1. Electron configurations

2. Electron configuration for elements in period 1: H and He
We know hydrogen (H) has a nucleus with one proton. We can draw a diagram of hydrogen by placing a circle in the center of the page, and then placing a +1 inside that circle. This represents the nucleus of hydrogen. Since there is one proton, there must be one electron (remember #of protons=# of electrons in a neutral atom)
We will draw a larger circle around the nucleus, add a small circle somewhere on the circumference of that circle- with a (-) inside of the circle and we now have a Bohr atom model of Hydrogn.

3. Helium
We can do the same thing for helium, with an atomic number of 2 and a mass of 4. Here, we have two protons and two neutrons in the nucleus, and will have a pair of electrons orbiting the nucleus in a circular path.
Draw the Bohr atom model for helium

4. Aufbau principle
We have an alternative method of writing a representation of these two elements: l
l XX 1s
L_______________
Each x represents an electron, the two x's represent a pair of electrons
The line the pair of electrons are sitting is called an "s" orbital
If the orbital is an "s" orbital, the electron must have a spherical shape.
We are calling the pair of electrons 1s2

5. Electron configuration of H and He
The electron configuration for Hydrogen is 1s1
The electron configuration for Helium is 1s2
Note the maximum number of electrons for period 1 where "n" =1 is two electrons. So, if we need more electrons we need to move onto the next energy level where "n"=2

6. LITHIUM 3 Li7
We know the atom has three protons ( how do we know this?)
We also know the atom has four neutrons (how do we know this?)
Since this is a neutral atom, we know the atom has three electrons.
So we can assign the first two electrons into the first energy level and have a pair of electrons in the “s” orbital. 1s2

8. So we can account for two electrons- place them into the 1s orbital.
But we need a third electron- and there is no room in the first energy level
So we will add the third electron into the second energy level, and it must be an “s” electron
The electron configuration for Lithium must be 1s2 2s1 The numbers after the “n” quantum number is an exponent and represents the number of electrons

9. Consider the following and identify the element
Suppose we have an element like Carbon 6C12.
Number of protons: ?
Number of electrons: ?
Number of neutrons: ?

10. 6C12
We need a total of six electrons for the atom’s electron configuration:
Remember the maximum number of “s” orbitals is one per energy level ( Lets rename the energy level as a
PRINCIPAL QUANTUM NUMBER
So in any principal quantum number, the first orbital is an “s” orbital and the maximum number of electrons that ever goes into any orbital is two electrons

11. Let’s write the electron configuration for 6C12
First two electrons go into the 1s orbital- and is written 1s2,
The next two electrons must go into the second principal quantum number where “n” =2
So then the configuration is 1s2 2s2
How many more electrons do we need for Carbon?

12. 6C12
We have 1s2 2s2 2px1 2py1
The two electrons are unpaired- one in the x axis and one in the y axis.
Since these electrons are found in different directions, they do not interact with each other.

13. 7N14 Write the electron configuration for nitrogen: # of protons=
# of electrons=
# of neutrons=
We need ? Electrons:
They can be found, two electrons in 1s, two electrons in 2s and three electrons in the p orbitals- but one is in the x direction, one in the y direction and one electron is in the z direction.

14. If the three p electrons are in different axis they do not interact.
The three p electrons are in 2px 2py and 2pz, with one electron in each p orbital
1s2 2s2 2px1 2py1 2pz1.
If you google the shape of a p electron, it is described as a dumb-bell shape.
See white board for the shape.

15. 6C12
Since we have two electrons in the 1s orbital, and two electrons in the 2s orbital, we need two more electrons to complete the electron configuration.
The maximum number of electrons in the second quantum number is eight electrons.
We have two electrons occupying the 2s orbital, so we need to create a different orbital for the balance of the electrons in the second quantum number
We called these electrons “p” shape electrons and we have them oriented into the three different directions of x, y and z

16. Nitrogen and its three p electrons
One consequence of these three p electrons, being unpaired, is the effect we call magnetism.
Unpaired electrons have magnetic properties- due to the fact that all unpaired electrons are spinning in the same direction.
QUESTION:
Are the two electrons in the 1s orbital paired, or unpaired? Is the orbital with these two electrons going to have magnetic properties or is it nonmagnetic.

17. In terms of electron pairing- define magnetism
Which of the following is a magnetic element?
He C Ne
Li N
Be O
B F