A.6 –A.8
In which you will learn about:
•The history of the periodic table
•Grouping elements by properties
A.6 The Periodic Table
• By the mid-1800s, chemists had identified about
60 elements
• Periodic table: an arrangement where items with
similar properties are placed near one another in
a chart
• In 1869, Dmitri Mendeleev published a periodic
table of elements
• Patterns of properties repeat themselves after
every row
– EX: Lithium is chemically similar to sodium
Properties Used to Determine
Groupings of Elements
• 1) Atomic mass
• 2) “combining capacity”
– EX: one K can combine with one Cl &
one Cs can combine with on Cl
(represented as ECl , one element E with one Cl)
--EX: one Mg can combine with two Cl &
one Sr can combine with two Cl
(represented as ECl2)
Modern Periodic Table
• In Mendeleev’s periodic table…
– Elements with similar chemical properties were
placed in the same vertical column (group)
– Horizontal arrangements (periods) were based on
increasing atomic weight
• In the modern periodic table…
– Elements with similar chemical properties are placed
in the same vertical column (group)
– Horizontal arrangements (periods) are based on
increasing atomic number [Henry Moseley figured this
out]
A.7 Grouping the Elements
• Alien Activity completed in class
A.8 The Pattern of Atomic Numbers
• Creators of the early periodic tables were
unable to explain WHY neighboring elements
had similar properties
– Answers weren’t found until 50 years after
Mendeleev’s work [Henry Moseley, see below]
• Atomic number: the number of protons in an
atom. Gives each element its identity. Also is #
of electrons for a neutral atom.
Neutrons to the Rescue!
• Using mass and properties works well for
small atoms, but falls apart for larger atoms
– Total mass of an atom is the protons AND
neutrons (both found in the nucleus)
– Mass number: # of protons + # of neutrons. THIS
IS NOT THE SAME AS ATOMIC MASS!!!
– Electrons are too small to contribute
Isotopes
• The same element can have different numbers
of neutrons = isotope
• Mass number – Atomic number = # of
neutrons
• Isotopes are the major reason for fractional
atomic masses found on the periodic table.
• More on this tomorrow when we do an
activity…
HOMEWORK (your hand-made colored periodic
table will help you with most of these)
• 1) Give another term for each of these features
on the periodic table:
– A) row
b) column
• 2) Give the names and symbols of two elements
other than lithium in the alkali metal family.
• 3) Consider the noble gas family:
– A) Where are noble gases located on the periodic
table?
– B) Name one physical property that noble gases share.
– C) Name one chemical property that noble gases
share.
HOMEWORK SOME MORE
• 4) Given a periodic table and the formulas
BeCl2 and AlN, predict the formula for a
compound containing
– A) Mg and F
B) Ga and P
• 5) The melting points of sodium (Na) and
rubidium (Rb) are 98°C and 39°C respectively.
Estimate the melting point of potassium (K).
MORE HOMEWORK
• 6) Complete the following table for each
electrically neutral atom:
Element
Symbol
Ca
# of Protons
# of Neutrons # of Electrons
6
6
6
6
7
6
21
117
U
146
78
FINAL HOMEWORK QUESTION
• 7) A student asked to explain the formation of
a lead (II) ion (Pb2+) from an electrically
neutral lead atom (Pb). The student says that
a lead atom must have gained two protons to
make the ion. How would you correct this
student’s mistaken explanation?