Bohr-Rutherford Diagrams

• In 1913, a Danish scientist named Neils Bohr (1885 – 1962), explained why in the atom electrons do not spiral into the nucleus.

02

• Bohr’s model equated the nucleus of an atom to the sun, and the electrons to the planets.

• The atoms positive nucleus exerts a strong force of attraction on the negative electrons.

02

• The electrons resist the attraction because they are moving rapidly in fixed regions around the nucleus.

• These regions are three dimensional and sphere like, and are called electron shells.

02

• The electrons orbit the nucleus in regions called shells.

• Shells are areas that surround the nucleus like layers around an onion.

02

• Each shell is a certain distance away from the nucleus.

• The first shell is closest to the nucleus; the second is further away.

• Shells are often called orbitals or energy levels.

02

• Each shell can hold a certain number of electrons and no more.– The first shell can hold 2 electrons– The second shell can hold 8 electrons– The third shell can hold 8 electrons– The fourth shell can hold 18 electrons

•

02

The First Shell

The Second Shell

You draw the third shell.

Remember, it has 8 electrons as well.

The Third Shell

• The outer shell is called the valence shell.

• Electrons in the outer shell are therefore called valence electrons.

02

• Valence electrons are important because they determine the element’s properties and how it will react.

• It is only the valence electrons that are involved in chemical reactions.

02

• The valence electrons correspond to the atomic number of an element.

Remember:

atomic number = # of electrons..

02

• Ex: Lithium has an atomic number of 3, so it would have 3 electrons:

02

• The # of valence shells = the # of the period the element is found in.

• The number of valence electrons (electrons in the outer shell) = the # of the group the element is found in.

02

How to Draw Bohr Diagrams1. Draw the nucleus as a solid circle.

2. Put the number of protons (atomic number) in the nucleus with the number of neutrons (mass number – atomic number) under it.

3. Place the number of electrons (same as protons) in orbits around the nucleus by drawing circles around the nucleus. In the second and the third shells, electrons should be drawn in pairs when they number more than four.

Ex: Carbon

Carbon: Period: 2, Group: 4

Bohr Diagram:

Complete Bohr Diagrams for the First 20 elements

of the Periodic Table

Adding to your Periodic Table...

• Write the period number along the sides. – Period 1 = Row 1– Period 2 = Row 2

• Write the group number across the top.– Group 1 = 1st column– Group 2 = 2nd column

Lewis Dot Diagrams

Electron Dot Diagrams

• Electron dot diagrams (also called Lewis Dot diagrams) are useful because they show only the valence electrons, since these are the important ones anyways.

• They include the element’s symbol and dots to represent the valence electrons.

How to Draw Dot Diagrams

1. Write the element symbol..

2. Determine the number of valence electrons the element has. Remember the valence electrons = group number.

3. Place the number of valence electrons as dots around the element symbol.

Electron Dot Diagrams

H C

O Ne

Draw Electron Dot Diagrams for the First 20 Elements on

the Periodic Table

This will be to be handed in.