At first glance, the periodic table looks very complex. In
fact it is a large grid of every element that exists. The elements are arranged
in order of their atomic number. The atomic number is the number of protons
each atom has in its nucleus. By arranging the elements in this way, those with
similar properties (characteristics) are grouped together. As with any grid,
the periodic table has rows running left to right, and columns running up and
down. The rows are called PERIODS and the columns are called GROUPS.
One element that Mendeleyev left a gap for in his periodic table was
gallium (element 31). Mendeleyev called it eka-aluminium because he predicted
it would have similar properties to aluminium. In 1875, French scientist Lecoq
de Boisbaudran discovered gallium. It has the exact properties that Mendeleyev
predicted. Gallium is a soft, silvery metal with a melting point of
29.8ºC (85.6ºF).
BIOGRAPHY: DIMITRI MENDELEYEV Russian, 1834-1907
This chemist was convinced there was an order to the elements. He
collected information on each one and, in 1869, he published a table of
elements on which the modern periodic table is based. He left gaps for elements
he predicted would be found, such as gallium, germanium, and scandium.
There are 18 groups (columns) in the periodic table.
Group 1 (also known as the alkali metals) is the column on the far left of the
table. Elements in the same group have similar, but not identical
characteristics. This is because they all have the same number of electrons in
their outermost shell. You can tell a lot about an element just by knowing
which group it is in.
As you move down one element in a group, there is a large jump in
the number of protons and neutrons in the nucleus, and a new shell of electrons
is added. The extra particles make the atom heavier and the extra shell of
electrons makes the atom take up more space.
An astronaut’s visor is gold-plated to reflect sunlight.
This shiny, hard-wearing metal does not corrode (rust), making it ideal for use
in space, where materials cannot be replaced easily. Gold, copper, and silver
belong to group 11. Group 11 metals are also called coinage metals, because
they are used to make coins.
The properties of the elements across a period (row)
change gradually. The first and last elements are very different. The first is
a reactive solid – it catches fire when it mixes with oxygen –
and the last is an unreactive gas. However, they have the same number of
electron shells. All the elements in the third period, for example, have three
shells for their electrons.
As you go across a period, the atoms get slightly heavier, but
they also get smaller. This is because the number of electron shells stays the
same across the period, but the number of protons in the nucleus increases. The
stronger, attractive force from the positively charged protons sucks the
negatively charged electrons tighter into the centre.
Phosphorus is a non-metal element. It is a yellowish, waxy,
slightly see-through solid. Like magnesium, it is very reactive. Because of
this, phosphorus compounds are used on the tips of matches. Phosphorus glows in
the dark, an effect called phosphorescence.
Argon is very unreactive and does not combine with other elements.
In arc welding, metals are melted surrounded by argon gas. The argon keeps
oxygen out, so that oxygen cannot react with the melted metals.
