Periodicity

Na reacts with water to form the hydroxide, NaOH

Mg reacts with steam to form the oxide, MgO

The reactions of the elements become less vigorous from Na to Si.

The reactivity then increases from Si onwards. 

All the elements: Na, Mg, Al , Si, P and S react with oxygen to form the oxide. 

Below are the formulae of the products:

Na: Na2O

Mg: MgO

Al: Al2O3

Si: SiO2

P: P4O10

S: SO2

The melting points of the period 3 oxides generally decrease as you across the period. This is due to the bonding and structure of the compounds:

Na2O: ionic lattice

MgO: ionic lattice

Al2O3: mostly ionic bonding

SiO2: giant covalent

P4O10: simple covalent

SO2 / SO3: simple covalent

Ionic lattices have high melting points because the electrostatic forces between oppositely charges ions are very strong and require a lot of energy to break. 

Simple covalent molecules have relatively low melting points because between the molecules there are van der Waals forces, which are relatively weak, therefore require little energy to break.

Na2O reacts with water to form the hydroxide, NaOH

MgO also reacts with water to form the hydroxide, Mg(OH)2

Al2O3 does not react with water

SiO2 does not react with water

P4O10 reacts with water to form phosphoric acid, H3PO4

SO2 reacts with water to form sulfurous acid, H2SO3

SO3 reacts with water to form sulfuric acid, H2SO4

As you can see the reactivity of the period 3 oxides with water generally decreases up to aluminium oxide and silicon dioxide which both do not react with water. However, phosphorus (V) oxide, sulfur dioxide and sulfur trioxide react to form the corresponding acids. 

Na2O Hydroxide Ionic

MgO Hydroxide Ionic

Al2O3 Does not react Ionic

SiO2 Does not react Covalent

P4O10 Acidic Covalent

SO2 / SO3 Acidic Covalent

As you can see the oxides form solutions which are generally get more acidic as you go from left to right, therefore the acidity increases and the pH decreases. 

The reason for this is due to the bonding. Ionic compounds generally form basic solutions, whereas covalent compounds normally form acidic compounds. 

Both NaOH and Mg(OH)2 react with acids to form a salt and water

Al(OH)3 is amphoteric, so reacts with both acids and bases:

Al(OH)3 + 3HCl = AlCl3 + 3H2O

Al(OH)3 + NaOH = Na[Al(OH)4]

Si(OH)2 reacts with bases to form the ion, Si(OH)3-

Si(OH)2 + NaOH = Na[Si(OH)3]

P4O10 reacts with a base to form a salt  and water:

P4O10 + 12NaOH = 4Na3PO4 + 6H2O

SO2 reacts with a base to form a salt (sulfite) and water, SO3 reacts with a base to form a salt (sulfate) and water:

SO3 + 2NaOH = Na2SO4 + H2O