Lab Activity 16.1 - Molecular Models

Purpose:

To construct models of molecular compounds and draw their Lewis dot structures.

Material:

1 model kit, coded as follows:

color of ball	element
black	carbon
light blue	nitrogen
yellow	hydrogen
red	oxygen
green	chlorine
orange	bromine
purple	iodine or fluorine
connector	bond type
1” wooden peg	C to H, C to Cl or other univalent linkage
2” wooden peg	C to C, C to N or other bonds
2” springs	for multiple bonds (double, triple)

Procedure/Questions:

1) Inventory your model kit on the sheet provided in the inside cover of the kit.

2) Examine a black ball which represents carbon.

a) What group is carbon in?

b) How many valence electrons does carbon have?

c) How many electrons does carbon need to gain to form an octet?

d) How many bonds can a carbon atom form? Why?

e) How does the number of holes in the black ball compare to how many bonds carbon can form?

3) How many holes would you expect to find in a ball representing each of the following elements and why? Check the balls in the kit – were you right?

a) nitrogen

b) hydrogen

c) oxygen

d) chlorine

e) bromine

f) iodine or fluorine

4) Build each of the molecules in the data table, connecting the balls with wooden pegs or springs if there are multiple bonds. Make sure no holes are left empty.

5) Draw the structural formula & Lewis dot structures for each. Refer to the attached sheet “Summary of Structural formulas & Lewis Dot Structures”.

6) Check your answers here:

http://www.stolaf.edu/depts/chemistry/courses/toolkits/121/js/lewis/

Data Table p. 1/2

molecular formula	structural formula	Lewis Dot Structure
molecular fluorine F2
molecular nitrogen N2
hydrogen peroxide H2O2
carbon dioxide CO2
ammonia NH3

Data table p. 2/2

molecular formula	structural formula	Lewis Dot Structure
water H20
ethyl ether C2H5OC2H5
methanol CH3OH
acetylene C2H2
nitrous acid HNO2

Conclusions and Extensions:

1) Which ball in the kit might be used as phosphorus? Explain.

2) When you make dot structures for polyatomic ions, you add to or subtract from the total number of valency electrons allowed by the group A numbers. (See the last example in the attachment “Summary of Structural formulas & Lewis Dot Structures”). Draw the Lewis dot structures for the following and check them here:

http://www.stolaf.edu/depts/chemistry/courses/toolkits/121/js/lewis/

a) phosphate ion, PO₄ ^3-

b) nitrate ion, NO₃ ^–

3) Some elements do not obey the octet rule when forming compounds. Boron, for example, in group 3A only has three valence electrons and can only form 3 covalent bonds. Draw the Lewis dot structure for Boron trifluoride (BF₃) and check it here:

http://www.stolaf.edu/depts/chemistry/courses/toolkits/121/js/lewis/

4) A few atoms, especially phosphorus and sulfur, sometimes expand the octet to include 10 of 12 electrons. Visit the link below and explain how. Also, draw the Lewis Dot structures for SF₂, SF₄, and SF₆.

http://wblrd.sk.ca/~chem20bb/covmolec/movies/expand.html

5) Resonance structures occur when it is possible to write two or more valid electron dot formulas that have the same number of the electron pairs for a molecule or ion.

a) Write two different Lewis dot structures for ozone, O₃ and check them here:

http://www.stolaf.edu/depts/chemistry/courses/toolkits/121/js/lewis/

b) Scientists used to think that the molecule oscillated rapidly between the two forms, but it was discovered that the two bond lengths between the oxygens are intermediate between characteristic single and double bond lengths between a pair of oxygens. Which bond would you expect to be shorter, a double bond or a single bond? Why?

Summary of Structural formulas & Lewis Dot Structures

The steps to draw a Lewis dot structure of a covalent molecule are:
1.	Write the symbol for the central atom in the molecule.
2.	Join on the other atoms using dashes.
3.	Calculate the number of bonds. The formula to use is:
	or more simply, this formula can be expressed as:
4.	Add dashes to your diagram to make the number of bonds calculated in step 3 equal to the number of dashes shown in the diagram.
5.	Add more dots to obey the octet rule. You may replace each dash by a pair of electrons to help show the octet around each atom.
6.	Do a final count that all atoms obeyed the octet rule. Also, make sure you haven’t used more electrons than are available according to the group numbers of the elements.

Example: Draw the Lewis dot structure for H2O.
Steps:		Work
1.	Write the symbol for the central atom in the molecule.	O
2.	Join on the other atoms using dashes.	H-O-H
3.	Calculate the number of bonds. Note: by the time you are finished making the lewis structure, each hydrogen will have 2 valence electrons and oxygen will have 8. This makes a total of 12 valence electrons needed. The current number of valence electrons before bonding is 1 on each hydrogen and 6 on oxygen, making a total of 8.	# of bonds = (12-8)/2 # of bonds = 2
4.	Add dashes to your diagram to make the number of bonds calculated in step 3 equal to the number of dashes shown in the diagram. This is called a structural formula.	H-O-H More dashes were not needed.
5.	Add dots to obey the octet rule. This is the Lewis dot structure. You may replace each dash by a pair of electrons to help show the octet around each atom.	or
6.	Do a final count that all atoms obeyed the octet rule. Also, make sure you haven’t used more electrons than are available according to the group numbers of the elements. (H is in 1A, O is in 6A so 1 + 1 + 6 = 8)

Example: Draw the Lewis dot structure for CO2.
Steps:		Work
1.	Write the symbol for the central atom in the molecule.	C
2.	Join on the other atoms using dashes.	O-C-O
3.	Calculate the number of bonds. Note: by the time you are finished making the Lewis structure, each atom will have 8 valence electrons making a total of 24. Currently, each oxygen atom has 6 valence electrons and carbon has 4. This means before bonding you have a total of 16 valence electrons.	# of bonds = (24-16)/2 # of bonds = 4
4.	Add dashes to your diagram to make the number of bonds calculated in step 3 equal to the number of dashes shown in the diagram. This is called a structural formula.	More dashes were added.
5.	Add dots to obey the octet rule. Note that the added dots on each oxygen atom are place on the corners of the imaginary box. This is done to accommodate the fact that the groups of electrons on the oxygen atoms are all negative in charge and will move as far apart from each other as is possible. This means that the electron groups around each oxygen atom will be 120^oapart from each other. This is the Lewis dot structure.	or
6.	Do a final count that all atoms obeyed the octet rule. Also, make sure you haven’t used more electrons than are available according to the group numbers of the elements. (O is in 6A and C is in 4A so 6 + 6 + 4 = 16.

Example: Draw the Lewis dot structure for S03^2-
Steps:		Work
1.	Write the symbol for the central atom in the molecule.	S
2.	Join on the other atoms using dashes. Put the ion in brackets and the charge outside as a superscript.
3.	Calculate the number of bonds. Note: by the time you are finished making the Lewis structure, each atom will have 8 valence electrons making a total of 32. Currently, each oxygen atom has 6 valence electrons and sulfur has 6. Add two electrons for the -2 charge. This means before bonding you have a total of 26 valence electrons.	# of bonds = (32-26)/2 # of bonds = 3
4.	Add dashes to your diagram to make the number of bonds calculated in step 3 equal to the number of dashes shown in the diagram. Put the ion in brackets and the charge outside as a superscript. This is called a structural formula.	More dashes were not needed.
5.	Add dots to obey the octet rule. Note that the added dots on each oxygen atom are place on the corners of the imaginary box. This is done to accommodate the fact that the groups of electrons on the oxygen atoms are all negative in charge and will move as far apart from each other as is possible. This means that the electron groups around each oxygen atom will be 120^oapart from each other. This is the Lewis dot structure.	or
6.	Do a final count that all atoms obeyed the octet rule. Also, make sure you haven’t used more electrons than are available according to the group numbers of the elements. (O is in 6A and S is in 6A and you add 2 for the -2 charge so 6 + 6 + 6 + 6 + 2 = 26.