Na, Mg,Al, Si, P,S, Cl are present in 3rd period of the periodic table. Among them Na, Mg and Al are metallic in nature. So they form metallic oxides. on the other hand P, S & Cl are non metal,so they form nonmetallic oxide. But oxide of Si has dual nature. We know that if we move from light to left the size of the atoms are decrease. So Size order of Na, Mg, Al are Na>Mg>Al. During the formation of oxide of these highly electropositive metal bonded with the highly electronegative atom oxygen and they form ionic bond. So for break the bond higher energy is required as result they have higher melting point.But Mg2+ and O2- has greater no of charge so they form more stronger ionic bond then Na2O.So the melting point order of first 3 metals are- Na2O<MgO>Al2O3.
Silicon dioxide has a high melting point that varies depending on the particular structure , but each is close to 1700°C. Very strong silicon-oxygen covalent bonds must be broken throughout the structure before melting occurs. Very strong silicon-oxygen covalent bonds must be broken throughout the structure before melting occurs. In Si two types of bonding are considered, it makes little sense to compare these values directly to those of the metallic oxides; suffice it to say that in both types of structures The melting point is high.And as the previous trends it will also decrease in melting point after Al2O3 after.
But from the P, we can see a sharp change(decrease) in melting point. This is because the attractive forces between these molecules include vander Waals dispersion and dipole-dipole interactions. These vary in size depending on the size, shape and polarity of the various molecules, but will always be much weaker than the ionic or covalent bonds in a giant structure. These oxides tend to be gases, liquids, or low melting point solids. But at the end of the period the melting point of Cl2O7 is again increases because in this molecule the oxidation state of Cl is +7 and the chlorine uses all of its seven outer electrons in bonds with oxygen. This produces a much bigger molecule, and so it would expect its melting point higher than SO3.
So the periodic 3 elements's oxide's melting point order is - Na2O<MgO>Al2O3>SiO2>P4O10>SO3<Cl2O7.
The melting points of the oxides of the third-period elements are given in parentheses: Na2O (1275°C), MgO (2800°C), Al2O3 (2045°C), SiO2 (1610°C), P4O10 (580°C), SO3 (16.8°C), Cl2O7 (291.5°C). Classi...