periodic table the how and why. history u 1829 german j. w. dobereiner grouped elements into triads...
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Periodic Table
The how and why
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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
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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
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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.
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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
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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
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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
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Horizontal rows are called periods There are 7 periods
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Vertical columns are called groups. Elements are placed in columns by
similar properties. Also called families
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1A
2A 3A 4A 5A 6A7A
8A0
The elements in the A groups are called the representative elements
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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
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Metals
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Metals Luster – shiny. Ductile – drawn into wires. Malleable – hammered into sheets. Conductors of heat and electricity.
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Transition metals The Group B
elements
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Non-metals Dull Brittle Nonconductors
- insulators
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Metalloids or Semimetals Properties of both Semiconductors
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These are called the inner transition elements and they belong here
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Group 1A are the alkali metals Group 2A are the alkaline earth metals
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Group 6A is called the chalcogens Group 7A is called the Halogens Group 8A are the noble gases
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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.
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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.
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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.
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Main group elements: These elements consist of groups 1, 2, and 13-18.