1. Electron Configurations- The Bohr Model of the Atom

2. How are particles arranged in an atom?
Scientists have debated for many years how the particles in an atom are arranged.
They know that protons and neutrons are in the nucleus and electrons are around the outside.
For a long time, they believed that the electrons went around the nucleus like the planets go around the sun.

3. Today scientists know…
Electrons do orbit the nucleus.
However, there is no exact path that they follow.
Instead they form a cloud around the nucleus.

4. Energy levels
Scientists think that electrons are arranged into energy levels or shells.
Each shell is named by a letter of the alphabet starting with “K”.
Each shell can only hold a certain amount of electrons.

5. “K” Shell The shell closest to the nucleus is called the “K” shell.
Electrons want to be near the positive nucleus so they will fill up the “K” shell first.
The “K” shell can only hold 2 electrons. K

6. “L” & “M” Shells The next shell out from “K” is “L”.
The “L” shell can hold 8 electrons.
After “L” is “M” which can also hold 8 electrons. K L M 2 8

7. Sodium’s atomic number is 11 so it has 11 electrons.
Filling shells
The number of electrons in an atom’s shells depends on how many electrons an atom has.
A shell must be full before a new shell starts.
Sodium’s atomic number is 11 so it has 11 electrons. 1 8 2

8. Drawing Bohr Models p = 3 e- = 3 n = 4
Calculate the number of protons, neutrons and electrons in the atom.
Put the right number of protons and neutrons in the nucleus.
Add electrons to shells filling the inner ones first. 1 2
p = 3
e- = 3
n = 4 3p 4n

9. Try this one… 3 8 13
p = e- = n = 2
13p
14n 13 14

10. Use your periodic table to draw the Bohr Model of each atom listed.
Hydrogen
Helium
Chlorine
Carbon
Magnesium
Phosphorous
Neon
REMEMBER:
Calculate the number of protons, neutrons and electrons in the atom.
Put the right number of protons and neutrons in the nucleus.
Add electrons to shells filling the inner ones first.