How to Draw a PEN Diagram

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

• 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

Protons

Neutrons

Shell 1:Holds 2 electrons

Shell 2:Holds 8 electrons

Shell 3:Holds 18 electrons

Drawing a PEN Diagram

1. Get out your periodic table2. 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

3. 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

4. Remember your number of protons and electrons– Forgot? This is written on the top line inside the

nucleus of your PEN diagram

5. 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

Example: Chlorine

1718287

Example: Lithium

3421