Chem bonding14
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Transcript of Chem bonding14
A: to be happy (stable)… atoms are social creatures. Some share, some take but there is almost always interaction… ok, so they are not really social but they do need to be stable
To be Happy is to be stable… to be stable is to have a full valence shell
All bonds form due to some level of electrostatic attraction:
Attractive force is proportional to: (+ q) (- q)
+ q = magnitude of the positive charge - q = magnitude of the negative charge r2 = distance between the charges Bigger charges means stronger bonds Charges that are closer together are stronger, farther apart
is weaker bond
r2
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Cl2 Chlorine gas is very toxic Na is a highly explosive metal, even in
water
But Na + Cl2 makes NaCl which is table salt.
So we take a poison gas and an explosive metal, react them, and then put it on our food.
How is this possible? Because they become STABLE
Atoms tend to gain, lose or share electrons in their valence shells in order to have a full electron valence shell. This is usually 8 electrons, hence the octet rule
When atoms valence shells are full, they are stable, and “happy”
Note that these are trends… THERE ARE ALWAYS EXCEPTIONS
One quick note: Transition metals seem to be a major exception to the octet rule. That is because when they form ions, they tend to lose e- from their outer s orbital first then as many as necessary d orbital e- to be stable… This is counter intuitive as it requires more energy to hold e- in the lower d orbital due to its angular momentum. This is why so many have more than one oxidation number
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Reactions which require energy from their environment to occur are Endothermic
Reactions which release a net amount of energy into the environment are Exothermic
Note: this energy is almost always in the form of heat, but is sometimes electrochemical (release or require electron flow) or chemiluminescent (require or release of light)
A bond formed between 2 or more atoms, by the sharing, or transfer of 1 or more electrons in order to satisfy the octet rule
Important note: All chemical bonding occurs in the valence shell electrons (VSE). That is those electrons that are found in the outer most shell
Whenever elements come in contact with each other, they compete for electrons all based on electronegativity: the ability of an atom to take and hold electrons. The results are chemical bonds
A: because the Valence shell electrons (VSE) are getting closer to the nucleus, so the attraction is greater
Electron shielding does not allow this to be perfect increase
A: because there are more protons in the nucleus so the positive force is greater
Again, Electron shielding does not allow this to be perfect increase
BY FAR, as the electrons get closer the attractive force gets greater. This has a far greater impact than simply adding a proton to the nucleus
Ionic Covalent
◦ (to a lesser extent) hydrogen Metallic
A bond formed from a SEQUENTIAL transfer of electrons and electrostatic attraction. The sequence:
1) Transfer of one or more electrons 2) Ion formation 3) Attraction
More highly charged ions will form stronger bonds (+3 and -3 is stronger than +1
and -1) Also those ions that are smaller in radius
will form stronger bonds than those that are larger in radius (Na+1 and Cl-1 will form a stronger bond than Rb+1 and Br-1)
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NaCl or LiF
CuO or CuCl
MnP or ZnO
CaBr2 or CaF2
For ANY bond , ionic character is determined by finding the difference in electronegativity between the two elements (regardless of how many of each)
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Tend to occur between metals and nonmetals (some occur between nonmetals and polyatomic ions)
Always occur between oppositely charged particles (called ions)
Tend to be VERY polar compounds Result as a difference in electronegativity
Usually dissociate easily in water, but remain stable ions
Usually good conductors of electricity in water, BUT NOT conductors of electricity as a solid
Tend to be brittle compounds with significant (and identifiable) crystalline structures, called lattices
1) Electrons are transferred: Na loses one electron and becomes positive
Na --------> Na+ + (Cation)
2) Ions form:Cl gains one electron and becomes negative
Cl + -------> Cl -1 (Anion)
Na atom single valence electron
Sodiums lost VSE transferred to Cl
Ions are now attracted to each other because of the opposite charge
Na+ + Cl -1 -------> Na Cl Notice how the positive one and negative one charges are
neutralized by each other, yet the compound itself is very polar
(+)NaCl(-)
Electrostatic attraction: Based on Coulomb's law, which describes attraction of oppositely charged forces AP
Oppositely charged ions are strongly attracted to each other but the compound is very polar: Referring to oppositely charged (poles) of force.
If referenced correctly an atom is the neutral (but not necessarily stable form of an element) such as ◦ Mg or Cl
Ion ALWAYS refers to a charged form of an atom or polyatomic ion such as ◦ Mg+2 or Cl- or NO3
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Notice when the VSE is transferred, the Na radius shrinks
When anions form from neutral atoms they actually get a little bigger because of the natural repulsion of el-
When cations form from neutral atoms, they get smaller because there are less electrons to repel each other AND because if the cation is stable it has shrunk to the next shell down
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Atomic: Single atoms with positive or negative charges- These can be found using the oxidation numbers on the periodic table
Polyatomic: 2 or more atoms bound together (usually covalently) acting together as an ion:
List of polyatomic ions (list is at top of link and also on next slide)
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Cations: Positively charged ions
Anions: Negatively charged ions
In atoms this is the charge an ion acquires after it becomes stable (these are sometimes referred to as ionic charges)… results from ionization process
Tendencies: Elements in groups tend to have common oxidation numbers in ionic compounds
GroupIA tend to be +1IIA tend to be +2VIIA tend to be -1VIA (nonmetals) tend to be -2 (ionic)VA (nonmetals) tend to be -3 (ionic)IVA (nonmetals) tend to be -4 (ionic)Boron tends to be +5 (ionic)Most transition metals can have more than one
Most transition metals can have more than one O.N. (refer to periodic tables)
Some very common transition metals have only one O.N. and you will need to know these: Ag=+1, Zn=+2, Al=+3
As a rule ions will arrange themselves to become a neutral compound
Ions possessing the same number of electrons…
N-3, O-2, F-1, Mg+2
These all have 10 electrons like the stable neon atom: 1S2, 2S2, 2P6
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A: yes, a wonderful place called the periodic table
Many elements have more than one ON
Click interactive table to find incredible amounts of information about all elements including oxidation numbers
Now that Ionic is over with, we get to the easy stuff!
Here elements, play nice and share valence shell electrons to become stable (satisfy the octet rule) and “happy”
Theoretically a nonmetal can form as many shared bonds as it has electrons in its valence shell
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Always occur between nonmetals Often form diatomic and sometimes polyatomic bonds
between atoms of the same element: O2 or O3
List of Diatomic molecules: H2, N2, O2, F2, Cl2, Br2, I2 This is how they naturally occur… free state
Tend to be very low and even NON-polar, and are therefore referred to as molecules (covalently bonded groups of atoms which tend to be mostly or completely neutral as a group)
Bond is very strong and can be single, double or triple bond
The first covalent BOND formed is called a sigma bond (all single-bonded molecules are sigma bonds)
Additional bonds between the SAME atoms are called Pi bonds
2 pi bond plus a sigma bond (triple bond) is the strongest and shortest
1 pi bond plus a sigma (double) is not quite as strong but not as short
A sigma (single) bond is weakest and longest
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Tend to be soft powders, liquids or gases at room temperature
Tend to have low melting points* points Do not dissociate easily in water Are not good conductors of electricity in
water
*Melting point: Point at which a pure substance goes from solid to liquid
Ionic compounds are always considered very polar
In general, the greater the ionic character, the greater the polarity
Covalent compounds are only nonpolar if they are exactly the same element, since one is not more electronegative than the other
Covalent compounds made of differing elements will have some polarity. The more electronegative element will exert more attraction to the valence electrons… keeping them more to one side of a molecule than another: we will discuss this later: intermolecular force
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Take note that in each of the following shared bonds, by sharing, they satisfy the octet rule and hence are stable
Exception: Hydrogen only needs 2 to have a full valence shell, but it is still stable at 2
single
H2 H2O
Oxygen gas O2 CO2
Nitrogen gas N2 6 shared electrons
As you might expect:◦ Single bond: weak◦ Double bond: stronger◦ Triple Bond: Strongest
Good conductor of heat Good conductor of electricity Many are highly malleable Many are highly ductile Many are hard and shiny
The bond itself forms as relatively fixed-position cations are surrounded by free-floating electrons. The free floating electrons are called the “sea of electrons.” As they surround the cations, they act as a flowing “glue” to hold the cations together but still give them the ability to flex and stretch without necessarily breaking… think of it like a Rice-Crispy treat…
Positive nucleii are
the rice Crispies
Free-floating
electrons are the
marshmallow “goo”
A blending of 2 or more metals or metal and Nonmetal NON-IONICALLY which leads to a new metal with different chemical and or physical characteristics
Many react with acids to produce salt and H2 gas
Oxidize fairly easily in presence of water or oxygen
Tend to form cations Some metals are more reactive than others
in a very predictable manner: see activity series on next frame
An important way to illustrate the process of bonding… particularly for covalent bonding
Process:◦ Count the total VSE◦ Adjust based on the charge of the molecule (PAI)◦ Design the structure with least EN element or the single
element in the middle◦ Start pairing up electrons (start with single bonds) between
bonds then place extra outside (starting with the central one) to satisfy the octet rule.
◦ Adjust the numbers in between the bonds to satisfy the octet rule
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1) Must be that correct number of e- 2) Must satisfy the octet rule 3) When there are more than one possible
structure that satisfy number 1 and 2 above, then use formal charge
CO3-2
Carbon: 4 = 4 Oxygen: 6 x 3 = 18 -2 charge means that there are 2 extra e-
Total: 24
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O
Are there 24 e- below? AND is the octet rule satsified?
O
O
C
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So we double bond one oxygen with the carbon:
Now the octet rule is satisfied AND there are 24 e-… You could put the double bond on any of the Oxygens… this means that this has resonance forms
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O
O
O
C
Used as a tool to help determine the best Lewis structure
When several Lewis structures are possible, the most stable one will be the one that:◦ Atoms have the smallest formal charges AND◦ Any negative charges are on the more EN atoms
1) All lone e- are assigned to that atom 2) Half of the bonding e- are assigned to it
Subtract the assigned e- from the ATOMS valence e-
The formal charges of all atoms should equal the compounds total charge
What is the formal charge of each of these Lewis dot structures?
Formal charge will equal molecular charge… if 0 then the formal charge should be 0… if -2 than it should be -2, etc
These are simple different forms of the same molecule
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Those elements that can be stable with less than 8 electrons. EG: H:2, Be (hydride):4, Li(hydride): 2B (freak): 6
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Atoms that have d subshells available, the central atom can have MORE than 8 electrons
Eg: PCl5
SF4
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There are some odd times when there is an odd number of e- in the valence shell… usually with nitrogen:
N O
N
O O
NO2 Has 2 resonance forms
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Valence shell electron pair repulsion model◦ Molecules with more than 2 atoms can have
varying shapes and are dictated by the electron pairs, regardless if they are in the bond or lone
◦ Each pair of electrons manifests a negative charge called an electron domain that repels other electrons which changes the molecular shape
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The best arrangement of a given number of electrons is the one that minimizes the repulsion among them
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Electron domain geometry: The arrangement of bonding or lone electron pairs around central atom ( a domain can be 1, 2, or 3 e- pairs in a bond)
Molecular geometry: the arrangement of the atoms which attach to the electrons around the central atom… Molecular shape as a result of the electron geometry
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1) diagram the Lewis dots model 2) count the e- domains and arrange them
in a way that minimizes the repulsion 3) Describe the geometry
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Table of molecular and electron pair geometry
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Orbitals around central atom which come from the combining of orbitals of differing atoms electrons…
SP orbitals
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1) Do the Lewis structure first2) Count the number of high density electron
areas (2 or more electrons, bonded or lone)
3) Each area represents an sp orbital
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A) SO2
B) NO3-
C) PCl3
Work these out
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A molecule has d2sp3 hybrid orbital… how many areas of high density electrons are there…
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d2=2 sp3=4
4+ 2 = 6
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Fundamental attractions between molecules… are weaker than intramolecular: Ionic, covalent, metallic
When there is a difference in EN of atoms in the same moment, it creates a dipole The side where electrons spend most of their time becomes partially negative compared with the other side which becomes partially positive
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The polarity of molecules is measured in the dipole moment. The larger the dipole moment the more polar the molecule is.
If the EN differential is greater on the two ends of the molecule then the dipole moment is greater
Also, the greater the distance between the charges, also the greater the value of the dipole moment
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Delta used for partial charge
Similar to the dipole-dipole attraction of molecules (although this is stronger than dipole-dipole). The positive hydrogen part of a molecule (like water) is attracted to the negative portion of another molecule which contains a highly electronegative element like F, O, N.
These have higher MP than dipole-dipole attractions because they are a stronger bond
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Fundamental attraction between neutral nonpolar molecules
Occurs because e- move randomly and at any instant there may be more on one side of a molecule than on the other, giving it momentary polarity
This is a temporary nonshielding by the electrons moving out of the way
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An attractive force between molecules often (not always) generated by (weak) partial polarity such as in water. Tutorial
Intermolecular bond: Here the delta (partial
charge0 is greater than dipole-dipole attraction
Intermolecular: Van DER Waals forces: dipole-dipole
Hydrogen London dispersion
Intramolecular: ionic, covalent, Metallic
Intramolecular is always stronger than intermolecular force
There are rules which vary slightly for Ionic and covalent compounds formula writing… fortunately the only rule for writing metallic formulas is the metal itself
Almost always write the cation (not the atom) first and the anion (not the atom) second
Mg+2Cl-1 (Note: Sometimes you will only see + or – next to the ion. It is an implied value of +1 or -1)
Now some basic math skills: Q: How many -1 charges are required to cancel out a +2
and make the net 0? A: 2
MgCl2 So for every Magnesium, there is 2 Chlorine
because we need 2 Chlorine ions to cancel out 1 Magnesium ion and make the whole thing neutral
Write the resulting formula for K and N Write ions: K+1N-3
It takes 3 positive ones to cancel out a negative three, so we need 3 K and 1 N
Result: K3N
There is a quick correlation here… do you see it?... See how the number 3 went from the N to the K?
Shortcut to follow…
Simply eliminate the charge signs, criss-cross the numbers down and simplify
K+1N-3
K3N Obviously there is no reason to show
the 1 from the K… That’s like saying 1X in algebra
Ca+2 S-2
◦ Ca2S2 should simplify to ◦ CaS◦ Don’t forget to simplify
Ba+2 N-3
◦ Ba3N2
◦ Done, since you can not simplify this any further
Same process
NH4+Cl-
NH4Cl
Treat Polyatomic ions as if they were their own element… don’t change them… just add more
Calcium ion and Phosphate Polyatomic ion Ca+2PO4
-3
Ca3(PO4)2
Notice how we used parenthesis around the polyatomic ion to multiply the phosphate to the number we needed?
UNLESS otherwise directed, do NOT change the polyatomic ion
Ammonium ion and Sulfate ion NH4
+SO4-2
What did you get? (NH4)2SO4
Covalent:
For all intent and purpose, you can use the same method as ionic. When two nonmetals bond to each other ONE possible formula can be made by criss-crossing. The more electronegative element will be Partially negative and the less electronegative element will be partially positive
By using the criss-cross method and simplifying you canDetermine ONE form of the covalent compound, although this compound doesn’t form the same way as ionic.
Trouble is, that since there single, double, and triple bondsThey can combine in different ways
Example
C + O
C+4 O-2
C2O4simplify
CO2 This is one form of carbon and oxygen but they also exist as just CO. CO has completely different physical characteristics than CO2
Covalent bonds are formed in necessary ratios to share electrons and satisfy the octet rule… the math is really tough!
If you can subtract single digits you should be OK
Ready?
Best to use an example to explain:
How would Nitrogen and hydrogen combine?
N-3 H+1 Yes this has ionic qualities but technically this is a covalent molecule because they are both nonmetals: NH3
Hydrogen is a weird element that can act as both a metal (H+) and a nonmetal (H-)
If you are not given the actual formula, then you can use a criss-cross method to determine ONE of the possible molecules.
Other forms will have to be named for you first… so let’s get to naming compounds
Different structural modifications of an element
This often leads to different chemical formulas of that element: S, S2, O2, O3
Again, naming ionic and covalent compound have different rules, but have some similarities
1) Name them in order of appearance (except acetate)… this is same for covalent naming
2) the cation is NOT modified at all unless it has multiple oxidation numbers. If so then you MUST state which one
3) The anion is modified to end with “ide” unless it is a poly atomic ion (never change the name of a poly atomic ion)
Let’s try some
1) Name the cation (only has one oxidation number): ◦ Calcium
2) Name the anion but change the end to “ide” since it is NOT a polyatomic ion◦ Oxide
3) Put it together: Calcium Oxide
Here’s some more…
NaSO4
Sodium Sulfate (Remember that you do not change the name of a polyatomic ion)
NH4NO3
Ammonium Nitrate (since they are both poly atomic ions, there is no change)
FeCl3 Iron has more than one Oxidation Number-so how do we write it?
It is still Iron but which species? Fe+2 or Fe+3
Here’s what you do: Look at the ANIONEvery Cl has a -1 in an ionic bond. Since there are 3
of them then there must be a total of -3 charge attached to iron, SO the iron HAS to be +3 to cancel out the -3 … Express this using roman numerals:
Iron (III) Chloride
1) BaBr2
2) Li3N
3) NaHCO3
4) Cu2O
5) (NH4)4C Answers on next frame
1) Barium Bromide
2) Lithium Nitride
3) sodium bicarbonate or sodium hydrogen carbonate
4) Copper (I) Oxide
5) Ammonium Carbide
Ionic naming games:
Naming covalent molecules/compounds requires more information because covalent compounds can have single, double or triple bonds:
Name each element in order but also adding a prefix which indicates how many there are unless there is only one:
From above example: hexacarbon decahydrogen
As with ionic compounds, the ending is changed to “ide”: hexoxide
So: hexacarbon decahydrogen hexoxide
Mono-1 Di-2 Tri-3 Tetra-4 Penta-5 Hexa-6 Hepta-7 Octa-8 Nona-9 Deca-10
1) C2H4
2) CH4
3) S2O2
4) SiO2
5) HF6) C6H8
Answers on next slide
1) Dicarbon Tetrahydride2) Carbon TetrahydrideDo not need to say monocarbon because if there is only ONE of the first element, it is
understood to be mono
3) Disulfur Dioxide4) Silicon Dioxide5) Hydrogen MonofluorideHere we are using covalent naming rules because they are both nonmetals but we can
(and usually do) use the ionic naming rules here because hydrogen acts as a metal. This would be hydrogen fluoride using ionic rules
6) Hexacarbon Octahydride
If Hydrogen is bonded to poly atomic ion, then use ionic naming
H2SO4 is hydrogen sulfate compound, where HSO4- is
hydrogen sulfate ion
If, however hydrogen is bonded to single nonmetalic elements, then use covalent rules:
H2O is dihydrogen monoxide
Finally, if hydrogen is bonded to a metal than it is an anion and uses ionic naming:
MgH2 is magnesium hydride
Games:
Lewis dots are a way to represent ionic and covalent compounds based on the valence shell electrons. Each valence electron is represented by a dot.
So where do we find the number of valence electrons???
Mg atom Mg
Mg ion Mg
Cl Atom Cl
Cl ion Cl