Bohr Models

Bohr’s Model of the Atom • Focuses on electrons and their

arrangement.

• Bohr stated that electrons move with constant speed in fixed orbits around the nucleus, like planets around a sun.

• Bohr correctly assigned energy levels to electrons, but electrons do not move in fixed orbits around the nucleus.

Bohr’s Model of the Atom

Electron Nucleus

Valence Electron

Electron Cloud Model • The electron cloud model replaced

Bohr's vision of electrons moving in predictable paths.

• An electron cloud is a visual model of the most likely locations for electrons in an atom. – probability of finding an electron is higher in the

denser regions of the cloud.

Electron Cloud Model • An electron cloud is a good

approximation of how electrons behave in their orbitals. –An orbital is a region of space around the

nucleus where an electron is likely to be found.

– The electron cloud represents all the orbitals in an atom.

Electron Cloud Model

Because we cannot see electrons in the Electron

Cloud Model, we will draw Bohr Models to

illustrate the number of electrons in the atom of

an element.

Before drawing a Bohr Model…

• Horizontal rows on the Periodic Table are called periods. – The row number tells you how many energy levels, or

rings, the element has.

– Elements in the same row have the same number of rings.

– The number of rings increases as you move DOWN the Periodic Table.

• Example: Sodium (Na) and Magnesium (Mg) are in the third row so they both will have three rings in their Bohr Model.

Horizontal Rows=Period=Number of Rings

1

6

5 4 3 2 Will not draw Bohr Models for Transition Metals

Number of Electrons on a Ring

• Each ring has a maximum number of electrons that can fit on it.

• Each ring CAN and will hold LESS than the maximum number of electrons.

Ring Maximum Number of Electrons

1 2

2 8

3 18

4 32

Examples:

Hydrogen is on row #1 of the periodic table so it has one energy level.

Lithium is on row #2 of the periodic table so it has two energy levels.

Sodium is on row #3 of the periodic table so it has three energy levels.

Before drawing a Bohr Model…

• Vertical columns on the Periodic Table are called groups. – Group numbers tell you how many valence electrons

each element in that column has. • Valence electrons are electrons that are located on the LAST

energy level.

– The number of valence electrons increases as you move ACROSS the Periodic Table.

• Example: Sodium and Potassium are both in the 1st column, so they both will have one valence electron in their last energy level. – If the group number is higher than nine, so 10-18,

drop the one to determine the number of valence electrons.

Vertical Columns=Groups=Number of Valence Electrons

1

2 3 4 5 6 7

2,8

Will not draw Bohr Models for Transition Metals

Examples: (Each “dot” represents an electron.)

Hydrogen is in column #1 of the periodic table so it has one valence electron. Of course, it only has one electron at all!

Lithium has three electrons. One valence electron because it is column #1.

Sodium has 11 electrons. One valence electron because it is in column #1.

Examples: (Each “dot” represents an electron.)

Helium has two electrons. It is in column #8 so one would think it should have eight valence electrons but it cannot because it only has two total electrons. It has one ring because it is in row #1.

Boron has five electrons. Three valence electron because it is column #3. It has two rings because it is in row #2.

Argon has 18 electrons. Eight valence electron because it is in column #8. It has three rings because it is in row #3.

Drawing Bohr Models

1. Determine the number of rings, or energy levels. (Look at the period, or row, number.)

2. Determine the number of protons, neutrons, and electrons.

3. Determine the number of valence electrons. (Look at the group, or column, number.)

4. Draw the correct number of rings.

5. Draw the correct number of electrons on each ring, or energy level. (Use Energy Levels, Orbitals, and Electrons table.)