Covalent Compounds
Ionic Covalent
Melting Pt
Solubility◦ (polar solvents)
Solubility◦ (nonpolar solvents)
Conductivity◦ (molten & aqueous
solutions)
High Low
Soluble Insoluble
Insoluble Soluble
High Low
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2 extremes in bonding
pure covalent bonds◦ electrons equally shared by the atoms
pure ionic bonds ◦ electrons are completely lost or gained by one of
the atoms
most compounds fall somewhere between these two extremes
Ionic vs. Covalent bonding
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# of atoms in the molecule◦ Monatomic = 1 atom Ex. He◦ Diatomic = 2 atoms Ex. O2
◦ Triatomic = 3 atoms Ex. O3
◦ Polyatomic = many Ex. H2SO4 or S8
Homonuclear: the mlcl is composed of only 1 kind of atom: O2, H2, P4
Heteronuclear: the mlcl is made up of more than 1 kind of atom: H2O
Terminology
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Lewis Dot Representations of Atoms
Li Be B C N O F Ne.... .. ..
..HeH
.
.. . .
.. ..
..
...
..
.. .
...
.
... ..
.
or Lewis dot formulas, a convenient bookkeeping method for valence electrons (electrons that are transferred or involved in chemical bonding) Only the electrons in the outermost s and p orbitals are shown as dots.
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Li & Na. .
N & P.. ..
..
. ..
. F & Cl...
....
.
... ..
.
elements in the same group have same Lewis dot structures
For groups IA – VIIIA, the group number equals the # of valence electrons
Valence electrons determine the chemical and physical properties of the elements as well as the kinds of bonds they form.
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metals react with nonmetals to form ionic compounds
cations or positive (+) ions (metals)◦atoms have lost 1 or more electrons
anions or negative (-) ions (nonmetals)◦atoms have gained 1 or more electrons
Ionic Bonding
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C842 MPsolid gas metal
white yellow silver
LiF2 F Li2
- VIIA IA
o
(s)2(g)(s)
We can use Lewis formulas to represent the neutral atoms and the ions they form.
Li + F...
.... .
Li+
F[ ]...... ..
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underlying reasons for LiF formation
1s 2s 2pLi ̄F ̄̄̄̄
becomesLi+ ̄ [He]F- ̄̄̄̄̄[Ne]
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Li+ ions contain two electrons◦ same number as helium
F- ions contain ten electrons ◦ same number as neon
Li+ ions are isoelectronic with heliumF- ions are isoelectronic with neon
Isoelectronic species contain the same number of electrons.
cations become isoelectronic with preceding noble gas
anions become isoelectronic with following noble gas
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....
... F..
. F....
Be .. Be2+
2 F
.... ....
IIA metals with VIIA nonmetals, mostly ionic compounds ~ exceptions - BeCl
2, BeBr
2, BeI
2
these are covalent compounds
Be(s) + F2(g) ®BeF
2(s) electronically this is
happening
similarly for all of the IIA & VIIAM(s) + X
2 ® M2+ X
2-
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IA + VIIA MX
IIA + VIIA MX2
IIIA + VIIA MX3
IA + VIA M2X
IIA + VIA MX
IIIA + VIA M2X
3
NaF
BaCl2
AlF3
Na2O
BaO
Al2S
3
SUMMARIZING TABLE Groups Gen. Form. Example
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IA + VA M3X
IIA + VA M3X
2
IIIA + VA MX
Na3N
Mg3P
2
AlN
SUMMARIZING TABLE Groups Gen. Form. Example
H forms ionic compounds with IA and IIA metalsLiH, KH, CaH
2, BaH
2,, etc.
other H compounds are covalent
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extended three dimensional arrays of oppositely charged ions
high melting points because coulomb force is strong
Structures of Ionic Compounds
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Coulomb’s Law
ions ofcenter between distance d
ionson charge of magnitude q
ionsbetween attraction of force F
whered
qqF
2
~ ions with high charges F is large
~ ions with small charges F is small
arrange these compounds in order of increasing attractions among ions
KCl, Al2O
3, CaO
K+Cl- < Ca2+O2- <Al23+O32-
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covalent bonds formed when atoms share electrons
share 2 electrons - single covalent bondshare 4 electrons - double covalent bondshare 6 electrons - triple covalent bond
attraction is electrostatic in nature◦ lower potential energy when bonded
Covalent Bonding
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Covalent bonding may be explained by 2 different theories
◦ Valence bond (VB) theory: each atom has electrons in atomic orbitals which overlap to form bonds (Ch. 8)
◦ Molecular orbital (MO) theory: the electrons belong to the molecule as a whole and are in molecular orbitals instead of belonging to each atom (Ch. 9)
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The element needing the most electrons to fill its octet is usually the central atom
The most symmetrical skeleton is usually correct
Halogens and H always share one electron to complete outer shell
In ternary acids, H are bonded to O (ternary acids are oxy-acids: they contain H, O, and another nonmetal)
General rules for Lewis Dot Diagrams for Covalent bonds
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Carbon always obeys the octet rule
Carbon rarely has lone pairs of electrons. Exception: If it’s at the end of a molecule or ion. Ex. CN- , CO, CNO
When forming multiple bonds between atoms, both atoms donate the same number of electrons
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Oxygen atoms normally bond to other nonmetals, not to each other
Oxygen can do several things depending on the mlcl.◦ Single bond by sharing an electron
◦ Single bond by accepting 2 electrons from another atom and not sharing at all
◦ Double bonds by sharing 2 of its electrons
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homonuclear diatomic molecules◦ hydrogen, H2
◦ fluorine, F2
◦ nitrogen, N2
Pure covalent bonds - Nonpolar Covalent Bonds
H HorH H..
F F.. .. ....
..
.. ..F F
.. .... ..
.. ..or
N N········ ·· N N·· ··or
nonpolar covalent bonds - electrons are shared equally
symmetrical charge distribution - must be the same element to
share exactly equally
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+H. H . H H.. or H2
Lewis dot representation H2 molecule formation
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heteronuclear diatomic molecules
hydrogen halides◦ hydrogen fluoride, HF
◦ hydrogen chloride, HCl
◦ hydrogen bromide, HBr
Polar Covalent bonds - Unequal sharing of electrons
or ··H F··
··H F..
······
or ··H Cl··
··H Cl..
······
or ··H Br··
··H Br..
······
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polar covalent bonds - unequally shared electrons
• assymmetrical charge distribution
• different electronegativities
Some bonds are very polar, Ex. HF
bondpolar very 1.9 Difference
4.0 2.1 ativitiesElectroneg
F H
1.9
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Polar Covalent Bonds
Electron density map of HF◦ blue areas - low electron
density◦ red areas - high electron
density
polar molecules have separation of centers of negative and positive charge
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bondpolar slightly 0.4 Difference
2.5 2.1 ativitiesElectroneg
I H
0.4
Some bonds are only slightly polar, ex. HI
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Polar Covalent Bonds
Electron density map of HI◦ blue areas - low electron
density◦ red areas - high electron
density
notice that the charge separation is not as big as for HF◦ HI is only slightly polar
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Representative elements achieve noble gas configurations in most of their compounds.
Lewis dot formulas are based on the octet rule.
H needs two electrons to have Helium's noble gas configuration, everything else wants 8
The Octet Rule
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water, H2O ammonia molecule , NH3
ammonium ion , NH4+ hydrogen cyanide, HCN
sulfite ion, SO32-
Lewis Dot Formulas for Molecules and Polyatomic Ions
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Two or more Lewis dot diagrams are needed to describe the bonding in a molecule or ion.
LDD for sulfur trioxide, SO3
Resonance
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Resonance
three possible structures for SO3
invoke resonance◦ Double-headed arrows are used to indicate
resonance formulas.
O S
O
O·· ······ ··
······
OS
O
O·· ···· ·· ··
··
······
O S
O
O·· ····
·· ··
····
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Resonance
flaw in our representations of molecules
no single or double bonds in SO3
all bonds are the same
best picture OSO
O
--- ---
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The concept of formal charges helps us choose the correct Lewis structure for a molecule. If a resonance structure has a high formal charge it’s not a very good one.
Formal charge = group # - e- you can assign
to that atom
Or F.C. = (valence e- ) – (# of bonds + # of
unshared e- )pg 289
Formal Charges
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Sigma bonds (σ) : result of head-on (end to end overlap, there is a free rotation around σ bonds.
Pi bonds (π) : result of side-on overlap of p orbitals. There is no free rotation around a π bond. The side –on overlap locks the molecule into place.
All single bonds are sigma bonds: 1σ bondAll double bonds: 1 σ bond, 1 π bondAll triple bonds: 1 σ bond, 2 π bonds
Sigma and Pi bonds
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Limitations of the Octet Rule for Lewis Formulas
species in which the central element must have a share of more or less than 8 valence electrons to accommodate all substituents
compounds of the d- and f-transition metals
In cases where the octet rule does not apply, the elements attached to the central atom nearly always attain noble gas configurations. ◦ The central atom does not
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Write LDD for BBr3
Write LDD for AsF5
Write LDD for XeF4
Limitations of the Octet Rule for Lewis Formulas
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As we all know, in the wintertime we are more likely to get shocked when we walk across carpet and touch the door knob. Here’s another experiment to perform. Turn on a water faucet until you have a continuous but small stream of water coming from the faucet. Brush your hair vigorously then hold the brush near the stream of water. You will notice that the stream bends towards the brush. Why does the water bend?
On a “infomercial” it claimed that placing a small horseshoe magnet over the fuel intake line to your car’s carburetor would increase fuel mileage by 50%. The reason given for the mileage increase was that “the magnet aligned the molecules causing them to burn more efficiently.” Will this work? Should you buy this product?