History of the
Periodic Table
During the nineteenth century,
chemists began to categorize the
elements according to similarities in
their physical and chemical properties.
The end result of these studies was
our modern periodic table.
Döbereiners’ Triads
• Johann Döbereiner (c. 1829) described
the idea of the triad
– Triads were groups of three
elements that shared similar
properties
– He also noted that when the
elements were arranged in order of
their increasing atomic mass, the
atomic mass of the middle element
was approximately half way between
that of the other two elements of
the triad
Döbereiners’ Triads
• He listed three triads:
– Ca, Sr, Ba
– S, Se, Te
– Cl, Br, I
Newlands Octaves
• John Newlands (c. 1864) arranged
the existing elements by increasing
atomic mass
• He noticed that properties
reemerge after 8 elements
• He called this the law of octaves
• The Law of Octaves states that the
properties of the elements seem to
repeat in a periodic fashion after
every 8.
Newlands Octaves
Newlands Octaves
• The pattern was perfect up to calcium then became less
convincing as some metals appeared unlike the non-metals to their
left.
• Note that Newlands did not always stick to a strict increase in
number. He exchanged the positions of Zn and Y, presumably
because he realised that Y resembled Bo (modern symbol B).
• On 1st March 1865, he described his ideas at a lecture at the
Chemical Society. Many critised his work saying he that he might
have equally well listed the elements alphabetically.
• Chemistry journals refused to publish his paper, supposedly
because it was of a purely theoretical nature. Humiliated,
Newlands went back to his work as chief chemist at a sugar
factory.
Dmitri Mendeleev
In 1869 he published a table of
the elements organized by
increasing atomic mass.
1834 - 1907
Mendeleev
• Russian teacher Dimitri
Mendeleev (c. 1869) proposed
the first true version of the
periodic table
• Mendeleev’s table was not
complete in that he left
“holes” where he thought
elements would later be
discovered.
Mendeleev
• For example, Mendeleev proposed that there was an
element with similar properties as silicon that should
exist, though it had not been discovered
– This element was called ekasilicon
– Later experiments isolated ekasilicon. It is now
known as Germanium today.
– Properties of germanium were almost the same as
predicted
Moseley
• English scientist Henry Moseley (c.
1913) proposed the modern periodic
table
• He arranged the periodic table in
order of increasing atomic number
as opposed to atomic mass
• Came up with periodic law
– (def) the physical and chemical
properties of the elements are
periodic functions of their
atomic numbers
Moseley
• The modern periodic law states: The
chemical and physical properties of the
elements are periodic functions of their
atomic numbers.
• Put in other words, when the elements are
arranged in order of increasing atomic
number, there is a periodic repetition of their
properties.
Moseley
His research was halted when the British
government sent him to serve as a foot
soldier in WWI. He was killed in the
fighting in Gallipoli by a sniper’s bullet, at
the age of 28. Because of this loss, the
British government later restricted its
scientists to noncombatant duties during
WWII.
Differences between Mendeleev's Periodic
Table and the Modern Periodic Table
Mendeleev’s Periodic Table
Modern Periodic Table
Fewer Elements (some not yet
discovered)
More Elements
Gaps present
No Gaps
Arranged in order of Atomic Mass
Arranged in order of Atomic
Number
Group 0 Elements missing
Group 0 Elements included
Mendeleev’s periodic table versus the
modern periodic table
• Mendeleev’s Periodic Table
• Modern Periodic Table
Our periodic table can be arranged into three
general sections
The horizontal rows of the periodic table are called
PERIODS.
The elements in any group of the
periodic table have similar physical and
chemical properties!
The vertical columns of the periodic table are called
GROUPS, or FAMILIES.
Alkali Metals
Alkaline Earth Metals
Transition Metals
These elements are also
called the rare-earth
elements.
Inner Transition Metals
Halogens
Noble Gases