The Third period in the periodic table presents an interesting chemistry in form of the elements and their oxides, which undergo a variety of reactions.
The Period 3 of the Periodic table is shown below:
Table - 1
|Period 3|| 11|
|Difference in electroegativity||2.6||2.3||2.0||1.7||1.4||1.0||0.5||/|
Trend in Electronegativity low → → → → high
Electronegativity is chemical property, a quantitative measure of how tightly an atom holds onto its electrons. The type of bond in oxides could be determined by assessing the difference in electronegativities between oxygen and the element. Electronegativity of oxygen is 3.5. Electronegativities of Period 3 elements (using Pauling scale of atoms electronegativity) and differences in electronegativity of each element and oxygen is shown in Table 1.
Across the period when we go from the left to the right side i.e. from Na to Ar, Electronegativity increases because of the number of charges on the nucleus increases.
In oxides oxygen attracts electrons to itself much more than the metals (Na, Mg and Al) so oxygen atom pulls electrons away from the metal atom resulting in the formation of two ions: oxygen anion, O2-; metal cation, Na+, Mg2+, Al3+ and ionic bonds. These are solid oxides and forces of electrostatic attraction keep ions of oxygen and metals in arranged 3-dimensional structures, lattices.
The metal oxides consist of giant structures with strong attractions between the ions. Breaking these attractions needs much heat energy and these oxides therefore are hard and have a high melting points.
The other Period 3 elements: Si, P, S and Cl are more similar to oxygen in electronegativity so electrons which make up the bond between atoms are more shared than attracted of one of them. So these elements make the covalent type of bond in their oxides.
SiO2 forms solid tridimensional network called silica similar in structure to that of the diamond. Silica is very hard solid at room temperature with a high melting point.
Oxides of phosphorous P4O10 a sulfur SO2 and SO3 and chlorine are all small discrete covalent molecules. Some of these molecules are individual and some are polymeric. Between these molecules acting intermolecular forces type Van der Waals dispersion forces or dipole-dipole interactions and they determine the low melting point of this kind of oxides, much lower than those of the metal oxides.
In reactions of elements with the oxygen, we get products –oxides.
Oxides of all Period 3 elements except Cl and Ar could be made on the same method: burning the element in the air in a combustion reaction when the element will react with the oxygen in the air to produce the oxide of the element. Ar is noble gas and it doesn’t form oxides and Cl form oxides (Cl2O, ClO2, Cl2O7) which are not stable and couldn’t be produced on this way.
When we observe Period 3 of the Periodic table from left to right we can come to some conclusions:
- Nature of bond in oxides of Period 3 elements is changing from ionic to covalent bonding.
- Oxides form ionic lattice to the covalent network to covalent molecules.
- Oxides of Period 3 elements are basic to amphoteric to acidic.
Water although very weak oxidising agent, very easy and quickly oxidized sodium. The resulting solution is strongly alkaline, and has a pH of 14:
2Na(s) + 2H2O → 2Na+ + 2OH- + H2(g)
Magnesium reacts slowly with cold water, but very exothermically with hot water and steam and gives the oxide:
Mg(s) + H2O(g) → MgO(s) + H2(g)
Reactions of oxidation are:
- Sodium 4Na + O2 → 2Na2O
- Magnesium 2Mg + O2 → 2MgO
- Aluminium 2Al + 3O2 → 2Al2O3
- Silicon Si + O2 → SiO2
- Phosphorus 4P +5O2 →2P2O5 & 4P +5O2 → P4O10
- Sulfur S + O2 → SO2 & 2S + 3O2 → 2SO3
Reaction of Periodic 3 oxides with water will be following:
- Na2O(s) + H2O(l) → 2Na+(aq) + 2OH-(aq)
- 2MgO(s) + H2O(l) → 2Mg2+(aq) + 2OH-(aq)
This metal hydroxides: NaOH and Mg(OH)2 are both bases.
- Al2O3 doesn’t react with H2O, the strength of ionic bonds holding lattice together is too strong.
- SiO2 doesn’t react with H2 The covalent bonds holding the silicon and oxygen atoms together in the 3-dimensional lattice are to strong to be broken by the water molecules.
- Oxides of non-metals react with water to form oxyacids (an acid in which oxygen is attached to the non-metal).
P4O10(s) + 6 H2O(l) → 4H3PO4(l)
- Sulfur trioxide reacts violently with water to produce sulfuric acid.
SO2(g) + H2O(l) → H2SO3(l)
SO3(s) + H2O(l) → H2SO4(l)
So solutions of metal oxides in water display properties of bases, while aqueous solutions of the non-metal oxides display the properties of acids.
The reaction of Period 3 oxides with acids and bases and their basic, amphoteric and acid character
The general trend in acidity in oxides of the Period 3 elements as we go across the period from left (Group 1) to right (Group 17):
basic oxides (Group 1, 2) → amphoteric oxide (Al2O3) → acidic oxides (oxyacids)
The same trend can be seen in each period of the Periodic table and we have:
Bases react with acids such is HCl:
- Sodium oxide reacts with dilute hydrochloric acid to produce salt and water:
Na2O(s) + 2HCl(aq) → 2NaCl(aq) + H2O(l)
- Magnesium oxide reacts with warm dilute HCl to produce salt and water.
MgO(s) + 2HCl(aq) → MgCl2(aq) + H2O(l)
- Aluminium oxide react with hot dilute hydrochloric acid and produce aluminium chloride and water.
Al2O3 + 6HCl(aq) → 2AlCl3 + 3H2O(l)
Aluminium oxide also reacts with bases. When concentrated sodium hidrokside solution is added to aluminium oxide, complex ions are formed such as the tetrahydroksoaluminate ion:
Al2O3(s) + 2NaOH(aq) +3H2O(l) →2NaAl(OH)4(aq)
In this reaction Al2O3 is acting as an acid.
Conclusion: Al2O3 has amphoteric character.
- Silicon dioxide, the oxide of metalloid is chemically very inert and weakly acidic. It reacts with hot concentrated sodium hydroxide to produce a sodium silicate and water.
SiO2(s) + 2NaOH(l) → Na2SiO3(l) + H2O(g)
Since the Period 3 non-metallic oxides are all acidic, they react with bases e.g. aqueous solution of sodium hydroxide as the base. Oxides of the phosphorus react with water to produce acids as we seen and these acids can react with the sodium hydroxide in the neutralization reaction.
- Then we have:
H3PO3(aq) + 3NaOH(aq) →Na3PO3(aq) + 3H2O(l)
H3PO4(aq) + 3NaOH(aq) →Na3PO4(aq) + 3H2O(l)
- Sulfur dioxide reacts with aqueous solution of sodium hydroxide and produces a salt (sodium
sulfite) and water. Sulfur trioxide reacts violently with water to produce sulfuric acid which will react with sodium hydroxide and produce salt.
SO2(g) + 2NaOH(aq) →Na2SO3(aq) + H2O(l)
H2SO4(aq) + 2NaOH(aq) →Na2SO4(aq) + H2O(l)