Chemistry (C3) - Development of the Periodic Table

1800s, 30 elements were known. People noticed some like sodium, potassium and lithium shared certain proprties.  

In 1864, over 50 elements were known, John Newland arranged them in order and noticed that every eighth element shared certain properties which he called the law of octaves. 

Unfortunately, after calcium, his law broke down as he did not know about transition metals.

Mendeleev then arranged a table and left gaps for elements that he said they did not know. He then predicted the elements that he said were not yet discovered and his predictions were correct for the next three elements.

Mendeleev did not keep to the order of atomic mass but just used it as a starting point and then rearranged some elements, like swapping iodine with tellurium as iodine matched the properties of flourine, bromine and chlorine than tellurium did. 

When protons were found in the 20th century, it was clear elements should be arranged by atomic number, not mass and Mendeleev's switching did exactly this. Mendeleev's table was accepted because he made accurate predictions about undiscovered elements.

The table is arranged by atomic number, this makes elements go into groups with similar properties. 

Transition metals separate vertical groups and horizontal periods after the first 20 elements. 

The period number tells you the number of energy levels while the group number tells you the number of elections on the outer energy level.

Three Quarters of the periodic table are metals which are found in the centre of the table and on the left, the left contains soft but very reactive metals while the middle contains every day metals. 

Non metals take up less than a quarter of the periodic table and are found in groups 6 and 7 (which are reactive non metals) and 0 (which are noble, unreactive, gases).

In groups three four and five the elements are undefined because at the top the are non metals but as you come further down the turn into metals.

Alkali metals are group 1 metals. They are soft and are conductors of heat and electricity. They are shiny but tarnish in air. They have low density and as you go down group 1 on the periodic table, the melting and boiling points become lower.

They are very reactive so to stop the reactions they are stored as oils. 

They have one electron on the outer shell which they lose once they react to form an ionic compound, which are white and dissolve in water to form colourless solutions.

The reactions become more vigorous as you go down the group. When reacted with water LITIUM fizzes, SODIUM melts from the heat, POTASSIUM gets so hot the hydrogen catches fire, CAESIUM explodes.

This is because as you get lower down the group there are more energy levels menaing the outer electron is further away from the positive nucleas making the electron easier to lose.

Therefore the bigger the atom the more reactive it is in Group 1. 

Transition metals have properties (compared to the alkali metals) of:

• higher melting and boiling points
• higher densities
• stronger and harder
• much less reactive.

Transition metals are useful catalysts. 

Compounds containing transition metals are coloured. 

Transition metals can form different ions with different charges.

Halogens are Group 7 in the periodic table.

They have coloured vapours, they don't conduct electricity or heat, the have low melting and boiling points.

They react with metals to form ionic compounds where they gain electrons from the metal. 

They react with non-metals to form compounds with covalent bonds where electrons are shared between the atoms. 

They become less reactive as you go down because the attraction of the nucleas for gaining an electron from a metal is weaker if there are more energy levels. 

A more reactive halogen will displace a less reactive halogen from a compound through displacement.