Biology

1. HISTORY OF THE PERIODIC TABLE (PT)
 Dmitri Mendeleev arranged the first periodic
table in 1872.
 At this time there were only about 70 known
elements.
 He arranged them in order of increasing atomic
mass and according to similar properties of
elements.

2. SKETCH OF FIRST DRAFT OF PERIODIC TABLE

3. HENRY MOSELEY
 In 1913, Henry Moseley discovered atomic number
and arranged the PT accordingly.
 Based on this discovery, the re-arranged the
table according to increasing atomic number.
This arrangement of the periodic table had
fewer inconsistencies and therefore is still
used today.

4. THE MODERN PERIODIC TABLE
 Periodic Law – “Physical & chemical properties of
the elements are periodic functions of their atomic
number.”
 Element properties show a repeated pattern when they
are arranged in order of increasing atomic number
(number of protons). This allows us to identify trends in
the periodic table.

5. GROUPS & PERIODS