Chapter 5:Chapter 5:The Periodic LawThe Periodic Law

Coach KelsoeCoach Kelsoe

ChemistryChemistry

Pages 132-169Pages 132-169

Section 5-1:Section 5-1:History of the Periodic TableHistory of the Periodic Table

Coach KelsoeCoach Kelsoe

ChemistryChemistry

Pages 133-137Pages 133-137

Section ObjectivesSection Objectives

Explain the roles of Mendeleev and Moseley Explain the roles of Mendeleev and Moseley in the development of the periodic table.in the development of the periodic table.

Describe the modern periodic table.Describe the modern periodic table. Explain how the periodic law can be used to Explain how the periodic law can be used to

predict the physical and chemical properties predict the physical and chemical properties of elements.of elements.

Describe how the elements belonging to a Describe how the elements belonging to a group of the periodic table are interrelated in group of the periodic table are interrelated in terms of atomic number.terms of atomic number.

Origins of the Periodic TableOrigins of the Periodic Table

In September of 1860, In September of 1860, chemists assembled at the chemists assembled at the First International Congress First International Congress of Chemists in Karlsruhe, of Chemists in Karlsruhe, Germany.Germany.

Italian chemist Stanislao Italian chemist Stanislao Cannizzaro presented a Cannizzaro presented a method for accurately method for accurately measuring atomic masses.measuring atomic masses.

Mendeleev and Chemical PeriodicityMendeleev and Chemical Periodicity

Russian chemist Dmitri Mendeleev started Russian chemist Dmitri Mendeleev started writing a chemistry book, using the values writing a chemistry book, using the values agreed upon at the Congress of Chemists and agreed upon at the Congress of Chemists and organized all this information.organized all this information.

He noticed that when the discovered He noticed that when the discovered elements were arranged in order of increasing elements were arranged in order of increasing atomic mass, certain similarities in their atomic mass, certain similarities in their chemical properties appeared at regular chemical properties appeared at regular intervals.intervals.

Mendeleev’s Periodic TableMendeleev’s Periodic Table

Mendeleev and Chemical PeriodicityMendeleev and Chemical Periodicity

Mendeleev created a table in which elements Mendeleev created a table in which elements with similar properties were grouped together.with similar properties were grouped together.

His original table was arranged in order of His original table was arranged in order of increasing atomic mass, not atomic number.increasing atomic mass, not atomic number.

This “periodic table” was first published in This “periodic table” was first published in 1869.1869.

Mendeleev originally set up his table Mendeleev originally set up his table horizontally and left gaps in the table where horizontally and left gaps in the table where he predicted the existence of new elements.he predicted the existence of new elements.

Mendeleev and His TableMendeleev and His Table

Mendeleev was credited as the discoverer of Mendeleev was credited as the discoverer of the periodic law.the periodic law.

““Periodic” means “having a repeating pattern.”Periodic” means “having a repeating pattern.” There were two questions remaining about There were two questions remaining about

the periodic table though:the periodic table though: Why could most of the elements be arranged in Why could most of the elements be arranged in

the order of increasing atomic mass but a few the order of increasing atomic mass but a few could not?could not?

What was the reason for chemical periodicity?What was the reason for chemical periodicity? Mendeleev never won a Nobel Prize!Mendeleev never won a Nobel Prize!

Moseley and the Periodic LawMoseley and the Periodic Law

The question of why elements The question of why elements were not always listed in order of were not always listed in order of atomic mass was unanswered atomic mass was unanswered for another 40 years.for another 40 years.

In 1911, English scientist Henry In 1911, English scientist Henry Moseley discovered that Moseley discovered that elements in the table fit better elements in the table fit better when arranged in order of when arranged in order of increasing nuclear charge increasing nuclear charge (atomic number).(atomic number).

Moseley and the Periodic LawMoseley and the Periodic Law

Moseley’s discovery was Moseley’s discovery was consistent with consistent with Mendeleev’s ordering of Mendeleev’s ordering of the periodic table by the periodic table by properties instead of properties instead of strictly atomic mass.strictly atomic mass.

Today, Mendeleev’s Today, Mendeleev’s principle of chemical principle of chemical periodicity is correctly periodicity is correctly stated as the periodic law.stated as the periodic law.

Periodic LawPeriodic Law

The The periodic lawperiodic law states “the physical and states “the physical and chemical properties of the elements are periodic chemical properties of the elements are periodic functions of their atomic numbers.”functions of their atomic numbers.”

In our terms, when the elements are arranged in In our terms, when the elements are arranged in order of increasing atomic number, elements with order of increasing atomic number, elements with similar properties appear at regular intervals.similar properties appear at regular intervals.

The Modern Periodic TableThe Modern Periodic Table

Today’s periodic table is a lot Today’s periodic table is a lot different from Mendeleev’s different from Mendeleev’s original table.original table.

The The periodic tableperiodic table is an is an arrangement of the elements arrangement of the elements in order of their atomic in order of their atomic numbers so that elements numbers so that elements with similar properties fall in with similar properties fall in the same column, or group.the same column, or group.

The Noble GasesThe Noble Gases

English physicist John English physicist John William Strutt and Scottish William Strutt and Scottish chemist Sir William Ramsay chemist Sir William Ramsay discovered argon, only 26 discovered argon, only 26 years after discovering years after discovering Helium.Helium.

Ramsey proposed Group 18; Ramsey proposed Group 18; gases that are generally gases that are generally unreactive. We call them unreactive. We call them Noble Gases.Noble Gases.

The LanthanidesThe Lanthanides

The next addition to the periodic table was The next addition to the periodic table was the lanthanides.the lanthanides.

The The lanthanideslanthanides are the 14 atomic elements are the 14 atomic elements with atomic numbers from 58 (Cerium, Ce) to with atomic numbers from 58 (Cerium, Ce) to 71 (Lutetium, Lu).71 (Lutetium, Lu).

The LanthanidesThe Lanthanides

The lanthanides belong in period 6.The lanthanides belong in period 6. Because these elements are so similar in Because these elements are so similar in

chemical and physical properties, the process chemical and physical properties, the process of separating them from the rest was a long of separating them from the rest was a long process.process.

The ActinidesThe Actinides

The next major development of the periodic The next major development of the periodic table came with the discovery of the table came with the discovery of the actinides.actinides.

The The actinidesactinides are the 14 elements with are the 14 elements with atomic numbers from 90 (Thorium, Th) to 103 atomic numbers from 90 (Thorium, Th) to 103 (Lawrencium, Lr).(Lawrencium, Lr).

The ActinidesThe Actinides

The actinides belong in period 7.The actinides belong in period 7. The actinides as well as the lanthanides are The actinides as well as the lanthanides are

usually set below the main portions of the usually set below the main portions of the periodic table.periodic table.

Both actinides and lanthanides fall between Both actinides and lanthanides fall between Groups 3 and 4.Groups 3 and 4.

PeriodicityPeriodicity

Periodicity with respect to atomic number can Periodicity with respect to atomic number can be observed in any group of elements.be observed in any group of elements.

Group 1 elements are very similar with Group 1 elements are very similar with respect to their properties, but are different respect to their properties, but are different from each other and different from those in from each other and different from those in other groups.other groups.

Groups 1-2, and 13-18 follow a pattern of 8, 8, Groups 1-2, and 13-18 follow a pattern of 8, 8, 18, 18, and 32 with the exception of 18, 18, and 32 with the exception of hydrogen.hydrogen.