The Periodic Table and Periodic Law

6.1 Development of the Modern Periodic Table

Objectives Trace the development and

identify key features of the periodic table.

History of the Periodic Table’s Development

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’s Periodic Table

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.

Text pgs. 178-179

Organization of the Periodic Table

Color of each square indicates how the element is classified - metal, nonmetal, or

metalloid.

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. Metals Non metals Metalloids

Metals Most 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 The noble gases

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.