KEY IDEAS

The chemical formula of a compound is information about its composition.
How that information is properly interpreted depends upon the category of substance of the compound – in particular whether it is categorised as a covalent molecular compound or an ionic compound.
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Substances that we categorise (on the basis of their properties) as covalent molecular compounds comprise identical molecules (each a group of atoms held together in a certain connectivity by covalent bonds between atoms).

In these covalent molecular compounds, the chemical formula tells us the actual number of atoms of each element in each of the molecules.

But it tells us nothing about the connectivity of atoms in the molecules (that is, what is bonded to what).

​Substances that we categorise (on the basis of their properties) as ionic compounds, comprise an orderly arrangement of positively charged ions (called cations) and negatively charged ions (called anions).

Each cation is surrounded by anions, and each anion by cations. The crystal is held together by the electrostatic forces of attraction between cations and anions (dominating the cation-cation and anion-anion repulsions.

We can think of this ionic bonding as a cooperative effect involving the whole crystal: there are no pairs of ions (or small groups of ions) that can be identified as more-or-less independent units: that is, it is not composed of molecules.

In ionic compounds, the chemical formula can only tell us the relative numbers (the ratio) of cations and anions in the crystal.
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And it tells us nothing about the particular orderly arrangement of the cations and anions among each other.
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SELF CHECK

Answers:  1 (c);  2 (a);  3 (c);  4 (a);  5 (d)


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