The trend in the melting point of the elements Na–S

• Melting point decreases across the period.
• The melting point increases as the charge on the metal ions increases as the size of the metal ion decreases. 
• Ionic compounds with smaller ions and higher charge ions have the highest melting points. 
• The melting point of aluminium oxide is lower than that of magnesium oxide due to the aluminium ion having a higher charge dentistry. It polarises the electrons in the oxide ion and aluminium oxide has a degree of covalent character, reducing the melting point.