Electrons &their arrangement

in an atom

• Electrons live in something called energy levels.

• Only so many electrons can be in any energy level.

• The electrons in the outer most energy level of any element are called valence electrons.

Electrons have special rules….

Bohr Diagrams1) Find the element on the periodic table.

2) Determine the number of electrons– in a neutral atom it is the same as the atomic number.

3) This is how many electrons you will draw.

Bohr Diagrams• Find out which period

(row) your element is in.

•Elements in the 1st period have one energy level.•Elements in the 2nd period have two energy levels, and so on.

1

2

3

4

5

6

7

The electron shells surrounding the nucleus each hold a particular number of electrons.

Shells are named with letters:

1 = K shell = 2 electrons2 = L shell = 8 electrons3 = M shell = 8 electrons4 = N shell = 18 electrons5-7 = O, P, Q = up to 32 electrons

Remember: The outer shell of an atom (no matter what letter) can only hold 8 electrons!

Bohr Diagrams1) Draw a nucleus with

the element symbol inside.

C 2)Write the number of Protons the element has inside the nucleus.

P = 6

Bohr Diagrams

1) Carbon is in the 2nd period, so it has two energy levels, or shells.

2) Draw the shells around the nucleus.

CP = 6

Bohr Diagrams1) Add the electrons.

CP = 6

2 e-

2) Carbon has 6 electrons.3) The first shell can only

hold 2 electrons.4) Write the number of

electrons that shell holds and e- for electrons and their charge.

5) How many electrons are in the outer shell?

Bohr Diagrams

1) Since you have 2 electrons in the first shell, you need to add 4 more.

CP = 6

2) These go in the 2nd shell.

3) Again, write e- and the number of electrons.

2e-

4e-