The compounds which are formed by ions are called ionic compounds. In general, cations are made by metals and anions are made by non-metals. So, it can be understood as compounds made by metals and non-metals are ionic compounds. Examples – Al2(SO4)3, CaCl2, K2SO4, KNO3, CaCO3, NaCl, MgCl2 etc.
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- Ionic compounds such as NaCl do not exist as individual molecule. The actual formula of sodium chloride can be given as (Na+)n(Cl–)n or (NaCl) n , where ‘n’ can be any very large number. Therefore no real molecular mass can be given for the actual formula.
- Further, to simplify the problem, we can write the simplest formula that shows the relative number of each kind of atom that produces an electrically neutral unit such as Al2(SO4)3, CaCl2, K2SO4, KNO3, CaCO3, NaCl, MgCl2 etc. This is called formula unit of the compound.
- So to differentiate substances like NaCl, that does not contain individual molecules, some chemist refer to their formula unit and instead of the molecular weight / mass, they use the term formula mass or formula unit mass.
The sum of the atomic masses of all the atoms in a formula unit of a compound is called the formula unit mass of the compound.
Formula unit mass is calculated in the same manner as we calculate the molecular mass. The only difference is that we use the word formula unit for those substances whose constituent particles are ions. For example,
the formula unit mass of CaCl2 = atomic mass of Ca + [ 2 x atomic mass of Cl]
= 40 + [ 2 x 35.5 ] = 111 u
the formula unit mass of NaCl = atomic mass of Na + atomic mass of Cl
= 23 + 35.5 = 58.5 u