Periodic Table The how and why

History u 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 u Not all elements had triads

u 1863 – John Newlands suggested another classification. u Put elements in order of increasing atomic masses. u Found repetition of similar properties every 8 th element. u He arranged the elements (known at that time) into 7 groups of 7. u Law of Octaves

u Russian scientist Dmitri Mendeleev taught chemistry in terms of properties u Wrote down the elements in order of increasing mass u Found a pattern of repeating properties u Difference – thought that similar properties occurred over periods (rows) of varying length.

Mendeleev’s Table u Grouped elements in columns by similar properties in order of increasing atomic mass u Found some inconsistencies - felt that the properties were more important than the mass, so switched order. u Found some gaps u Must be undiscovered elements u Predicted their properties before they were found

Predicted Properties - EkasiliconActual Properties - Germanium Atomic mass Melting Point high958 Density 5.5 g/cm g/cm 3 Dark gray metalGray metal Will obtain from K 2 EsF 6 K 2 GeF 6 Slightly dissolved by HClNot dissolved by HCl Will form EsO 2 Does form oxide (GeO 2 ) Density of EsO g/cm 3 Density of GeO 2 = 4.70 g/cm 3

The Modern Table u Elements are still grouped by properties u Similar properties are in the same column u Late 1800’s added a column of elements Mendeleev didn’t know about. u Henry Moseley recognized increasing nuclear charge was a better order for arranging elements

u Horizontal rows are called periods u There are 7 periods

u Vertical columns are called groups. u Elements are placed in columns by similar properties. u Also called families

1A 2A3A4A5A6A 7A 8A 0 u The elements in the A groups are called the representative elements

1A2A 3A4A5A6A7A 8A 3B4B5B6B7B8B 1B2B IAIIA IIIBIVB VB VIBVIIB VIIIB IIIAIVAVA VIAVIIA VIIIAIBIIB Other Systems

Metals

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

Transition metals l The Group B elements

Non-metals l Dull l Brittle l Nonconductors - insulators

Metalloids or Semimetals l Properties of both l Semiconductors

u These are called the inner transition elements and they belong here

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

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

u Alkali metals (group 1): Extremely reactive, soft metals with low density that form ions with a +1 charge. u 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.

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

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

u Main group elements: These elements consist of groups 1, 2, and