'A' levels Qn ('O' levels optional)
Q1) Draw the Kekule and/or dot-&-cross structure to
represent sodium nitrate(V).
Q2) Draw the Kekule and/or dot-&-cross structure to
represent carbon monoxide.
Solution :
For 'O' level students, you have a choice - either bo hiu / bo
chap such questions, and don't bother about drawing structures of
compounds that you won't be asked to draw at 'O' levels (assuming
you *are* at 'O' levels... are you?), or if you want to learn them,
then self-educate (this is the age of the internet - google be thy
sword, wikipedia be thy shield) yourself about Kekule structures,
Lewis structures, lone pairs of electrons, bond pairs of electrons,
dative covalent bonding, electronegativity, electron orbital
hybridization, etc.
You can also easily google the Kekule and/or dot-and-cross
structures of any compound you want. But it makes more sense, if
you learn and understand the underlying principles, so you can
make better sense of any structure you google out.
You may or may not understand the following (its ok not to
understand if you're 'O' levels (since it's not required at
'O' levels), but if you're 'A' levels, then you definitely must
know how to draw the Kekule and dot-and-cross structures of
polyatomic ions like nitrate(V) ion), which is the reasoning and
thought process that 'A' level students have to do go through, in
drawing structures like the polyatomic nitrate(V) anion.
To draw the nitrate(V) or (NO3)- ion (it's recommended the
student draw the Kekule structure first, then translate to
dot-&-cross notation) :
1) First draw the central atom, N.
2) Then draw 3 O atoms, single bonded to it.
3) The fact that the anion is uninegative, means at least one of
the O atoms must be negatively formal charged.
4) In other words, that O atom must have 3 lone pairs and one
bond pair.
5) Hence, try drawing 1 single bonded O, and the 2 other O atoms
be double bonded to N.
6) But in which case, N has violated its octet. Being in period
2, it does not have empty 3d orbitals with which to expand its
octet.
7) Consequently, you realize that 2 of the O atoms must be
single bonded (ie. each with 3 lone pairs and 1 bond pair).
8) Consequently in turn, you realize that the N atom must have a
postive formal charge, because
8ai) you have 4 bond pairs for N, ie. 4 valence electrons near
the nucleus of N, and indeed, N is in Grp V (ie. 5 valence
electrons for no charge); so the N atom should
indeed have a +ve formal charge.
8aii) the entire anion is uninegative, and since 2 of the O
atoms have a -ve charge; so to cancel out the 2nd -ve
charge, the N atom should indeed have a +ve formal
charge.
8b) Since (ai) and (aii) agree with each other, this supports
the validity of your Kekule structure.
9) Your N atom has trigonal planar electron geometry with
sp2 electron orbital hybridization (the unhybridized p orbital of N
overlaps with a p orbital from the double bonded O atom, to form
the pi bond).
Once you've drawn your NO3- anion, with individual formal
charges inside the brackets, and a overall uninegative charge
outside the square brackets; draw the unipositive Na+ cation,
also with square brackets. The electrostatic attraction
between the cation and anion represent ionic bonding. So obviously,
sodium nitrate(V) has ionic bonding between the cation and anion,
but covalent bonding within the anion.
For carbon monoxide.
The dative (ie. "donated") covalent bond is from oxygen to
carbon. Meaning, one lone pair from oxygen becomes a bond pair
between oxygen and carbon.
Your Kekule or dot-&-cross structure should show :
Carbon having 1 lone pair and 3 bond pairs.
Oxygen having 1 lone pair and 3 bond pairs.
Hence, carbon has a -ve formal charge, and oxygen has a +ve
formal charge.
This is due to the dative (ie. "donated") covalent electron bond
pair; oxygen has lost an electron, carbon has gained an
electron.
The reason why the dative bond occurs or exists, is to allow
both C and O to have a stable octet.
For physical chemistry (but not organic chemistry*), reflect the
nature of the dative bond with an arrow-headed line (as
opposed to a normal line for a normal or non-dative covalent bond),
for the Kekule structure.
For the dot-&-cross structure, the dative bond should be
represented as xx or oo (where x or o represent electrons from
oxygen), as opposed to xo or ox for a normal or non-dative covalent
bond.
(* In O.C., curved arrows represent electron flow mechanism. And
once the dative bond is formed, it is no different from any other
covalent bond. Hence in O.C., you only show the formation of the
dative bond using curved arrows; thereafter, normal lines to
represent the bonds.
In P.C., the arrow-headed line to represent the dative bond, is
actually the mechanism for the formation of the dative bond. If in
the exam, you're unsure which diagram to submit (ie. you're unsure
whether it's an O.C. or P.C. type question), do what the
intelligent exam candidate would do - draw both alternative answers
or forms, as long as both are correct, you'll get your marks.)
Hope you had fun. Everyone loves Chemistry! 