chapter 3 the periodic table the how and why. history u dmitri mendeleev - russian scientist taught...
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Chapter 3The Periodic Table
The how and whyThe how and why
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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.
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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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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.
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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.
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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
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Determining Valence Electrons Valence electrons
determine how a chemical will react.
For group A elements, the group number is the number of valence electrons
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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
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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
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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.
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Write the electron dot diagram for
Na Mg C O F Ne He
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Periodic Table Setup
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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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The group B are called the transition elements
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 7A is called the Halogens Group 8A are the noble gases
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Transition Metals Chromium, gold, silver,
nickel, zinc, iron, etc. Transition metals do not
behave predictably Their atomic structure is
more complicated
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Inner Transition Metals Two rows “under” main
periodic table First row is lanthanides – rare
earth metals Second row is actinides –
radioactive
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Nonmetals Don’t conduct electricity Brittle as solids Low melting points
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Metalloids Between metals and
nonmetals Semiconductors – conduct
some electricity
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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
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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
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Semiconductors P- type have one less
electron and are positive Can combine types to form pnp- or npn- type
Allow electronic devices to be small: laptop, hearing aids, cell phones