Big Idea #1: Chemical Formulas

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One of the most fundamental skills in Chemistry is being able to look at a chemical formula and understand quickly what it means.

e.g., NaCl, K, SiO2, K2O, SiO2, CO2, Cl2, Ar, Xe, Mg(OH)2, Al2(SO4)3.18H2O, I2(s), HCl(aq)

Have a little think about the examples above and try to define what we mean by “formula”. It’s incredibly difficult, isn’t it? This is because the specific meaning of “formula” changes with the type of chemical structure we’re talking about, e.g.,

  • giant ionic lattice (NaCl, Mg(OH)2, Al2(SO4)3.18H2O)
  • giant metallic lattice (K)
  • giant covalent lattice / macromolecular (SiO2)
  • simple molecule (H2O, Cl2)
  • monoatomic (Ar, Xe)
  • molecular lattice (I2(s))
  • aqueous solution (HCl(aq))

You must keep this in mind for the structure and bonding topic and remember to start every answer with a very clear and unambigous statement about the structure of the substance.

For chemical calculations, however, the most important thing we need to be able to do is to look at a formula and count how many atoms of each type there are in the formula unit.

Here are a few examples from above:

H2O

  • The subscript after H means there are two H in the formula unit.
  • No subscript after O means there is one O in the formula unit.
  • In total – two H and one O per formula unit.

If we add in what we know about structure, we can also say that H2O is a simple molecule with covalent bonds between H and O atoms and intermolecular forces between the H2O molecules.

Mg(OH)2 – a giant ionic lattice

  • No subscript after Mg means there is one Mg in the formula unit.
  • The subscript after the bracket means there are two OH in the formula unit.
  • In total – one Mg and two OH per formula unit.

If we add in what we know about structure, we can also say that Mg(OH)2 is a giant ionic lattice of Mg2+ ions and OH– ions.

Al2(SO4)3.18H2O – another giant ionic lattice

  • The subscript after Al means there are two Al in the formula unit.
  • The subscript after the bracket means there are three SO4 in the formula unit.
  • The dot means there are 18 H2O in the formula unit.
  • In total – two Al, three SO4 and eighteen H2O per formula unit.

If we add in what we know about structure, we can also say that Al2(SO4)3.18H2O is a giant ionic lattice of Al3+ ions and SO42- ions with H2O molecules within the structure as the water of crystallisation.

I2(s) – a molecular lattice

  • The subscript after I means there are two I in the formula unit.
  • In total – two I per formula unit.
  • The state symbol tells us that I2 is a solid. This may be relevant to some calculations.

If we add in what we know about structure, we can also say that solid I2 is a molecular lattice of I2 molecules with intermolecular forces between the I2 molecules.

Why is all this so important and worthy of Big Idea #1?

Simple – if you cannot count up the atoms in a formula accurately, you will not be able to correctly balance symbol equations or calculate the Relative Formula Mass (RFM or Mr) of a substance. This idea is incredibly fundamental and underpins absolutely everything else on the chemical calculations topic.