The Periodic Table of the Elements
In 1867, a Russian chemist, Dmitri Mendeleev wrote down all
the then known elements on separate cards with their
chemical and physical properties. He ordered these cards
into chart, the first Periodic Table.
Mendeleev’s
First Chart
Mendeleev
arranged the
element in order
by their then
known weights.
When he came to
an element that
had similar
properties to a
previous element,
he put them sideby-side.
Mendeleev’s
First Chart
When the next
heaviest element
did not fit side-byside in terms of its
properties,
Mendeleev
continued down
leaving spaces
until it was next to
an element with
similar properties.
Mendeleev then
predicted that new
elements would be
found to fill those
spaces.
New Elements Were Found As Predicted by Mendeleev
The order that
Mendeleev found in
the elements helped to
discover new
elements like
germanium. He stated
the Periodic Law
which today is that if
elements are arranged
in order by their
atomic numbers,
elements with similar
properties to earlier
elements will
periodically be found.
The Modern Periodic Chart Arranged by Increasing
Atomic Number
The Atomic Number of an element is the number of protons
its atoms have in their nuclei.
The Modern Periodic Chart
In the Modern Periodic Chart, elements are arranged in
increasing order by their atomic numbers in horizontal rows
called periods.
The Modern Periodic Chart
Whenever an element has chemical and physical properties
like a previous element, it is placed underneath the previous
element.
The Modern Periodic Chart
Thus vertical columns represent elements that have similar
physical and chemical properties. These vertical columns
are called chemical families.
The Atomic Mass – An Average
• The atomic masses listed on the periodic chart are
averages of different atoms of the element called isotopes.
The Atomic Mass Unit (amu)
One atomic mass unit is about the weight of one proton.
Exactly it is 1.66 x 10-27 kg. The Atomic Mass of each element
in the Modern Periodic Chart is an average of its various
isotopes (forms of the same element with different numbers
of neutrons) in amus.
The Ion Charge
Neutral atoms have the same number of electrons as protons.
But most atoms like to gain or lose electrons since this
makes them more stable. If a neutral atoms loses an electron
(1- in charge), then it becomes a +1 ion. A charged atom or
group of atoms is called an ion. The ion charge is the electric
charge that forms when an atom gains or loses electrons to
become more stable.
Ion Charges in the Periodic Chart
Elements in the same families have similar ion charges.
Metals, Non-metals and Metalloids (Semi-metals)
The Alkali Metal Family
The alkali metal elements are silvery, have a low density (float
on water), react very strongly with water to form hydrogen
gas and very strong bases (alkalis).
The Alkaline Earth Family
The alkaline earth metal family is less reactive than the alkali
metals but alkaline earth metals burn brightly in air, some
very colourful (strontium is used in red fireworks). Alkaline
earth elements also react with water and are very common
elements in earth rocks (calcium and magnesium especially).
The Halogen Family
The halogens are very reactive and corrosive, especially with
metals (Na + Cl  NaCl {table salt}). At room temperature,
fluorine and chlorine are gases while bromine is a liquid and
iodine is a solid. Chlorine, bromine and iodine are very good
anti-bacterial agents.
The Noble Gas Family
The noble gases are non-reactive and stable by themselves.
They are all colourless and odourless gases. They glow with
distinctive colours (Ne is reddish) when electricity is passed
through them. Their ion charges of zero indicate that they do
not form charged ions.
Alphabetical List of Elements
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