The Periodic Table: Chemical periodicity II

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7) Variation of oxidation number in chlorides and oxides

The oxidation numbers of elements in their oxides is always positive because oxygen is a very electronegative atom. The maximum oxidation number of each element is the same as its group number. The group number of the element corresponds to the number of electrons in its outermost shell.

Most of the elements have their usual oxidation states. Just take note that phosphorous has an oxidation state of +5 in phosphorous oxide and +5 in phosphorous chloride. Sulphur has an oxidation state of +4 in sulphur dioxide and +6 in sulphur trioxide.

The oxidation number becomes more positive across the period because the number of valence electrons increase across the period. These valence electrons can take part in bonding with chlorine and oxygen to form chlorides and oxides.

8) Reaction of oxides with water
Metallic oxides reacts with water to form alkaline solutions.
Sodium oxide react vigourously with water to form an alkaline solution of sodium hydroxide.

Magnesium oxide reacts less readily with water to form magnesium hydroxide. Its low reactivity with wtaer is due to the high charge density of Mg ions which holds the oxygen ion more firmly.

Aluminum oxide does not react with water.

Silicon oxide does not react with water.

Non metalllic oxides react with water to form strong acids.
Phosphorous (V) oxide react with water to form phosphoric (V) acid.

Sulphur dioxide react with water to form sulphurous acid.
Sulphur trioxide react with water to form sulphuric acid.

9) Reaction of oxides and hydroxides with acid and NaOH

Metal oxides and hydroxides (Na, Mg) are ionic compounds. They are basic in nature and reacts with acid to form salt and water.

Non metal oxides and hydroxides (Si, P, S, Cl) are covalent compounds. They acidic in nature and reacts with NaOH to form salt and water.

Aluminum oxide is not souble in water but it is amphoteric in nature; i.e. it reacts with both acid and base. Aluminium oxide reacts with HCl to form aluminum chloride and water. Aluminium oxide reacts with NaOH to from an a complex ion (aluminate).

10) Reaction of chlorides with water

Simple ionic chlorides like NaCl and magnesium chloride simply dissolve in water. The solutions of ionic chlorides are neutral.

Aluminum chloride reacts with water to form a complex ion. The aluminum complex can polarize water molecule due to its high charge density. The highly charged aluminium ion draws electrons away from surrounding water molecules, causing them to give up H+.

Silicon chloride reacts with water to form silicon dioxide and HCl.

Phosphorous (V) chloride reacts with water to form phosphoric (V) acid and HCl.

Take home message:
Compounds of Na and Mg are ionic compounds Thus they have high melting points and form basic oxides. Ionic compounds do not react with water, they simply dissolve.

Aluminum compounds are ionic with strong covalent character and form amphoteric oxides. Aluminum compounds react with water to form acidic solutions.

Si compounds has a giant molecular structure and have high melting points. They form acidic oxides.

Phosphorus and sulphur compounds have simple molecular structures and low melting points. They form acidic oxides that react with water giving rise to acidic solutions.

Side note: The syllabus for this section looks very demanding and difficult. However this section isnt really being asked in long answer type of questions. So dont panic

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