Ionic compounds and shapes

Ionic compounds do not have shape, on the other hand molecular compounds have a definite shape. Ionic compounds have a crystal structure, the inter ionic attraction being electrostatic the force of attraction is directed in space in a three dimensional area, in NaCl each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions effectively the positive charge is satiated by the negative charges surroundings it. If you look closely you would see that if the positive charge is placed at a point where the three axis intersect the chloride ions would be on either ends of the X, Y and Z axis. This potentially would lead to an FCC type of unit cell, and in each unit cell we would have 4 formula units of NaCl.

The formula of sodium chloride is represented as the simplest ratio in which the positive and negative ions combine to form a neutral molecule. Hence MgO and not Mg2O2. Every ionic compound would belong to one of the 7 crystal systems.
FCC represents face centered cubic unit cell. There are 4 formula units in one unit cell of NaCl, yet the formula is represented as a 1:1 ratio of positive to negative compound.





  1. What if a 2 substances are mixed together and both have a mass that of which is equal to one another. For example H2O and HCl and both have a mass of 100g, where we do not know which is the solvent. To find q would we use 100g x 4.184 x 6oC or 200g x 4.184 x 6oC?

    1. Here you make an assumption the heat capacity of HCl is the heat capacity of water 4.18J/goC. So the total mass would be 100+100 = 200g