C3.1 The Periodic Table

In 1863 John Newlands tried to arrange a periodic table of 63 elements, many were still undiscovered. Newlands arranged the known elements in order of their atomic weights and found similar properties wit every eighth element. He noticed periodicity although missing elements caused problems.

Dimitri Mendeleev realised that some elements had yet to be discovered, so when he created his table in 1869 he left gaps to allow for them. He used his periodic table to predict the exsistence of other elements.

The discovery of subatomic particle (protons,neutrons & electrons) and electronic structure in the early 20th century provided evidence that could be used to create the table. The periodic table was arranged in order of atomic(proton) numbers. So, the modern Peridoic Table is an arrangemnet of the elments in terms of their electronic structure.

The elements are arranged in periods (rows) according to the number of electrons in their outer energy level.

This arrangement resulted in elements with the same number of electrons in their outer energy level being in the same group.

Elements that have the same number of electrons in their outer energy level have similar properties. The table is called a Periodic table because similar properties occur at regular interavals.

The six elements in Group 1 are known as the alkali metals.

Alkali metals :

• have low melting and boiling points that decrease as you go down the group.
• have a low density (lithium,sodium and potassium are less dense than water.
• become more reactive as you go down the group.

Alkali metals have similar properties to each other because they have the same number of electrons in their outer energy level. E.g One electron.

Alkali metals becom more reactive as you go down the group becuase the outer energy level gets further away from the influence of the nucleus, and so an electron is lost more easily.

The alkali metlas are stored under oil beacuse the react very vigourously with oxygen and water. When alkali metals react with water, a metal hydroxide is formed and hydrogen gas is given off.

Potassium + Water -> Potassium hydroxide + hydrogen.

 2K (s)    + 2H20(l) -> 2KOH (aq)  + H2(g)

If a metal hydroxide is dissolved in water, an alkaline solution is produced.

Alkali metals react with non-metals to form ionic compounds. When this happens, the metal atom loses one electron to form a metal ion with a positive charge. The products are white solids that dissolve in water to form colourless solutions.

There are fove elements on Group 7. They are known as Halogens. They are non-metals.

The Halogens..

• have melting and boiling points that increase as you go down the group (at room tempertaure, fluorine and chlorine are gases and bromine is a liquid)
• have coloured vapours
• exsist as molecules made up of pairs of atoms
• become less reactive as you go don the group.

They become less reactive as you go down the group because the outer energy level gets further away from the influence of the nucleus and so an electron is gained less easily.The more energy levels an atom has... the more easily atoms are lost... the less easily electrons are gained.

Halogens react with metals to produce ionic salts. The halogen atom gains one electon to form a halide ion that carries a negative charge.

Lithium + chlorine -> lithium chloride

2Li + Cl2 -> 2LiCl

Halogens react with other non-metallic elements to form molecular compounds.

Hydrogen + chlorine -> Hydrogen chloride

A more reactive halogen will displace a less reactive halogen from an aqueous solution of it's salt.

• chlorine will displace both bromine and iodin
• bromine will displace iodine.

In the centre of the Periodic Table, between groups 2 & 3, is a block of metallic elements called the transition metals.

Many transition metals...

• form coloured compounds
• have ions with different charges
• can be used as catalysts to speed up chemical reactions.

Like all other metals, transition metals..

• are good conductors of heat and electricity
• can be easily bent or hammered

In comparison to Group 1 metals, transition metals ...

• have higher densities and higher melting points and are less reactive