Group 1 and 7

All alkali metals have characteristic properties due to the single electron in their outer shell and the fact that they belong to the same group
They react with air which is shown by the fact that they tarnish easily when exposed to air (for this reason, they are stored under oil)
They react with water vigorously to create an alkaline solution and hydrogen
They are soft (can easily be cut with a knife)
They are low density (the first three elements can float on water)
They react with oxygen but different types of oxide will form depending on the element (lithium forms lithium oxide, sodium forms a mixture of sodium oxide and sodium peroxide, potassium forms a mixture of potassium peroxide and potassium superoxide)
They are good conductors of heat and electricity
They have low melting and boiling points compared to other, heavier metals such as copper

sodium + water --> sodium hydroxide + hydrogen

2Na + 2H2O --> 2NaOH + H2

The elements in Group 1 get more reactive as you go down the group (as the atomic number increases)
This can be seen by the rate of reaction (the more reactive an element is, the quicker it will react)
This happens because the further down Group 1 you go, the easier it is for elements to lose electrons and form positive +1 metal ions
This is due to the increase in the number of electron shells between the outer shell and the nucleus, meaning that the distance between them is further, so the attraction is weaker and the electron is more easily lost

Lithium bubbles slowly, floats and moves slowly across the water surface
Sodium fizzes steadily and floats across the water surface
Potassium bubbles very quickly, melts into a ball, floats, moves very quickly across the water surface and produces a flame

A dense, yellow-green gas
Fairly reactive
Soluble in water (solution formed is highly acidic- pH 2)

Red-brown liquid
Gives off an orange vapour
Soluble in water (solution formed is slightly acidic- pH 5)

Grey-black solid
Not soluble in water

Most reactive element in group 7
A gas at room temperature

Halogens exist as diatomic molecules (Cl2, Br2, F2)
Trend in state from gas to liquid to solid down the group because the melting and boiling points (at room temperature) increase as you go down the group
From this you can predict that any halogens above chlorine will be gases (lower boiling points) and any below iodine will be solids (higher melting points)
The higher up group 7 an element is, the more reactive it is
All halogens have 7 electrons in their outer shell and can gain one electron to form a -1 ion
The easier it is for a halogen to attract an electron, the more reactive it will be
As you go down the group it is harder to attract the extra electron as the outer shell is further from the nucleus so the electrostatic attraction is weaker so the halogens become less reactive

A more reactive halogen can displace a less reactive halogen in an aqueous solution of its salt
Chlorine displaces bromine and iodine
Bromine displaces iodine but not chlorine
Iodine does not displace chlorine or bromine
When bromide is displaced from a compound and becomes bromine, it forms an orange solution
When iodide is displaced from a compound and becomes iodine, it forms a brown solution

Chlorine + Potassium Bromide --> Potassium Chloride + Bromine

Cl2 (aq) + 2KBr (aq) --> 2KCl (aq) + Br2 (aq)