2. The Atom and the Periodic Table  Electron Cloud Structure  Energy Levels  Rows on the Periodic Table  Bohr Models  Electron Dot Diagrams

3. Review  The vertical columns in the periodic table are called groups.  Elements in each group have similar properties.  For example, in Group 11, copper, silver, and gold have similar properties. Each is a shiny metal and a good conductor of heat and electricity.  What is responsible for the similar properties?

4. Electron Cloud Structure  Scientists have found that electrons within the electron cloud have different amounts of energy.  Scientists model the energy differences of the electrons by placing the electrons in energy levels as shown below.

5. Electron Cloud Structure  Energy levels nearer the nucleus have lower energy than those levels that are farther away.  Electrons fill these energy levels from the inner levels (closer to nucleus) to the outer levels (farther from nucleus).

6. Electron Cloud Structure  Elements that are in the same group have the same number of electrons in their outer energy level.  It is the number of electrons in the outer energy level that determines the chemical properties of the element.  The electrons in the outer energy level are called valence electrons.  The electrons available to the be lost, gained, or shared in the formation of chemical compounds are referred to as valence electrons.  These electrons are what determines an atom’s chemical properties.

7. Energy Levels  The energy levels are named using numbers one to seven.  The maximum number of electrons that can be contained in each of the first four energy levels is shown below.

8. Rows on the Periodic Table  Remember that the atomic number found on the periodic table is equal to the number of electrons in a neutral atom.  The first row has hydrogen with one electron and helium with two electrons both in energy level one.  Energy level one can hold only two electrons. Therefore, helium has a full or complete outer energy level.

9. Rows on the Periodic Table  The second row begins with lithium, which has three electrons—two in energy level one and one in energy level two.  Lithium is followed by beryllium with two outer electrons, boron with three, and so on until you reach neon with eight outer electrons.  Do you notice how the row in the periodic table ends when an outer level is filled?

10. Bohr Models  A Bohr model of the atom is a simplistic representation using an element’s symbol as the nucleus and circles to represent the energy levels in the atom.  Dots are used to represent electrons in the energy levels.  Remember, in reality, scientists cannot tell exactly where an electron is at a given moment or where it is going.

11. Bohr Models  The picture shows a Bohr model.  For simplicity, remember we are going to use the element’s symbol as the nucleus.  In the picture, the nucleus is too large.  Or, put another way, if the nucleus were going to be that large, the electrons are too close.  If the picture were accurate, we would have to place the electrons about a mile away.

13. Electron Dot Diagrams  Remember elements that are in the same group have the same number of electrons in their outer energy level.  These outer electrons are so important in determining the chemical properties of an element that a special way to represent them has been developed.

14. Electron Dot Diagrams  An electron dot diagram uses the symbol of the element and dots to represent the electrons in the outer energy level.  Electron dot diagrams are used also to show how the electrons in the outer energy level are bonded when elements combine to form compounds.

15. Electron Dot Diagrams  To draw electron dot diagrams, determine the number of electrons in the outer energy level.  Chlorine contains seven electrons in its outer energy level.  The element symbol is written and dots are placed around the symbol to indicate the valence electrons.  The element symbol represents the nucleus and the inner electrons.

16. Groups and Valence Electrons  Notice in the figure that all the elements in group 1 have the same number of valence electrons thus, their electron dot diagrams are similar.  So generally speaking, the number of valence electrons stays the same as you go up or down a group.  This holds true for groups 1, 2, 13, 14, 15, 16, 17, and 18 (except for He).

17. Groups and Valence Electrons  Group numbers are related to the number of valence electrons for Groups 1, 2, 13-18.  These groups are referred to as the main- group elements.  The figure at the right shows the elements of Group 1. Notice they have one valence electron.  Group 2 has two valence electrons.  For Groups 13-18, to determine the number of valence electrons, subtract 10 from the group number.  For example, Group 13 has three valence electrons and Group 14 has four valence electrons.