(bondtype.gif) As shown in Feynman, a 1% error in electron accounting would lead to a totally awesome force between everyday objects. Nature does a much better job of balancing its currency (electrons) than we humans do ours (dollars). It is by electrons that chemistry happens. We are ready to use our new-found knowledge about atomic electron configurations to see how molecules (at last!) are put together. We will:
If you are working from a graphics-capable browser, you should try to draw some of the Lewis structures before clicking on the link. See if you get them right. In many cases, you should try to draw structures showing how the structure being formed from atomic dot structures: e.g., H* + *H = H**H (* = electron).
(bondtype.gif) Define: An ion is an atom or molecule that carries a net charge.
Cation = + ion
Anion = - ion
Na ---> Na + + e-
Note balance of charge! No charge on left side of arrow. None on right.
Now look at Magnesium
Mg = 1s2 2s2 2p6 3s2
Mg ---> Mg++ + 2e-
Mg++ = 1s2 2s2 2p6 Like Neon!
If atoms like Na and Mg give up electrons, where do they go?
Ans..... to elements on other side of the periodic table!
Consider
Cl = 1s2 2s2 2p6 3s2 3p5
Cl + e- ---> Cl- (likear.gif)
Or....consider this one:
N = 1s2 2s2 2p3
Let it gain three electrons to become nitride: (nitride.gif)
Ionic Compounds
Suppose we put Na (a metal) near
F (nonmetal). Here is what happens
Na ---> Na+1 + e-
e- + F ---> F-
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Partial shell representation for formation of NaF: (naf.gif)
If Na loses an electron & F gains it they both become neon-like.
THIS IS CALLED IONIC BONDING. One atoms takes, the other provides.
Suppose we repeat with Mg and F
Mg ---> Mg2+ + 2e-
2e- + 2F --->2F-
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Net Reaction: Mg + 2F ---> Mg2F
It takes TWO Fluorines to soak up the TWO electrons from the ONE magnesium.
One oxygen could serve same "soakup" function as 2 fluorides
Mg ---> Mg2+ + 2e-
2e- + O ---> O2-
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Mg + O ---> MgO Magnesium Oxide
Let's do one that takes more than 1 metal
3 Na ---> 3Na+ + 3e-
3e- + N ---> N3-
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3Na + N ---> Na3N Sodium nitride
Let's do one that takes more than 1 metal & more than 1 nonmetal
e.g. Ca + N
Ca ---> Ca2+ + 2e-
3e- + N ---> N3-
Doesn't balance!
Multiply top by 3 and bottom by 2
3 Ca ---> 3Ca+2 + 6e-
6e- + 2N ---> 2 N3-
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3Ca + 2 N ---> Ca3 N2 (Calcium nitride)
How many electrons an element wants to give up or gain.
Valence of Na is 1
Valence of Cl is 1
Valence of Mg is 2
Valence of N is 3
etc .....
This definition is somewhat unfortunate, because "valence" has two meanings. For example, the "valence" of nitrogen is 3 in the sense that it often reacts with three of some other atom. However, it has five "valence" electrons--electrons in the highest shell. Lots of words have two meanings. Only in science is it very confusing.
e.g. NaCl, BaCl2, CsI2, Na3N
etc.
implies shared electrons; neither atom has a full charge
Covalent bonds require the skill of a chemist to break.
Breaking an ionic bond is often as simple as adding water
NaCl ---> Na+ + Cl-
H2O
Breaking a Covalent bond often requires the skill of the chemist to break, and we spend much time rearranging bonds.
However, if you pull real hard on rubber band until it breaks, you will have broken some covalent bonds.
Not everything is black and white like the all or nothing electron transfer of ionic bonds.
Ionic bonds can't explain molecules like Cl2. Both atoms in a Cl2 molecule want the electrons equally.
.....But the octet rule still can!
So you don't believe atoms are like people? Try cats instead. (cats.gif)
Chlorine (chlorine.gif)
If you don't understand these pi bonds, don't worry about it. We'll come back to it. Just understand why
there are two bonds.
Covalent Bonds Between Different Atoms, like HCl
Cl wants electrons more than H.
...... Cl is more electronegative
We could say that the HCl molecule is partly polarized
A dipole is two charges separated by a distance.
Dipoles don't collapse, because the charges are held fixed at a distance.
The dipoles of each bond add as vectors to give the net dipole of the whole molecule.
The self-aligning properties of dipoles cause molecules with net dipoles to be attracted to each other. They vaporize and melt at higher temperatures than molecules without dipoles.
.
Remember it's electrons that take space.
Nonbonding electrons take as much space as the bonded
As a result, the electrons around the oxygen are tetrahedral, and the molecule (described by where the atomic nuclei go) is bent. As a result of this, water has all its magnificent properties: high melting point and boiling point.
Are all covalent molecules made of nonlike atoms polar?
No! However, all bonds between nonlike atoms are polar. The reason is SYMMETRY. You must add the polarity vectors. If they cancel, the molecule is not polar. Vector addition is a physics topic. But if little pointy vectors are directly opposite, they obviously cancel. If they point out of a sphere in all directions equally, this also cancels. To see why: use silly putty to glue some pencils on a baseball. If you get it right, the baseball will not wobble (much) when you toss it in the air with a spin. If the pencils are not symmetrical, the ball will wobble. If you make a baseball with four tetrahedrally oriented pencils, it won't wobble.
Consider Methane:
Consider CO2
So far, we have seen single ions Na2O
NaCl
CaCl2
There are also "molecular ions" - covalent molecules can have a charge !
e.g. Sulfate ion
SO42-
How many valence electrons ?
S : 6 x 1 = 6
O : 6 x 4 = 24
Charge : 2- = 2
32 e- total
What does this molecular ion look like?
You need to be told that the sulfur is in center. (When in doubt, assume the least prevalent atom is centered.)
put electrons in to bond to S (takes 8)
satisfy octet for oxygen (takes 24)
total used: 32 electrons = OK
So this is sulfate:
How to treat a molecular ion: (likesmal.gif)
Let's do another: phosphate PO43-
P : 5 valence e-
4 x O : 24 valence e-
charge : 3 valence e-
32 total electrons
So this is phosphate:
Why is this -3 charged when SO4 ion is -2 ?
Answer: One less proton: P compared to S
Where are these electrons coming from to make the negative molecular ion?
Answer: Same place as for simple binary ions; the electrons come from metals !
e.g. Na3 PO4 sodium phosphate
Let's do another anion: cyanide, CN-1
1st rule of Lewis structures: ensure octet
2nd rule: "formal charges" must be minimized and sum to total charge on the molecule.
FC = Number of Valence Electrons on Free Atom
- (1/2)(Number of electrons in bonds)
- any nonbonded electrons.
Why is it defined this way?
1) Recall valence electrons is not the same as the "valence" of the atom. (See above note on valence.)
2) We could just as well define the FC this way:
FC = Number of Protons in atom
- electrons in inner core of atom
- (1/2)(Number of electrons in bonds)
- any nonbonded electrons.
Thus, the "valence electrons" in the first formula really represents the number of positively charged protons that are are not "cancelled out" by inner shell electrons.
Can we see why the Lewis structure for cyanide has -1 total charge?
FC on C is 4 - 0.5(6) -2 = -1
FC on N is 5 - 0.5(6)-2 = 0
Total is 0
