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Chapter 3The Periodic Table
The how and whyThe how and why
History Dmitri Mendeleev - Russian scientist
• taught chemistry in terms of properties.
Mid 1800’s - molar masses of elements were known.
Wrote down the elements in order of increasing mass.
Found a pattern of repeating properties.
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.
The modern table Elements are still grouped by
properties. Similar properties are in the same
column. Order is in increasing atomic number. A column of elements Mendeleev didn’t
know about was added later. The noble gases weren’t found because
they didn’t react with anything.
Why? The part of the atom another atom
sees is the electron cloud. More importantly the outside orbitals. The orbitals fill up in a regular pattern. The outside orbital electron
configuration repeats. Outside electrons are called valence
electrons The properties of atoms repeat.
Electron Arrangements repeat The shape of the periodic table is a
representation of this repetition. When we get to the end of the row
the outermost energy level is full. Full energy level is the most stable
• Noble gases do not react because they are already stable
Determining Valence Electrons Valence electrons
determine how a chemical will react.
For group A elements, the group number is the number of valence electrons
Valence Electrons All elements would like to
have 8 electrons (except H and He)
8 electrons means full outside shell
8 electrons means more stable
Electron Dot diagrams A way of keeping track of
valence electrons. How to write them:
• Write the symbol.
• Put one dot for each valence electron
• Don’t pair up until they have to
X
The Electron Dot diagram for Nitrogen
Nitrogen has 5 valence electrons.
First we write the symbol. NThen add 1 electron at a time to each side.Until they are forced to pair up.
Write the electron dot diagram for
Na Mg C O F Ne He
Periodic Table Setup
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
The group B are called the transition elements
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 7A is called the Halogens Group 8A are the noble gases
Transition Metals Chromium, gold, silver,
nickel, zinc, iron, etc. Transition metals do not
behave predictably Their atomic structure is
more complicated
Inner Transition Metals Two rows “under” main
periodic table First row is lanthanides – rare
earth metals Second row is actinides –
radioactive
Nonmetals Don’t conduct electricity Brittle as solids Low melting points
Metalloids Between metals and
nonmetals Semiconductors – conduct
some electricity
Semiconductors Electricity is the flow of
electrons Metals conduct electricity
because their electrons are free to move
Nonmetals do not conduct because their electrons are locked in place
Semiconductors Semiconductors work best
when a small amount of another element is added
Called doping Si (4 valence e-) doped with P
(5 valence e-) gives an extra electron
Called n-type for negative charge
Semiconductors P- type have one less
electron and are positive Can combine types to form pnp- or npn- type
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