Shape of the molecules

Question 1

What is used to predict the shapes of molecules and polyatomic ions?

Answer 1

VSEPR (valence shell electron pair repulsion)

Question 2

How can the number of electron pair surrounding a central atom be found?

Answer 2

• taking the total number of valence (outer) electrons on the central atom and adding one for each atom attached
• adding an electron for every negative charge
• removing an electron for every positive charge
• dividing the total number of electrons by two to give the number of electron pairs

Question 3

How are electrons pairs arranged?

Answer 3

(electron pairs are negatively charged and repel each other)

They are arranged to minimise repulsion and maximise separation

So the bond angle is as apart as possible

Question 4

The arrangement of electric pairs around a central atom for different numbers of electron pairs:
(ALL e- pairs are bonding)

Answer 4

2-linear
3-trigonal planar
4-tetrahedral
5-trigonal bipyramidal
6-octahedral (90degrees)

Question 5

What are the shapes of molecules and polyatomic ions determined by?

Answer 5

the shapes adopted by the atoms present based on the arrangement of electron pairs.

(electron dot diagrams can be used to show these arrangements)

Question 6

order of electron pair repulsion strength
(decreasing strength)

Answer 6

non-bonding pair/non-bonding pair
>
non-bonding pair/bonding pair
>bonding pair/bonding pair

Question 7

What does the shape of a molecule determine?

Answer 7

its properties and uses

Question 8

shapes of molecules where all e- pairs are bonding pairs

Answer 8

CO2 -> 2 bonding e- pairs ->linear
BF3 -> 3 bonding e- pairs ->trigonal planar
CH4 -> 4 bonding e- pairs -> tetrahedral
PCl5 -> 5 bonding e- pairs -> trigonal bipyramidal
SF6 -> 6 bonding e- pairs -> octahedral (appears most) 90 degrees

If there are any lone pairs, then the shapes of those molecules will be derived from these shapes