Electrolyte means a solute that allows a water solution to conduct electricity. Electrolytes produce ions when they dissolve in solution. Salts are usually electrolytes, while molecular substances usually aren't, unless they are acids or bases.
Many textbooks incorrectly state that all
salts or ionic compounds are strong electrolytes. However, many
ionic compounds or salts of transition metals or alkaline earth
metals are not strong electrolytes. When they dissolve, some of the
ions separate, but some stay together. Here are some examples:
K2SO4(s) + water → K+(aq) (70%) +
KSO4–(aq) (30%) +
SO42–(aq) (for a 0.36M solution)
CdI2(s) + water → Cd2+(aq) (2%) +
CdI+(aq) (22%) + CdI2(aq) (76%) +
I–(aq) (for a 0.5M solution)
As you can see, these salts are electrolytes (they do produce ions)
but if you do calculations assuming that they separate completely
into sulfate, potassium, cadmium(II) and iodide ions, you might get
very wrong answers! They are not strong electrolytes. In general,
the lower the concentration and the lower the charges on the ions,
the "stronger" the electrolytes will be. Alkali metals other than lithium are usually strong electrolytes especially when
the anion also has a small charge, and in dilute solutions
(<0.1M). Alkaline earth metal compounds are weaker electolytes, and
other metals are even weaker still. We'll talk more about why this
is later.
Solubility can cause some confusion here. For instance, it's possible that a compound is a strong electrolyte, but just not very soluble. For this reason, it would not be able to produce a solution with lots of ions, because it isn't soluble, not because the ions are still attached to each other in the solution. In general, it makes sense to guess that if the ions in a compound are very strongly attracted to each other, the compound will be less soluble, and also it might be a weaker electrolyte because even in solution the ions will be bonded to each other. However, electrolytes also look stronger at lower concentrations, because if the ions split up, they are less likely to find each other again.