The modern periodic table is based on Mendeleev's
arrangement, but instead of being arranged
by atomic mass, as Mendeleev's was, it is
arranged by elements ATOMIC NUMBER.
These are different because the ATOMIC MASS
is the total mass of the atom determined by
its number of protons AND neutrons, whereas
the ATOMIC NUMBER is the number of protons
in the atom.
Arranging the elements in order of their atomic
number solved some of the problems associated
with Mendeleev's table. For example in Mendeleev's
table Iodine, which has a mass of 127, should
be put before Tellurium which has a mass of
128. Why? Because Iodine has a smaller
mass than Tellurium.
But Mendeleev noticed that Iodine has similar
properties to Fluorine, Chlorine and Bromine,
so he broke his own rule and put Iodine within
this column.
In the modern periodic table, where the elements
are ordered by their atomic number, Tellurium,
which has an atomic number of 52, would be
put before Iodine, which has an atomic number
53 so they fit into their correct positions
without breaking any rules.
In the Modern Periodic Table there are currently
around 115 elements, and although new elements
aren't really being discovered anymore, new
ones are being made by scientists.
These new elements are highly radioactive.
Radioactive elements have an incredible short
existence, because they decay very rapidly,
but if we know where they would be in the
periodic table, we can make really good predictions
about the properties that they would exhibit.
In the most basic form, the periodic table
can be split into metals and non-metals, with
the Metals dominating most to the left side
of the table, and the non-metals on the right.
There are sub-categories within these which
we will learn about throughout the course.
