Periodic Table

The how and why

History 1829 German J. W. Dobereiner

Grouped elements into triads• Three elements with similar

properties• Properties followed a pattern• The same element was in the middle

of all trends• Example: Ca, Ba, Sr

Not all elements had triads

1863 – John Newlands suggested another classification.

Put elements in order of increasing atomic masses.

Found repetition of similar properties every 8th element.

He arranged the elements (known at that time) into 7 groups of 7.

Law of Octaves

Russian scientist Dmitri Mendeleev taught chemistry in terms of properties

Wrote down the elements in order of increasing mass

Found a pattern of repeating properties

Difference – thought that similar properties occurred over periods (rows) of varying length.

Mendeleev’s Table Grouped elements in columns by similar

properties in order of increasing atomic mass

Found some inconsistencies - felt that the properties were more important than the mass, so switched order.

Found some gaps Must be undiscovered elements Predicted their properties before they

were found

Predicted Properties - Ekasilicon Actual Properties - Germanium

Atomic mass 72 72.6

Melting Point high 958

Density 5.5 g/cm3 5.36 g/cm3

Dark gray metal Gray metal

Will obtain from K2EsF6 K2GeF6

Slightly dissolved by HCl Not dissolved by HCl

Will form EsO2 Does form oxide (GeO2)

Density of EsO2 4.7 g/cm3 Density of GeO2 = 4.70 g/cm3

The Modern Table Elements are still grouped by properties Similar properties are in the same

column Late 1800’s added a column of elements

Mendeleev didn’t know about. 1911 - Henry Moseley recognized

increasing nuclear charge was a better order for arranging elements

Horizontal rows are called periods There are 7 periods

Vertical columns are called groups. Elements are placed in columns by

similar properties. Also called families

1A

2A 3A 4A 5A 6A7A

8A0

The elements in the A groups are called the representative elements

1A 2A

3A 4A 5A 6A 7A

8A

3B 4B 5B 6B 7B 8B 8B 8B 1B 2B

1 2

13 14 15 16 17

18

3 4 5 6 7 8 9 10 11 12

IA IIA

IIIB

IVB

VB

VIB

VII

B

VII

IB

IIIA

IVA

VA

VIA

VII

A

VII

IA

IB IIB

Other Systems

Metals

Metals Luster – shiny. Ductile – drawn into wires. Malleable – hammered into sheets. Conductors of heat and electricity.

Transition metals The Group B

elements

Non-metals Dull Brittle Nonconductors

- insulators

Metalloids or Semimetals Properties of both Semiconductors

These are called the inner transition elements and they belong here

Group 1A are the alkali metals Group 2A are the alkaline earth metals

Group 6A is called the chalcogens Group 7A is called the Halogens Group 8A are the noble gases

Alkali metals (group 1):

Extremely reactive, soft metals with low density that form ions with a +1 charge.

Alkaline earth metals (group 2): Slighly less reactive than alkali metals, they are somewhat denser and less soft. They form ions with a +2 charge.

Halogens (group 17): Highly reactive and electronegative nonmetallic elements that form ions with a -1 charge. They are diatomic, volatile, and very difficult to handle safely.

Noble gases (group 18): Very stable nonmetallic gases that react poorly with other elements.

Transition metals (groups 3-12): Dense, hard metallic elements that usually form ions with more than one possible positive charge.

Lanthanides and actinides (the two rows at the bottom of the periodic table): The lanthanides are the top row and are reactive, dense metals. The actinides are the bottom row and include mainly radioactive elements that are produced artificially.

Main group elements: These elements consist of groups 1, 2, and 13-18.