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.