1. How to Draw a PEN Diagram

2. Background Info
Electrons’ movements are arranged into energy levels called shells
Hydrogen and Helium have only 1 electron shell. All other atoms have 2 or more shells
The shells can hold different amounts of electrons
The first shell (closest to the nucleus) can hold 2
The second shell can hold 8
The third shell can hold 18
The fourth shell can hold 32

3. The number of shells an atom has depends on its number of electrons
Each shell must have its full number of electrons before a new shell starts
The outer shell of most atoms is not full
Only the atoms in Group 18 have full outer shells

4. Protons
Neutrons
Shell 3:
Holds 18 electrons
Shell 2:
Holds 8 electrons
Shell 1:
Holds 2 electrons

5. Drawing a PEN Diagram Get out your periodic table
Determine the number of protons and electrons
Identify the element’s atomic number
This number is also the number of protons and electrons
Write this number on the top line in the nucleus of your PEN diagram

6. Determine the number of neutrons
Locate the element on the periodic table
Identify the element’s atomic number and atomic weight
Round the atomic weight to the nearest whole number (mass number)
Subtract the number of protons from the mass number
Mass Number = protons + neutrons
Write this number on the bottom line in the nucleus of your PEN diagram

7. Remember your number of protons and electrons
Forgot? This is written on the top line inside the nucleus of your PEN diagram
Determine the number of electrons in each shell
Put 2 electrons in the first shell. Subtract 2 from your total number of electrons
The second shell can hold up to 8. Put up to 8 remaining electrons in this shell
The third shell can hold up to 18. Put up to 8 remaining electrons in this shell

8. Example: Chlorine 17 7 8 2 18

9. Example: Lithium 3 1 2 4