Transcript File - singhscience

Elements – different types of atom
Elements are the simplest substances. There are about 100
different elements.
Each element is made up of very tiny particles called atoms, and
each element is made up of just one particular type of atom, which
is different to the atoms in any other element.
Gold is an element
made up of only gold
atoms.
Carbon is an element
made up of only carbon
atoms.
The Periodic Table is a big list of the
elements!
The beginnings of the
periodic table
• By the 1860’s 63 elements were known.
Chemists were keen to make a table that
organised the elements in a helpful way,
but this proved difficult.
• In one table of elements, every 8th
elements had similar properties.
Unfortunately this worked only as far as
calcium so it was a success.
Dmitri Mendeleev (1834-1907)
• Then in 1869, the Russian chemist
Dmitri Mendeleev (1834-1907) was
busy writing the second volume of his
chemistry textbook.
• He was having troubling deciding which
elements to write about next.
Mendeleev’s solution was to construct
a table.
The Periodic Table
• Mendeleev’s table is now called the
periodic table. To make his original
table, Mendeleev used the latest
measurements of atomic masses
(called ‘weights’ then) available.
• He also carefully considered the
properties of the different elements.
Atomic Mass
• Mendeleev arranged the elements in order of
increasing atomic mass. Other chemists had tried this
before, but Mendeleev sometimes broke this rule.
• He did so that elements was similar properties lined
up in his able. For example iodine should come
before tellurium according to its atomic mass.
• Mendeleev swapped the positions of these two
elements that they lined up with the elements with
similar properties.
Leaving Gaps
• Mendeleev put elements with similar
properties into horizontal rows in his first
table.
• However with similar properties organised
into vertical columns, just as in the modern
periodic table. Unlike other chemicals,
Mendeleev thought that there must be
elements still to discover, so he left gaps for
them.
Making Predictions
• Mendeleev used the gaps in his table to
make predictions about the properties of
undiscovered elements. One of these
predictions was for an elements that he
called eka-aluminium.
• When gallium was discovered shortly
afterword in 1875, its properties closely
fitted those he had predicted for ekaaluminium.
Key Points about the Modern Periodic
Table
• Elements are arranged in order of increasing
atomic number (the number of protons) rather
than in order of atomic mass as Mendeleev did.
• Horizontal rows are called periods
• Vertical Columns are called groups
• Each group contains elements with similar
properties, the main groups are numbered 1 to 7
left to right. The group on the far right is group 0.