Chemistry - Topic 6

• They have one electron on the outer shell, so they have similar chemical properties Their physical properties are similar they have low melting and boiling points and they are very soft and can be cut with a knife
• They make ionic compound and do not form covalent bonds 
• They are very reactive metals and as you go down the group they get more reactive. This is beacsue the single outer electron is further from the nucleus so it is less strongly attracted to the nucleus and less energy is needed to remove it and when they lose their outer electron so they form a 1+ ion
• They react vigourously in H2O (caesium & rubidium tend to explode when wet)
• The reaction in water produce hydrogen gases and a hydroxide of the metal 
• Example:  2Na + 2H2O -> 2NaOH + H2    
• (sodium + water -> sodium hydroxide + hydrogen)
• Lithium moves on the surface and fizzes, Sodium and Potassium do the same but melt and Potassium get hot enough to ignite the hydrogen produced

Fluorine, Chlorine, Bromine, Iodine, Astatine

• They have 7 electrons in their outer shell
• The Halogens exist as diatomic molecules (e.g. Cl2, Br2 , I2) they are like this because they share one pair of electrons to form a covalent bond
• They get less reactive as you go down the group
• They gain one electron to become stable so they form a 1- ion, the easier it is for the atom to attract an electron the more reactive it will be
• As you go down the group the melting and boiling points increase
• At room temperature: chlorine is a poisonous, fairly reactive gas; Bromine is a poisonous liquid and it gives off a vapour and iodine is a crystalline solid and gives off a vapour when heated
• They react vigorously with metals to form salts: 2Na + Cl2 -> 2NaCl (Sodium chloride)
• Halogens react with hydrogen to form hydrogen halides and they dissolve in water to from acidic solutions like hydrochloric acid

Reactions with iron wool:

• Fluorine = Reacts almost instantly 
• Chlorine = Reacts with heated iron wool very quickly 
• Bromine = The Bromine has to be heated and the wool too but it reacts quickly 
• Iodine = The wool and Iodine has to be heated strongly, the reaction is slow 

Displacement Reaction = The more reactive halogen displaces (pushes out ) the other less reactive halogen out of the compound.

Halogens react with metals to form salts (called metal halides) this is the compound used in a displacement reaction. If displacements takes place there will be a colour change (see table). Chlorine displaces BOTH Bromine and Iodine, Bromine displaces Iodine NOT chlorine and Iodine DOESN'T displace Chlorine and Bromine.

• They are inert (unreactive more or less) and are colourless at room temperature and this make them hard to find.
• The reason why they are inert is because they have full outer shells so they don't easily lose or gain electrons in their outer shells.
• Because they are inert they are not flammable.
• It took years for all the Noble gases to be discovered and this is due to the fact that there is so little of them, they are un-reactive and they are colourless.
• Argon has the largest percentage in air at 0.93%
• Xenon is the most reactive (but is still extremely unreactive) because it reacts with Flourine.
• The boiling points, melting points and density all increase as you go down the group

• Providing an unreactive or inert atmosphere 
• Argon is uses in filiment lamps, since its non-flammable it stops the hot filiment from buring away
• Flash Photography uses the same principle - agron, krypton and xenon can stop the flash filament from buring away during the high temperature flashes 
• Argon and Helium can be used to protect metals that are being welded, the inert atmosphere it creates stops the hot metal reaction with the oxygen in the atmosphere
• Helium is used in airships and ballons, because of its low density that air it floats. Helium is non-flammmables so it is a lot safer than hydrogen which was originally used in airships until the Hindenberg Disaster.