Chapter 1: Covalent Bonding And Shapes Of Molecules
Table of contents of main topics:
Electron Configuration
Step 1: Divide Periodic Table into s, p, d and f blocks as shown. Move helium next to hydrogen.
Step 2: Number the rows on the Periodic Table on the left and right sides. Number the d block 1 less then S block as shown. Number f block as well.
Step 3: Locate the element on the periodic table. Travel starting from the top left to the right through each row until you reach the element needed. As you travel through the table, write out the electron configuration. For example, as you travel though the first row, you will get 1s^2. 1 is the number of the row, s is the name of the block and the superscript 2 is the number of squares in the block you are getting through.
Let's try some examples:
Oxygen electron configuration
As I go through the first row, I get 1s2. Then, I have to start at the second row and I get 2s2, followed by 2p4 (since oxygen is in the fourth square on the p block row 2).
We can also write out the electron configuration as a noble gas configuration. All we need to do is find the noble gas (Group 18 on the very right) that is before our element. We put that noble gas into brackets and follow by writing the rest of the electron configuration. For oxygen, we would have [He] 2s^2 2p^4.
Note: In the textbook, the authors breakdown p sub shell into 3 orbitals: px, py and pz. Instead of writing 2p4 for oxygen, we would write 2px^2 2py^1 2pz^1. More on electron distribution in the part below.
Bromine electron configuration
Orbital Energy Diagram
Follow electron configuration to write out the orbital diagram.
S subshell has one orbital. P has 3 orbitals.
D has 5 orbitals and f has 7 orbitals. Each orbital has a maximum of 2 electrons. We represent orbitals by dashes and electrons by arrows.
Rules:
Follow electron configuration filling out orbitals from lowest energy to highest: Aufbau Rule
In the same subshell, first distribute electrons one in each orbital with parallel spins before pairing them: Hund's Rule
Let's show orbital energy diagram for chlorine
Lewis Dot Structures
Mayya's Tip:
Calculate the total number of valence electrons in a molecule (group number = number of valence electrons)
Draw our your molecule and put lines between atoms (bonds)
Subtract electrons used in bonds (2 electrons per bond) from the total number of valence electrons
Use the remaining electrons to make outer atoms happy (octet). Keep in mind hydrogen only wants 2 electrons.
If there are any electrons left, place them on the central atom.
Check if everyone is happy(octet). If not use neighboring atom's lone pairs to make double and triple bonds.
Lewis Dot Structure for CO2
Shapes of Molecules and Their Bond Angles
We need to use this chart and memorize it to determine the molecular geometry/shape of a molecule. First, we must start with the correct Lewis Dot Structure. Next, we have to find the central atom and identify how many other atoms it is attached to and how many lone pairs it has. In the chart, the purple dot represents the central atom, blue dots are atoms the central atom is attached to and two dots represent a lone electron pair.
For example, in CO2 example above, carbon is the central atom that is attached to two other atoms (oxygens) and has no lone pairs. If we check our chart, we see that case on the top right, making this molecule linear. The bond angle would be 180 degrees according to the chart.
Formal Charge
Formal Charge = number of valence electrons - (number of bonds + number of lone electrons)
Let's take a look at the example above and calculate the formal charge for the oxygen on the right. Oxygen has 6 valence electrons (it's in group 6 on the periodic table). We subtract from it the number of bonds it has (1) and the number of lone electrons it has (6). 6-(1+6) = -1
Functional Groups
Hybridization
Steric Number = number of attachments + number of lone pairs
We can use the chart above to determine hybridization of an atom in a molecule. For example, let's take a look at CO2 we have shown above and determine hybridization of carbon. The steric number for carbon is 2 because it is attached to two other atoms and has no lone pairs. According to the chart, hybridization is sp when steric number is 2.
Resonance
Many molecules are best described by multiple Lewis Dot structures. The molecule is a hybrid of all of these structures at the same time. These are called resonance structures.
Rules for writing Resonance Structures:
Do not give hydrogen more than 2 electrons and do not give second period elements more than 8 electrons.
You are allowed to move electrons only and nothing else.
Note: The total charge of each resonance structure must be equal.
We show arrows to draw resonance structures. An arrow represents a movement of two electrons. It must be coming from something that has electrons such as a double bond or a lone pair. It must move to something that can accept a pair of electrons.
Common Resonance Patterns
Do you need help in your OCHEM CLASS?
Book a session with our Online Organic Chemistry Tutor