Reminder, tomorrow is your Unit 9 Test. The Practice test and it’s answer key are posted for you to review with.
On top of that, you’ll be receiving your first review for semester 2. Today’s version is in outline form as a guideline for you to start pilfering through your handouts, notes, and other materials that may help you review for the final since I’m sure by now you sorta feel like this guy on the left.
We have one Unit left to sort through before we take on this exam fill boar. But until then, use your time wisely as we prepare for our last Test and Final of this School year.
Now that you have completed the simple VSEPR diagrams and made your models of Linear, Trigonal, and Tetrahedral molecular geometry… it’s time to break the rules.
The rule we’re breaking is the Octet rule. To this point you have limited the central atom to sharing up to 8 electrons. But, If your Lewis Dot Diagram has too few electrons, you add pairs of electrons to the central atom.
Let me show you what I mean. Let’s say that we have XeF2.If you calculate the valence electrons you should have 22.
But when you make the Lewis Dot Diagram, it only has 20 ve–.
So, we add a pair of electrons to the Xe central atom giving us a total of 22 ve–This creates a new Ideal Geometry known as Triangular Bipyramidal, which means it has placement for 5 parts (attached atoms or free electron pairs) around the central atom.
There is also an Ideal Geometry for 6 parts known as Octahedron. Think of the central atom as a 6 -sided die. So atoms can be bonded in 6 places and in 3-dimensions!
Now that we know how to find the general structure of a molecule using Lewis Dot diagrams, we can actually find the three-dimensional shape of a molecule AND BUILD IT! We can find the shapes of molecules using the VSEPR (Valence Shell Electron Pair Repulsion) Theory.
The idea is that atoms and electrons found around a central atom are repelled by each other because of the negative charges of the electrons surrounding each (remember opposites attract +/- and similar charges repel -/- or +/+). In other words, everything bonded or attached to the central atom wants to be as far away from everyone else as possible.
So let’s look at NH3 for example.
Step 1: Calculate the valence electrons
1 x N = 1 x 5 = 5
3 x H = 3 x 1 = 3 5+3 = 8 ve–
Step 2: Draw the Lewis Dot Structure
Step 3: Calculate the ABE type
Each letter of ABE stands for a part of the molecule
A= A central atom. If it has a central atom, write an A
B= Attached atoms. Count the number of atoms attached to the central atom and write that as a subscript of B example: B3
E= Free Electron Pairs. If their are extra electron pairs on the central atoms we count them as pairs and write that as a subscript of E (if there are no pairs, do not write E. If there’s only 1 pair, just write E) example: E2
NH3 has a central atom, three attached atoms, and 1 electron pair. So the ABE structure would be AB3E
Step 4: Find the Ideal Geometry.
I like to think of the ideal geometry as places on the central atom with thing happening. If there is only 2 places where things are happening (attached atoms or free electrons) then it’s Linear. If there’s three places, it’s Trigonal. If there’s four, it’s Tetrahedral.
Below is a great link to a video explaining the different shapes and where they come from.
Since NH3 is an AB3E structure there are 4 places where things are happening (B 3 + 1 E) so NH3 is Tetrahedral.
Step 5: Molecular Shape.
Along with the ABE chart is the list of Ideal Geometry and Molecular shape. The shape is based on the idea that the other atoms want to be as far apart as possible AND that free electron pairs need a lot of space to roam.
NH3 has 1 free pair of electrons that float to the top of the molecule as if they were in a balloon. The three Hydrogens then act as a tripod for the entire molecule holding it up. Therefore we say the shape of NH3 is pyramidal.
Seriously though, here is the video on Bonding. Use this link if the video does not play –> Bonding: YouTube
Unit 9 Discovery Quiz <- Click Here
Lots to do today so lets go over our agenda:
1. Turn in assignment from yesterday
2. Take the Unit 9 Discovery Quiz below.
3. Copy the table on the board to your notes.
4. Take The last of our notes
5. Complete today’s assigmnent.
If you do not see the quiz below, click the link at the top of this post to be directed directly to the quiz.
By the way… this is the 100th post on our blog so… congrats to us.
Here’s to the next 100!
I’ve been feeling ill all week, and it has finally gotten the best of me… so I’ll be taking the day off to hopefully get well soon. Below is a description of what you’ll be working on on Friday in my absence. Please Email me if you have questions.
Get your graded scantron and test. For each incorrect answer, if you explain what the correct answer is AND why, you will earn 1 point of the 2 pts missed back. So if you missed 5 questions, you can earn 5 pts. The responses should be as detailed as possible and in complete sentences to get credit. Please do this in pages or Notability on the iPad and email me the finished product as a doc or PDF and I will be able to grade it while recouping at home.
Note: You DO NOT have to correct #’s 16, 17, 34, & 37. Those points have been given back to you on powerschool.
The news is that our Unit 8 Test is scheduled for this Thursday April 11th. You’ll be given a pre-test, based on the real test today. Also this week, we’ll be heading to the lab to do our “Flame Test”. The date we’re in the lab is yet to be determined… but it won’t be Wednesday or Thursday… obviously. The Pre-Test is posted on the Sem2 Docs page and the key will be up later today. Good Luck!