Molecular Shapes

Hello!  I hope everyone had a great week and a great quiz!  The answers for Friday's quiz are as follows:

1.  Draw the Lewis Dot Structure for Calcium Chloride:

2.  Draw the Lewis Dot Structure for water

3.  Draw the Lewis Dot Structure for COCL2 indicating formal charges

4.  Draw all resonance structures for Sulfur Trioxide

I hope you all did well!  One reason for drawing Lewis electron dot structures is to be able to predict the three-dimensional geometry of molecules and ions.  Because the physical and chemical properties of compounds are tied to their structures, the importance of this subject cannot be overstated.  The valence shall electron-pair repulsion (VSEPR) model is a reliable method for predicting the shapes of covalent molecules and polyatomic ions.  Simply put, electrons want to stay as far away from other electrons as possible.  The simplest way to understand this model is to assume the electron pairs around the central atom are involved in only single covalent bonds.  These follow a AXn formatting where A is the central atom and X is the atoms connected to the center atom with n being the number of these atoms connected.  Here are some basic shapes:

Linear = AX2

Trigonal-planar = AX3

Tetrahedral = AX4

Trigonal bipyramidal = AX5

Octahderal = AX6

I would suggest studying these.... hint.... hint.....  Remember this is only if the central atom is surrounded by single bonds!

For Parents:

We will be having our science project fair in a month.  All students are required to pick a topic that is chemistry related, but can't be dangerous.  Some good ideas can be found at this web site : Chemistry project ideas.  Please have your child pick a topic by this Friday.  Thanks!





Formal Charges and Resonance in Lewis Structures

For Students:
Today we are going to discuss two important features of Lewis dot structures.  The first is called the formal charge.  The formal charge is given to an individual atom in a molecule or polyatomic ion.

The sum of the formal charges for the atoms in a species equals the overall charge on the ion or is zero (for an uncharged molecule).  The formal charge for an atom is calculated based on the Lewis structure.
Formal charge = group number of atom - (LPE + 1/2(BE))
In this equation LPE stands for the number of lone pair electrons on an atom and BE is the number of bonding electrons around the atom.
So in the above example for the top oxygen on the left molecule, the formal charge = 6 (group number) - (2 (lone pair) + 1/2(6) (bonding electrons))----> which is 6 - (2+3) = 1.  That is why there is a + above the atom.

Next, what happens when you come up with two different ways to write a Lewis structure, and both ways are correct?

This is called resonance, and was first proposed by Linus Pauling.  Resonance structures are used to represent bonding in a molecule or ion when a single Lewis structure fails to describe accurately the actual electronic structure.  Resonance structures differ only in the assignment of electron-pair positions, never atom positions.  Also, there will always be at least one multiple bond in each resonance structure.

For Parents:
We will be having another exam in 3 weeks.  Be sure that your child understand all the material we will be going over.  This is the section of the course that gets difficult and I am more than happy to help anyone who needs it.  Thanks!

Quiz results and the begining of bonding and molecular structure

For Students:
The quiz results are as follows:
1.  Chromium = 1s2 2s2 2p6 3s2 3p6 3d5 4s1
2.  What is the maximum number of electrons that can be identified with each of the following sets of quantum numbers?
a.  n = 4, l = 3 ml = 1 1/2? --> 2
b.  n = 6, l = 1, ml = -1, ms = -1/2  --> 1
c.  n = 3, l = 3, ml = -3  -->  none because l cannot equal n

3.  Arrange the following atoms in order of increasing ionization energy: Si, K, P, and Ca
K < Ca < Si < P

The picture shown above is called a Lewis dot structure.  These are for CH4, C2H4 and C2H2.  Structure refers to the way atoms are arranged in space, and bonding describes the forces that hold adjacent atoms together.  When a chemical reaction occurs between two atoms, their valence electrons are reorganized so that a net attractive force - a chemical bond - occurs between atoms.  There are two general types of bonds.  Ionic bonds form when one or more valence electrons are transferred from one atom to another, creating positive and negative ions.   In contrast, a covalent bond involves sharing of valence electrons between atoms.

The american chemist Gilbert Newton Lewis introduced a useful way to represent electrons in the valence shell of an atom.  The element's symbol represents the atomic nucleus together with the core electrons.  Up to four valence electrons, represented by dots, are placed one at a time around the symbol; then, if any valence electrons remain, they are paired with ones already there.  These pictures are refered to as Lewis electron dot symbols, and the pictures above are great examples!

For Parents:

The exam last week went very well.  The average grade was a B-, which is quite good!  If your child did not receive the grade they wanted or if there is something they don't understand, please have them come and see me.  Thanks again for all you do!

Atomic properties and periodic trends...

For Students:

The picture shown above gives and more or less accurate depiction of the atomic size trends going across the periodic table.  As you can see the largest atoms are in the bottom left of the periodic table.  Francium must be huge!  It is one of the biggest atoms, but not really huge, remember that atoms are approximately 10^-11 m in diameter.

Next we will look at electron affinity which is the energy required to add an electron to an atom in the gas phase.  In other words, how much that atom resists aquiring a new electron.

As you can see atoms in the top right corner really don't want any more electrons.  Helium is not going to be taking on any more electrons any time soon.  And this make sense because helium's shell is already full and it would have to make a new shell to hold any more electrons.  This would take a huge amount of energy to do!
Finally we will look at ionization energy which is the amount of energy required to remove an electron from a particular atom in its gas state.

It is essentially the same as the electron affinity but you have to remember that for electron affinity the values are getting more and more negative as you move up and right in the periodic table.  The energy required to add an electron is negative if something really resists it.  The ionization energy goes up when you move up and right in the periodic table in a positive way.  This means that it would require more and more energy to remove an electron  as you move up and right.

For Parents:
There will be a quiz tomorrow instead of Friday and on Friday we will be having our 2nd test of the semester.  Please make sure your child understand what is going on and please have them ask questions if they don't understand something on the study quide.  Hopefully things go well Friday!  Thanks!

Aufbau and Hund!

For Students:
Today we are going to discuss the other two very important principles to understand when it comes to doing orbital box diagrams.  Last time we discussed Pauli's principle, which states that no two electrons in an atom can have the same 4 quantum numbers.  When doing an orbital box diagram, another thing you should remember is the aufbau principle.  Aufbau literally means "building up".  The idea is to keep the energy in an atom as low as possible.  If you start at the bottom of our box diagram and fill in electrons from the bottom up, then you are essentially keeping the energy as low as possible.  (Remember that the box diagram has the lowest energy at the bottom and increases energy levels as you move up).    The final rule to remember is called Hund's rule.  Simply stated, this rule says that, for example in a p row of your energy orbital diagram, you should fill in the first three electrons, one in each box, all with the same spin first.  Then, you can fill in the boxes with the next 3 electrons, with the opposite spin.  Since this may sound a little confusing, lets look at some examples of all three rules for orbital box diagrams put together:

1.  Nitrogen would look like:

2.  Carbon would look like:

This is an idea of how to do these....

For Parents:

The second exam is next week.  I have sent home study guides today, so make sure if your child needs any help; have them come see me.  Have a great week!



The structure of atoms and periodic trends.

Hello!  I hope everyone had a great weekend.  This week we begin to develop some ideas about what the quantum numbers mean.
For Students:
Here are the answers to last week's quiz:
1.  Which of the following radiation involves less energy, x-rays or microwaves?  microwaves
2.  Place the following types of radiation in order of increasing energy per photon?
fm station < yellow light < x-rays
3.  An electron moves with a velocity of 2.5 x 10^8 cm/s.  What is its wavelength?
.29 nanometers
4.  How many subshells occur in the electron shell with the principal quantum number n=4?
four
5.  State which of the following orbitals cannot exist according to the quantum theory?
2d and 3f orbitals can't exist (n=2 subshell can only have a s or p subshell and n=3 can only have s, p or d subshells)

I hope everyone did well.  We now move on to new material.  To make the quantum theory work, the Austrian physicist Wolfgang Pauli stated in 1925 his exclusion principle: no two electrons in an atom can have the same four quantum numbers.  The result of this is that no more than two electrons can be in the same orbital.  One with an "up" charge and one with a "down" charge.  We will depict this using orbital box diagrams:

For example, when you want to depict the two electrons that make up a helium atom you would start at the bottom level, the 1S level and draw one arrow up and one arrow down inside the box.  That tells us that you have two electons in the 1S orbital, one spinning in the positive direction and one spinning in the negative direction.  Please note that these directions are completely arbitrary, it just helps us understand that there can't be two identical electrons in an atom.  Also it is important to note that s orbitals have one box, p orbitals have 3 boxes and d orbitals have 5 boxes.  How many boxes do you think an f orbital would have??  Seven is right!  The order of increasing energy must be memorized.  This will be on your next quiz.  The order is (starting at the bottom and working our way up): 1S, 2S, 2P, 2S,3P,3D, 4S, 4P, etc.

For Parents:
There have been a few difficulties lately regarding significant figures and scientific notation.  An error in this regard will not make the answer to a problem wrong, but I will have to start taking off a point of two.  If your child needs help in this area I will be available after school as usual and I will be giving out a review sheet to help with this.  If you have any questions let me know.  Thanks and have a great day!

No quiz last week, lucky you!

For Students:
You all know that we didn't have a quiz last Friday due to the field trip on Wendesday and the assembly Friday afternoon.  Don't get used to that!  Another one is coming up this Friday.
Anyway, this week we will be talking about quantum numbers and orbitals (and what that means for us chemists).  Quantum numbers are used to identify the energy states and orbitals (paths around the nucleus) available to electrons.  There are three principle quantum numbers.  The first is called the principal quantum number and its symbol is n.  The principle quantum number can have any whole number value from 1 to infinity.  It is the primary factor in determining the energy of an orbital and it defines the size of the orbital.  If more than one electron in an atom has the same (n) number, they are said to be in the same shell.  Shells can be broken down into subshells as well.  The second quantum number is called the azimuthal quantum number and its symbol is l.  This quantum number determines the actual shape of the orbital and it can have any whole number value from 0 to n-1.

If l = 0, you have an s orbital.

If l = 1, you have a p orbital.

If l = 2, you have a d orbital

And finally, if l = 3, you have an f orbital.

Our last quantum number is called the magnetic quantum number and it has the symbol m.  This number refers to the orientation in space of the orbitals within a subshell.  You can see from the above pictures that an s orbital can be oriented in just one way.  P orbitals can be oriented in 3 ways, d orbitals in 5 ways and f orbitals in 7 ways.  The magnetic quantum number can have the value of -l to l.

So now you can visualize exactly where the electrons have a good probability of being located around the nucleus.



For Parents:

Grades are looking very good so far this semester.  If you or your child have any questions don't hesitate to ask.  You can post on this blog, write me an email or see me after class everyday until 4:30pm.  Thanks again!