ch 5 notes part 1

History of the Periodic

Table

Chapter 5

Chapter 5

History of the Periodic TableThe Periodic LawThe Periodic Law –

Physical and Chemical Properties

of Elements are Periodic Functions

of their Atomic Numbers

History of the Periodic TableThe Periodic LawThe Periodic Law –

Physical and Chemical Properties

of Elements are Periodic Functions

of their Atomic Numbers

The development of the periodic table begins with German chemist

Johann Dobereiner (1780-1849) who 

grouped elements based on similarities. 

Calcium (atomic mass 40), strontium (atomic mass 88),

and barium (atomic mass 137) possess similar chemical

properties. (Find them on the

Periodic Table)

Dobereiner noticed the atomic mass of strontium fell midway between the masses

of calcium and barium:      Ca Sr   Ba     

40      ??     137(40 + 137) ÷ 2 = ??

Dobereiner  noticed the same pattern for the alkali metal triad (Li/Na/K) and

the halogen triad (Cl/Br/I). Li  Na  K   Cl   Br   I 7    ??  39    35    ??  127

(Was this merely a coincidence or did some pattern to the arrangement of the

elements exist?)

1829 Dobereiner proposed the

Law of Triads: The middle element in the triad had atomic mass that

was the average of the other two members.

1829 Dobereiner proposed the

Law of Triads: The middle element in the triad had atomic mass that

was the average of the other two members.

(Soon other scientists found chemical relationships extended

beyond triads. Fluorine was added to Cl/Br/I group; sulfur, oxygen,

selenium and tellurium were grouped into a family; nitrogen, phosphorus, arsenic, antimony, and bismuth were classified as

another group.)

First International First International Congress of ChemistsCongress of Chemists

Karlsruhe, GermanyKarlsruhe, Germany 1860 1860-to discuss uniform ways to

measure Atomic Mass and to solve other communication

problems.-Only 60 Elements at this timeOnly 60 Elements at this time

First International First International Congress of ChemistsCongress of Chemists

Karlsruhe, GermanyKarlsruhe, Germany 18601860-to discuss uniform ways to

measure Atomic Mass and to solve other communication

problems.-Only 60 Elements at this timeOnly 60 Elements at this time

Mendeleev insisted on ordering elements by

atomic mass, and grouping them by their

PROPERTIES.PROPERTIES.

Demetri Mendeleev 1869 Demetri Mendeleev 1869 FATHER OF THE FATHER OF THE

MODERN PERIODIC TABLEMODERN PERIODIC TABLE

Mendeleev insisted on ordering elements by

atomic mass, and grouping them by their

PROPERTIES.PROPERTIES.

Demetri Mendeleev 1869 Demetri Mendeleev 1869 FATHER OF THE FATHER OF THE

MODERN PERIODIC TABLEMODERN PERIODIC TABLE

Mendeleev insisted on ordering elements by

atomic mass, and grouping them by their

PROPERTIES.PROPERTIES.

Demetri Mendeleev 1869 Demetri Mendeleev 1869 FATHER OF THE FATHER OF THE

MODERN PERIODIC TABLEMODERN PERIODIC TABLE

This resulted in several "gaps" in the Periodic

Table. Both Gallium (Ga) and Germanium (Ge)

were unknown at the time Thus there was a gap under Aluminum (Al) and a gap

under Silicon (Si)

Mendeleev concluded Mendeleev concluded therefore that there therefore that there must be two elements, must be two elements,

which he called which he called "eka-Aluminum" and "eka-Aluminum" and "eka-Silicon" which "eka-Silicon" which must fill these gaps must fill these gaps

Mendeleev made predictionspredictions on some of the vacant spaces

in the Periodic Table. Such as:

Eka-silicon, (Germainium) Eka-aluminium, (Gallium)

Gallium was discovered in 1875

by Paul Emile. Its general

chemistry matched Mendeleev's

predictions for Eka-aluminium.

72.32

5.5

13

Outer Shell Electrons (Dots)

Property Ekasilicon Germanium

Atomic Mass 72

Density (g/cc) 5.47

Atomic volume 13.22

44

Boiling point of GeCl4

/degrees Celsius86<100

Germanium (ekasilicon) was discovered in 1886 by Clemens Winkler.

The agreement with Mendeleev's predictions are shown in the table below

Henry Moseley 1913Assistant to Rutherford

(killed at Gallipoli at age 28) -discovered a mathematical

relationship between the frequency of X-rays and the

atomic number.

Henry Moseley 1913Assistant to Rutherford

(killed at Gallipoli at age 28) -discovered a mathematical

relationship between the frequency of X-rays and the

atomic number.

He noticed that when struck by the cathode rays, different metals gave off x-rays with

distinct wavelengths.

Moseley realized that the atomic numbers were not just a convenient numbering scheme for the elements, but had a real physical meaning - ultimately

realized as being the number of protons

(and electrons) in an (neutral) element

Law of Octaves (1837-1898), English chemist John Newlands

having arranged the 62 known elements in order of increasing atomic

masses, noted that at in intervals of eight, elements had

similar physical/chemical properties. 

Newlands was the first to formulate the concept of

periodicity in the properties of the chemical elements.

In 1863 he wrote a paper proposing The Law of Octaves:

Elements exhibit similar behavior to the eighth element following it

in the table.

Noble GasesNoble Gases1864 HeHe discovered on the Sun1894 Englishman John Williams Strutt (Lord Rayleigh) and Scottish Sir William Ramsey discovered Argon1895 HeHe discovered on Earth1898 Krypton and Xenon-Ramsey1900 Radon discovered by German Friedrich Ernst Dorn