Download Intro to Periodic Table and Lewis Structures

The Periodic
Table
An Introduction
• I CAN describe the organization of the periodic
table and draw a Lewis Electron Dot Structure
for an element.
Development of the Periodic Table
• By the 1800s, scientists starting looking for
ways to organize the elements known at the
time [about 68] into a meaningful
arrangement.
• A number of scientists developed tables of the
elements. Since most had shortfalls, none of
them were very good.
Mendeleev’s Table
• In 1869, a RUSSIAN chemist by the name of DMITRI
MENDELEEV made a successful table of the element
[about 68 by this time].
• Mendeleev made cards with all the known
information about each element, including its atomic
mass number.
• He placed elements in horizontal rows from lowest to
highest atomic mass number [amn].
• When an element appeared that had similar
properties to one already in a row, Mendeleev placed
that element BELOW the previous one: this formed
VERTICAL COLUMNS of elements.
Increasing ATOMIC MASS NUMBER
• As Mendeleev continued placing elements
into his table, sometimes he had to leave
BLANK SPACES in order to keep all elements
with similar properties in the same vertical
column.
• The blank spaces represented elements that
were NOT YET DISCOVERED!!
• He predicted where several other elements
would fit.
• Within his lifetime, 3 other elements were
discovered that fit where he had predicted:
– Scandium (Sc), Germanium (Ge) and Gallium (Ga)
Problems with Mendeleev’s Table
• After Mendeleev died, more new elements
were discovered that did not fit into his table
as he had predicted.
• Often, they were only off by a vertical column
to the right or left of where he had predicted.
• This problem was not solved for nearly two
decades.
Moseley Rescues Mendeleev’s Table
• A ENGLISH scientist named HENRY MOSELEY
fixed the problems with Mendeleev’s table
with a simple rearrangement.
• By Moseley’s time (1913), it was possible to
determine the atomic number of an element.
• He rearranged the table by INCREASING
ATOMIC NUMBER rather than by AMN.
• When he did this…ALL the elements fit
into the table!
• The modern PT is still organized in this
way!
The Concept of PERIODICITY
• Mendeleev had discovered an important
principle of chemistry:
•
PERIODICITY
• Periodicity simply means that a
property or characteristic REPEATS
by a pattern.
• In Mendeleev’s table, it turns out
that the properties of the elements
were repeated by every 8th element.
The Periodic Law
• Mendeleev’s discovery lead to the proposal of
the PERIODIC LAW.
• The Periodic Law simply states that
“when the elements are arranged by
increasing ATOMIC NUMBER, their physical
and chemical properties repeat at regular
intervals”.
• This is where the name PERIODIC TABLE
comes from!
Valence Electrons
• The periodic table arrangement is based on
the arrangement of electrons in an atom’s
electron cloud.
• The physical and chemical properties of an
element are a result of the arrangement of
electrons in the outermost energy level of the
atom.
• The outermost energy level is referred to as
the VALENCE ENERGY LEVEL and the electrons
in this outermost level are called VALENCE
ELECTRONS, or Ve- for short!
• The number of Ve- an atom has can easily be
determined from the GROUP NUMBER THE
ELEMENT IS IN on the periodic table.
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Group 1
1 V eGroup 2
2 V eGroup 3-12 TRANSITION ELEMENTS [skip for now]
Group 13 3 VeGroup 14 4 VeGroup 15 5 VeGroup 16 6 VeGroup 17 7 VeGroup 18 8 Ve-
Lewis Electron Dot Diagrams
• Valence electrons [Ve-] are represented by
Lewis Electron Dot Diagrams, or Lewis
Structures for short.
• A Lewis Structure consists of the element’s
chemical symbol with dots (sometimes an x is
used) placed around it to represent Ve-.
Writing a Lewis Structure
• Find the element on the PT.
• Go to the TOP of column to the group number
and determine the number of Ve- the
element has.
• Write the element’s CHEMICAL SYMBOL.
• Place DOTS (or Xs) around starting on the
RIGHT SIDE and working COUNTERCLOCKWISE
in a diamond pattern (side, top, side, bottom).
• If you have MORE THAN 4 Ve-, make a second
trip around, forming PAIRS OF Ve-.
• There will NEVER BE MORE THAN 8 Ve-.
• This is an atom’s MOST STABLE FORM and is
known as an OCTET.
Practice Problem
• Draw a Lewis Structure for Oxygen
• 1. Write the chemical symbol:
O
• 2. Add dots to represent the Ve– Oxygen is in column 16, so it has 6 Ve-
O
More Practice Problems
• Draw a Lewis Structure for each of the
following elements:
• 1. Calcium
2. Aluminum
3. Chlorine
• 4. Potassium 5. Carbon
6. Sulfur
• 7. Boron
9. Magnesium
• 10. Nitrogen
8. Xenon