Matias psy193 Laboratory 4 molecular geometry PDF

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Laboratory 4: It’s All in the Shape: Discovering Molecular Geometry Structure begets function. How molecules behave or interact with one another or other molecules is an important part of chemistry. Molecular structure controls properties such as solubility or boiling point. So let’s explore the simple world of molecular geometry. It would be helpful to build some molecular models. If you have some toothpicks and clay, playdough, or gummy bears, you can make some models - the clay or gummy bear will be the central atom and the toothpicks will represent the electron pairs. Predict the geometry for the three situations given in the table below. How do you think electron pairs will behave towards each other? This behavior will control how they arrange around a central atom depending on the number of electron pairs. Number of electron pairs around central atom

Predict a possible arrangement - sketch it!

What is the angle Are all the angles the between the electron same? If not, point pairs? out the differences.

2 pairs

180°

Yes

3 pairs

120°

yes

4 pairs

90° / 180°

No, the one located at the coronal plane angles 90° and the other located at the transverse or horizontal area angle’s 180°

Discovering Molecular Geometry (2020 version, Sinex)

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Beware of molecular roadkill (flattened molecules)!!! distance. They want to get away from each other! This is the basis of valence shell electronpair repulsion theory or VSEPR. Let’s see how well your predictions came out above and add two more geometries to the list. Go to the following PhET html5 simulation: https://phet.colorado.edu/sims/html/molecule-shapes-basics/latest/molecule-shapesbasics_en.html

Click on the Model box as shown above. When the screenshot below opens, follow the instructions below.

1. 2. 3.

Click the “Remove All” button. Check the Show Bond Angles Option Check the Name Molecular Geometry

Discovering Molecular Geometry (2020 version, Sinex)

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Now if you click the single bond (top entity in the Bonding box (upper right corner), you can build the geometries in the table below around the central purple atom. WATCH carefully as you do this and you will see repulsion in action!!! If you click and hold in the space away from the boxes, you can rotate the molecule to examine the bond angles and general shape. Number of electron pairs around central atom 2 pairs

Sketch the arrangement of the atoms

What is the angle Are all the angles the between the electron same? If not, point pairs? out the differences. 180°

Yes

120°

yes

109.5°

yes

Name of molecular geometry: Linear

3 pairs Name of molecular geometry: Trigonal Planar

4 pairs Name of molecular geometry: tetrahedral

Discovering Molecular Geometry (2020 version, Sinex)

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5 pairs

90°/120°

No, the one that located above, angle’s 90°, the ones that surround the central angle’s 120°

90°

Yes

Name of molecular geometry: Trigonal Bipyramidal

6 pairs Name of molecular geometry: Octahedral

Draw the Lewis dot structures for the molecules below. BeCl2

BF3

PCl5

CH4

SF6

sion, Sinex)

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The number of electron pairs on the central atom, first atom in each formula above, determines the molecular geometry. Predict and illustrate the molecular geometries of the structures above. BF3 (Trigonal Planar) BeCl2 (Linear)

CH4 (Tetrahedral) PCl5 (Trigonal Bipyramidial)

Discovering Molecular Geometry (2020 version, Sinex)

SF6 (Octahedral)

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Click on the Real Molecules box at the bottom of the screen.

Check your predictions for the molecules. Did you get the correct answers? Revise your illustration if needed. Some help with illustrations is on the next page!

To help with the illustration of the geometries, wedge diagrams, which are used in organic chemistry, are introduced below. The solid triangular shape is in front of the plane of the paper and the dashed triangular shape is in back of the plane of the paper.

On these three geometries, are there any opposite positions (place where atoms are 180o apart)? Circle them. The trigonal bipyramidal geometry (tbp) needs a little further explanation before we go on. This geometry has two distinct positions - axial (180o apart) and equatorial (120o apart). This will be important later.

Discovering Molecular Geometry (2020 version, Sinex)

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