Uploaded by Kimette Witt

lab Making Covalent Molecules simple

advertisement
Making Covalent Molecules
1. Purpose: Construct models of covalent compounds.
2. Materials: Molecular Model Kit
3. Introduction:
A covalent bond is created when atoms share electrons. In covalent molecules, single, double, and triple
bonds form between atoms. Let's look at how molecular formulas, electron dot diagrams and
structural formulas are related to one another for several simple compounds. Molecular
formulas show only the type and number of atoms in a molecule. Structural formulas show the
atoms in their correct placement in the molecule. Electron-dot formulas resemble structural
formulas but also include all the non-bonding outer electrons.
First look at CH4 (also called methane). In the electron dot diagram you can see how the four
valence electrons of carbon match up with the one valence electron of each of the four hydrogen
atoms to give eight electrons around carbon. The electrons are arranged in four pairs representing
four covalent bonds. The structural formula emphasizes the bonds rather than the electrons and
shows one line for each of the bonds that connect the atoms together. Each line represents one
shared pair of electrons (2 electrons).
Molecular Formula
Molecular Model
Electron Dot Diagram
Structural Formula
CH4
Next you can see the same things for HC2H. However, in this case, the resulting structural formula
is different in an important way. Since there are three pairs of electrons shared between the two
carbon atoms, a triple line is drawn.
Molecular Formula
HC2H
Molecular Model
Electron Dot Diagram
Structural Formula
H:C:::C:H
4. Procedure:
a) Use the ball-and-stick model kit to construct the model that represents the molecule, based on these
guidelines:
A. Non-bonding electrons: Each unshared pair of electrons ("lone pair") will be
represented by 1 short, rigid connector, with no ball at the other end of it.
B. Bonding electrons:
i. Single bonds will be represented by 1 connector--use the short, rigid connectors for
bonds involving hydrogen atoms and use the longer, more flexible connectors for all
other bonds.
ii. Double bonds will be represented by 2 longer, more flexible connectors
iii. Triple bonds will be represented by 3 long, flexible connectors.
Name: ______________________________
Block: _____
Date: ____________________
Making Covalent Molecules
ATOM
NUMBER OF BONDING SITES
(holes)
Hydrogen and
alkali metals
1
Oxygen family
2
Carbon family
4
Halogens
1
Nitrogen family
3
COLOR
 Sometimes atoms form 2 bonds with another atom. This is called a double bond.
 Triple bonds are 3 bonds (for a total of 6 electrons) between the same two atoms.
 *Note: the structural formula should be written so that it matches, as much as possible, the
same shape of the molecule that you constructed (see the examples).
Molecular
Formula
Name
HF
hydrogen
monoflouride
O2
molecular oxygen
HCl
hydrogen
monochloride
BrCl
H2O
dihydrogen monoxide
CO2
carbon dioxide
SiCl4
Electron-dot formula
Structural formula
Molecular
Formula
Name
Electron-dot formula
Structural formula
H2S
C2H2
dicarbon dihydride
NH3
(also called ammonia)
NBr3
N2H4
C2H6
(also called ethane)
C4H10
tetracarbon
decahydride
Analysis
1. How many electrons are shared in a…
a) single bond ______
b) double bond ______
c) triple bond ______
2. Explain how molecular formulas, electron dot diagrams and structural formulas are related to
one another
__________________________________________________________________________________________
__________________________________________________________________________________________
__________________________________________________________________________________________
3. What is a dipole? Give an example of a molecule with a dipole and draw the dipole on its
Lewis structure.
4. What factors determine whether a molecule is polar or non-polar? Explain.
5. Both water and carbon dioxide are triatomic molecules. Explain why one is polar and the
other is nonpolar.
6. Both hydrogen monofluoride and nitrogen gas were two-atom molecules AND linear. Explain
why one of these is polar and the other is nonpolar.
7. Both carbon tetrachloride and methyl chloride were tetrahedral shaped. Explain why one
of these is polar and the other is nonpolar.
8. Why did we not make NaCl, BaO, Al2O3 or Li3N in this activity?
9. Why are there four holes in the model for the atom nitrogen?
10. Compare the molecules of N2 and H2. Which molecule has the longest bond? Why?
11. If you have a molecule with a double or triple bond in it that molecule could be what two
shapes? What determines which of those shapes the molecule is?
12. When you make the diatomic molecule for nitrogen, which way are the open holes facing and
why.
13. a. If a molecule has two atoms its shape is _________________________.
b. If a molecule has all single bonds and four atoms it takes the shape of
_______________________ or _________________________.
14. What shape is the molecule SiS2?