C3.1 The Periodic Table

In the 19th century, Scientists tried to classify elements based on their atomic weights and properties.

In 1863, Newlands proposed his law of octaves which stated that similar properties were repeated in every eighth element (when all 62 known elements were arranged into seven groups according to their atmoic weight). However, after calcium their properties did not match very well, and so other Scientists rejected his theory.

In 1869, Mendeleev produced a better table. He left gaps for elements that had not yet been discovered, so that the group's known elements would have similar properties. He then predicted the properties of missing elements and when these predictions were confirmed, the other Scientists accepted his theory. Mendeleev's table was used as the basis for the modern day periodic table.

At the start of the 20th century, Scientists found out about the electronic structure of elements.

They arranged the elements in order of their atomic numbers and then lined them up into verticle groups. The groups of elements have similar properties because they have the same number of electrons in their outer shells.

Within a group, the reactivity of the elements depend on their total number number of electrons. Going down a group, there are more occupied energy level. The higher the amount of energy levels, the less the outer shell electrons are attracted to the nucleus- due to shielding. 

Reactivity of metals increases going down the group.

Reactivity of non-metals decreases going down the group.

The Group 1 elements are known as the alkali metals, and will readily react with air and water. At room temperature they are soft solids with low melting and boiling points that decrease going down the group. They also have low densities, meaning lithium, sodium and potassium float on water.

Alkali metal + water --> (alkali) metal hydroxide + hydrogen

Alkali metals have one electron in their outer most shell- which they lose in a reaction to form ionic compounds. They react with Halogens to form salts which are white or colourless crystals. Compounds of alkali metals dissolve in water to form solutions which are usually colourless

Going down Group 1, the reactivity of the alkali metal increases. This is because when going down group 1, the elements have more and more occupied energy levels, meaning that the outer shell electron is less attracted to the nucleus- the nucleus is positive whereas the electrons are negative, so the inner electrons repel the outer one (shielding)- meaning that the outer shell electron is more easily lost and therefore the metal is more reactive.

The transition elements are all metals and are found between Groups 2 and 3 in the periodic table.

They all (except mercury) have higher melting and boiling points than the alkali metals. They are malleable, ductile, strong and dense, they are good conductors and react very slowly, or not at all, with oxygen and water at regular temperatures- making them very useful as building materials and often alloys.

They form positive ions with various charges, their compounds are often brightly coloured and are used as catalysts for chemical reactions

The Halogens are non-metallic elements in Group 7. They exist as small molecules made up of pairs of atoms, and have low melting/boiling points that increase going down the group.

At room temperature:

• Fluorine is a pale yellow gas
• Chlorine is a green gas
• Bromine is a red-brown liquid
• Iodine is a grey solid- which easily vaporises to a violet gas

The Halogens form ionic compounds with metals in which the halide ions have a charge of 1-, they can also bond covalenty with non-metals.

The reactivity of the halogens decreases going down the group- as the seven outershell electrons become less attracted to the nucleus so it is harder to gain the eigth electron and attain a full outer shell.

A more reactive halogen is able to displace a less reactive one from an aqueous solution.