The Atom and the Periodic Table

Electron Cloud Structure

Energy Levels Rows on the

Periodic Table Bohr Models Electron Dot

Diagrams

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?

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.

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).

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.

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.

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.

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?

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.

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.

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.

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.

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.

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).

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.