GCSE Chemistry 3.1

One of the first suggestions came from JOHN DALTON in 1808. He arranged elements in order of masses. 

Then in 1864 JOHN NEWLANDS built on DALTONS idea with 'the law of octaves' - he arranges element in order of atomic masses and noticed the properties of every 8th element seemed similar. However he failed to take into account element that were left unknown at the time---> this is why his idea was intialy ridiculed and refused.

Later on in 1869 DMITRI MENDELEEV cracked the problem. He arranged all 50 elements in a table in order of atomic weight. and he left gaps for element yet to be discoverd.

The modern periodic table is based on MENDELEEV's periodic table. MENDELEEV's periodic table produced regular patterns and was accepted...

HOWEVER ----> Not all element fitted into the pattern. e.g Argon and Potassium. Although potassium has a lower atomic mass than argon, potassium is put above in the periodic table.. This is because Argon has a lower atomic number than potassium does.We now arrange elements in order of their atomic (proton) number.

Reactivity increases going down group 1 because the further away the electrons are from the nucleus the weaker the bond. So the outer electron is easier to lose.

Reactivity decreases going down group 7 because the further away the electrons are to the nucleus the weaker the bond. So this means extra electrons are less easily attracted into the outer shell going down the periodic table.

 REACTIVE METALS: -React vigorously with other elements. -Soft and can be cut.        NON-METALS: -Low melting and boiling points. -Many are liquid or gases at room temp.  TRANSITION ELEMENTS: Not very reactive.                                                            NOBLE GASES: -very unreactive, -difficult to combine with other elements 

You will probably only see that the first 3 of these as others are extremely reactive and highly dangerous. Francium is radioactive!! 

Properties of the alkali metals:

• All the metals are extremely reactive. They have to be stored in oil to stop them reacting with the oxygen in the air. Their rectivity increases as we go down this group 
• Alkali metals have a very low density in comparision to other metals
• They are very soft and can be cut. They are very shiny when first cut but soon go dark as metal react with oxygen in air. This forms a layer of oxide on shiny surface.
• They can boil at very low temperatures. 
• They have 1 electron on outer shell whichgives them similar properties and makes them very reactive. Reactivity of the alkali metals increase going down the group.

When we add lithium, sodium or pottasium to water the metal floats, moves and fizzes on the surface. This is because the metal reacts with the water and forms hydrogen gas and produces metal hydroxide.

In the centre of the periodic table there is a large block of metallic elements.

Properties:

• Good conductors of electricity and energy
• Hard and strong
• High densities 
• High melting points (with exception of mercury which is liquid at room temperature)

Transitional elements have high melting points compared to group 1. They are also harder, stronger and denser. 

The transition elements often form coloured compounds and are useful industrial catalysts. 

Halogens are a group of poisionous non-metals that have coloured vapours. They have fairly typical properties of non-metals

Properties:

• Low melting and boiling points                          Halogens gain 8 electrons 
• Poor conductors of energy and electricity.             going down the group

They all have 7 electrons on their outer shell, so need to gain 1 electron to acheive a stable electronic structure of a noble gas. Halogens take part  in both ionic and covalent bonding.