SMU - resonance and formal charge

Question 1

what are valence electrons, and how are they calculated

Answer 1

they are the electrons that take part in the chemical bond,
- caclulated by adding up the no. of electrons in the outer shell of each atom in a molecule (dont forget if there are any charges).

Question 2

what are lewis structures

Answer 2

they are just pictoral representations that hsow valence electrsons as dots around an atom, and bonds as lines

Question 3

how many valence electrons are in the following -
1. NO3^-
2. CF4
3. PH3

Answer 3

1. 24 e-
2. 32 e-
3. 8 e-

Question 4

when drawing a lewis structure of a molecule, which atom will be the central atom.

example - CF4

Answer 4

the least electronegative molecule, or the one with the lewest subscript, will be the central molecule.

here the C atom is least electronegative, and lowest in number. so carbon is the cetra atom.

Question 5

what are resonance structures
- give example

Answer 5

when a molecule has more than one lewis structure, with morepossible ways or arranging the electrons.

NO3^- the double bond between N=O can be formed between either of the 3 available oxygen atoms.

Question 6

what is a hybridised resonance structure

Answer 6

it is a combination of all 3 resonacne structures, tht contribute to the hybird structure.
sometimes, one of the available resonance structures might contribute to more to the hybrid than the others.

Question 7

other than NO3^- give examples of other resonance structures. 2 examples

Answer 7

ozone, and benzene

Question 8

what is the octect rule

Answer 8

the tendency of atoms to prefer to have 8 electrons in their valence shell.

Question 9

what atoms are an exception of the octect rule

Answer 9

1. incomplete octects
2. expanded octect
3. odd number of electrons

Question 10

why are third period elements exceptions to the octect rule

Answer 10

cos elements in the thrid period and below, can accomodate more than an octect of electrons. however, its better to assume a octect strucutre here, as it;s their most stable form.

Question 11

what is the formal charge

Answer 11

the charge assigned to an atom in a molecule, assuming that electrons in all chemical bonds are shared equally between atoms, regardless of their electronegativites.

Question 12

why is formal charge used by chemists

Answer 12

to analyse molecules. and specifically to find which resonance structure contributes mostly to the resonance hybrid.

Question 13

what is equation to work out the formal charge

Answer 13

FC = VE - [LE + 1/2(BE)]

FC - formal charge
VE - valence electrons
LE - lone electrons
BE - bonding electrons