Limitations of models

Question 1

What can the formula for a substance be written as?

Answer 1

an empirical formula
a molecular formula
a structural formula

Question 2

empirical formula

Answer 2

simplest whole-number ratio of atoms of each element

not usually the actual number of atoms

Question 3

molecular formula

Answer 3

number of atoms of each element

does not show how they are arranged

Question 4

structural formula

Answer 4

number of atoms of each element does give an idea of how they are arranged

(e.g. CH3COOH)

Question 5

What does a stick diagram show and not show?

Answer 5

• which atoms are covalently bonded together
• how many covalent bonds are in the molecule
• the chemical symbols of each element
• the molecules 3D shape
• the bonding and non-bonding electrons

Question 6

What does a ball and stick model show and not show?

Answer 6

• how each atom is bonded to other atoms
• the molecules 3D shape
• bonding and non-bonding electrons
• each elements chemical symbol

Question 7

What does a space-filling model show and not show?

Answer 7

• similar to ball and stick models
• accurately represent the size of atoms relative to their bonds
• may not be able to see all atoms in a complex model
• same as ball and stick model

Question 8

What do dot and cross diagrams show and not show?

Answer 8

• covalent bonds in the molecule
• bonding and non-bonding electrons
• chemical symbols

-3D structure

Question 9

Describe the information a dot and cross diagram provides and state a limitation of this model.

Answer 9

• shows the symbol for each atom in the molecule
• shows how each atom is bonded to other atoms
• pairs of electrons are represented by dots and crosses
• non-bonding pairs of electrons in the outer shell are included

-does not show the three-dimensional shape of the molecule

Question 10

State one limitation of these models:

a) stick diagrams
b) ball and stick models
c) space-filling models
d) empirical formula
e) molecular formula
f) structural formula
g) dot and cross diagram

Answer 10

a) 3D shape of molecule
bonding/non-bonding electrons
size of atoms relative to bonds

b) element symbols
bonding/non-bonding electrons
size of atoms relative to bonds

c) element symbols
bonding and non-bonding electrons
some atoms in complex models

d) actual number of atoms
how atoms are arranged
3D shape of molecule
bonding/non-bonding electrons
size of atoms relative to bonds

e) how atoms are bonded
3D shape of molecule
bonding/non-bonding electrons
size of atoms relative to bonds

f) 3D shape of molecule
bonding/non-bonding electrons
size of atoms relative to their bonds

g) 3D shape of molecule
size of atoms relative to bonds