1. Atoms: Electron Configuration
An overview

2. Electron Arrangements
The arrangement of electrons in an atom is very important
This arrangement determines how an element will react with/bond with other elements
We will examine a simple model of the electron arrangement of the atom
This model has electrons located in regions around the nucleus called shells

3. Electron Shells In all there are 7 energy levels or electron shells
These are numbered 1 to 7
They are given letters to denote them
1(K) 5
2(L) N 6
3(M)
4(N) 7

4. Electron Shells
Generally, the further the shell is from the nucleus, the higher its energy level
The closer the shell from the nucleus the lower its energy level
Different shells can hold different numbers of electrons
Higher energy shells can hold more electrons than lower energy shells

5. Capacity of Electron shells
Maximum number of electrons per shell = 2n2

6. Maximum number of electrons per shell
Shell number
Formula] [2 (n)2]
Maximum e- 1
2 x (1)2 2
2 x (2)2 8 3
2 x (3)2 18 4
2 x (4)2 32 5
2 x (5)2 50 6
2 x (6)2 72 7
2 x (7)2 98

7. Filling of electrons into shells
Shells are filled with electrons(e-) in order
Lower shells are filled before higher shells
BUT
The outer shell never holds more than 8 e-, regardless of its maximum capacity
(exception is helium whose outer shell can’t hold more that 2 electrons)

8. Filling of electrons into shells
This is referred to as the OCTET rule
The outer shell refers to the outer most shell holding electrons
It is also referred to as the valence shell
Electrons in the outer shell are called valence electrons

9. Lets look at lithium ( Li )7 3
An atom of Lithium has 3 electrons
2 e- fill the first shell
Here is its nucleus
Here the 2nd shell is the outer shell + o
The 3rd e- is in the second shell

10. Now for potassium ( K) An atom of Potassium has 19 electrons39 19
An atom of Potassium has 19 electrons
But all 9 can’t go here as this would result in an outer shell with > 8 e-
The next shell has a maximum capacity of 18 e-
2 e- fill the first shell
N:20
P:19
19-2=17
8 e- fill the second shell
1 of these e- will occupy the next shell
17-8=9

11. Now for potassium ( K) An atom of Potassium has 19 electrons39 19
An atom of Potassium has 19 electrons
8 e- occupy the 3rd shell
2 e- fill the first shell
9-8=1
P:19
N:20
Outer shell
8 e- fill the second shell
1 e- occupies the 4th shell

12. Electronic configuration
Describes the arrangement of electrons in each shell of an atom
Lithium’s electronic configuration is: 2,1

13. Electronic configuration
Potassium’s electronic configuration is:
2,8,8,1

14. Draw the arrangement of electrons in an atom on Magnesium
Atomic number :12

15. Check your answer
Atomic number :12
2,8,2

16. Check your answer
Complete the drawing of the electron arrangements in the table provided
Complete the table of electron configurations