Section 4.4—The Periodic Table

Objectives•Explain how the modern periodic table was developed•Describe the key features of the periodic table•Explain why elements in the same group have similar properties

History of the Periodic Table

In the 1790’s Antoine Lavoisier compiled a list of 33 known elements.

The 1800s brought the industrial revolution and the invention of electricity; the number of known elements grew to 70.

Johan Dobereiner (1780-1849)Noticed that some elements

could be arranged in groups of three.

He called these groups triads.The elements within each

triad had very similar chemical properties.

John Newlands (1837-1898)In 1864 J. Newlands arranged the

elements by increasing atomic mass.The properties of the 8th element

were like those of the 1st, the 9th like those of the 2nd, and so on. The properties were periodic.

Because this pattern repeats every 8 elements, Newlands called this pattern the law of octaves.

In 1869, Dmitri Mendeleev arranged elements by increasing atomic mass into columns with similar properties.

Mendeleev left blank spaces for undiscovered elements. He could predict their properties.

Mendeleev published the first periodic table. It was widely accepted because he was able to predict the properties of yet undiscovered elements.

Arrangement by atomic mass caused problems, however. Elements were placed in groups with differing properties.

In 1913, Henry Moseley discovered that each element has a unique number of protons in its atoms. (This number is equal to its atomic number.)

He arranged elements by increasing atomic number instead of atomic mass.

There was now a clear periodic pattern of properties.

Periodic Law: There is a periodic repetition of chemical and physical properties of elements when arranged by increasing atomic number.

Organization of the Periodic Table

Most periodic tables give the above information, but it can be in a different location. Some tables do not indicate the state of matter; some tables have a color-coded squares

to indicate the classification of the element – metal, nonmetal, or metalloid.

Periodic Table: Text pg. 141

Rows are called periods.Columns are called groups or

families

Groups are numbered 1 - 18

Groups can also be labeled with a number 1-8 and an A or B designation

“A” groups are referred to as main group or representative elements.

“B” groups are referred to as transition elements.

Classification of the Elements

There are 3 main classifications for the elements.MetalsNon metalsMetalloids

MetalsMost elements are metals –

they are left of the “staircase”.

Lustrous when smooth and clean.

Solid at room temperature.Good conductors of heat

and electricity.Most are ductile and

malleable.

Group IA

Group IIA

Group B metals

Group B metals

Nonmetals

Located on right side of “staircase”Generally gases or brittle, dull-

looking solids.Only liquid at room temperature is

Bromine.Poor conductors of heat and

electricity

Group VIIA

Group VIIIA

Metalloids (Semimetals)

Metalloids

Metalloids are elements that have a mix of the physical and chemical properties of both metals and nonmetals.

For example, silicon is a lustrous solid (like a metal) yet is very brittle (like a nonmetal). It is a conductor but does not conduct as well as most metals. It is often referred to as a semiconductor.

Classification by valence electrons

Valence electrons are the electrons an element has in its outer most energy level.

Atoms of the same group have similar properties because they have the same number of valence electrons.

The number (in the group number) that accompanies the “A” designations tells you the number of valence electrons for that element.

There is one exception . . .

Valence Electrons

Oxygen has 6 valence electrons Group 6A) and aluminum has 3 (Group 3A).

These are the electrons involved in chemical reactions.

They can be represented by drawing a Lewis dot (or electron dot) diagram.

Electron (or Lewis) Dot Diagrams

Dots represent the valence electrons. The rest of the atom is represented by the chemical symbol.

To find the number of valence electrons:Find the element in the periodic tableRead the label at the top of the column it

is found in - look for the number that accompanies the “A” label

That number IS the number of valence electrons the element has.

Example: C is in column 4A so it has 4 valence electrons.

Practice

Draw Lewis dot diagram for:SiliconCalciumChlorine

Classification by energy level

The period number corresponds to the energy level of the valence electrons.

Valence electrons will always be in the highest energy level.

Where are the valence electrons for Ga?

How many valence electrons does Ga have?