eli5 Why are Ionic Compounds not considered molecules?
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A molecule has a definite size, shape and number of atoms. H2O is exactly two hydrogen atoms bonded to exactly one oxygen atom in a bent shape. NaCl always has a ratio of one Sodium ion to one Chloride ion, but the actual numbers are not fixed. A visible salt crystal is kind of like one giant molecule, large enough to be visible to the naked eye, but the crystal beside it would have different numbers of ions in it. There is no fixed number of sodiums and chlorides, so the formula gives the ratio, not a true representation of the actual object. The ions bond in a repeating pattern that can be any size.
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Tough to “eli5” but the simplest way I could explain it using limited chemistry/physics terms is:
Ions (the pieces that make up ionic compounds) are not bound together as strongly as the atoms that make up molecules are. Ionic compounds often readily separate under the right circumstances without requiring or giving off much energy. Example table salt, an ionic compound (NaCl), separating into free Na+ and Cl- (sodium and chloride ions) in water.
edit: fixes typo
As others have noted, this is difficult to explain while keeping it simple enough for ELI5. However, let’s consider the ion everyone is most familiar with, table salt (NaCl). The thing that makes an ion an ion is that it both the sodium (Na) and the chroride (CL) have a different number of electrons than the periodic table shows they have. Sodium ALWAYS has 11 electrons, except when something like chloride steals one. This turns sodium into a +1 ion that has ten electrons, and chlorine (which usually has 17 electrons) now has 18 and has a -1 charge.
This is also true for polyatomic ions such as sulfate (SO4^- 2). Sulfer has 16 electrons usually, while oxygen has 8. So, with 4 oxygen and 1 sulfer, we would expect sulfate to have 48 electrons (8+8+8+8+16), but instead it has 50! Sulfate stole two electrons from another atom (meaning it ionized another atom) to become sulfate.
TL/DR: ions have a number of electrons that differ from the number of electrons indicated by the periodic table.
Molecules are atoms that share electrons with each other.
Ions are atoms that don’t share, but give/take electrons from each other.
A very pedantic answer would be that ionic compounds are held together by ionic bonds and not by molecular bonds.
Molecules are held together by molecular (also called covalent) bonds, which are weaker than ionic bonds.
When you glue 2 things together, they’re not taped together. Molecular and ionic bonds are similar.
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