molecular orbitals & bond order

can anyone explain how:
O2 has a bond order of 2
N2 has a bond order of 3
what would the bond orders of N and F (NOT N2 and F2) be?
what would the bond order of NF be?
I realise that they don`t all exist in real life.
Thank`s.

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Re: molecular orbitals & bond order

Elements try to have a stable electronic configurations- 8 or 0 electrons in the last electronic level. Nitrogen has 5 valentic electrons (5 electrons in the last energy level) and it tries to have 8. Because of that, a bond between two N atoms in the N2 molecule is triple [in the covalent bond (bond between non-metal and non-metal) atoms in the molecule share one, two or three electronic pairs, so when two N atoms (with 5 valentic electrons) share 3 electronic pairs ( one electron from the pair comes from from the each atom), every one of them has 8 electrons.]
Oxygen has six valentic electrons, so the bond between atoms in O2 molecule is double.

[quote="electonegative"]what would the bond orders of N and F (NOT N2 and F2) be?
[/quote]
A bond between what?

In nitrogen fluorides bond is also covalent. :)

Hi,thank`s for reply.
I thought bond order was down to bonding & non-bonding electrons,as in the 2 pi orbitals?
thanks.

what you have to remember is that there's not actually any such thing as electrons - they're just made up lol

What you just got there was an explanation in terms of good 'ole GN Lewis' valence bond theory.

You're right, you could also give a more sophisticated and mathematical description in terms of MO (molecular orbital) theory,
with sigma & sigma-star, pi and pi* etc etc orbitals.

it's just another way of looking at it.

Whatever theory you try, you're bound to find it breaks down a bit with more complicated molecules.......

So can anyone shed any light on what I`ve asked?
As in the anti-bonding & bonding electrons giving the bond order.
thank`s.

The standard way to do it is to draw a molecular orbital diagram for your molecule you have three columns - the 2 on the outside have the atomic orbital (2s, 2p, whatever......)
and in central column has these mapped to molecular orbitals (sigma, sigma*, etc)

Then you draw in your electrons, as supplied by the atoms (as little up-arrows and down-arrows)...... fill in the molecular orbitals in order of lowest energy first....

every pair of electrons in a bonding molecular orbital counts as "1 covalent bond",
every pair of electrons in an anti-bonding orbital counts as -1 bond.

Add 'em up and see what you're left with....

It's practically impossible to illustrate in this ASCII forum though! Look in any half-decent text-book, you'll see......

aah,I ve been missing the point on [quote]every pair of electrons in a bonding molecular orbital counts as "1 covalent bond",
every pair of electrons in an anti-bonding orbital counts as -1 bond. [/quote]

thank`s.

Bond order is simple enough if you know how to draw lewis structures. Draw the lewis structures for the molecules and count how many bonds are made, that will tell you what bond order it is. This only works for the most common cases, O2 is a rare example of a diradical species that can only be properly deduced by using more advanced theory than lewis dot structures.

How about SO3 lewis structure?
S
/
o o-
\ /
o-
s = sulfur and o = oxygen
but can u do this?
S
/ \
o o
\ /
o
can u do that?
So the valence is nothing.

well, just drawing lines on a page doesn't really explain much -
if you want a more sophisticated explanation which correlates
better with spectroscopy results on the molecule,
use MO theory....

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