# Day 37: Writiing Chemical Formulae

Science 9 Today the students continued with working on how to name chemical formlae, given the name of a covalent compound. There was an added twist to it when the students were introduced to the idea of multivalent ions.  Since we don’t go deep into the concepts behind multivalent compounds, I almost feel that these should be easier because you are explicitly told the ion charge of the metal.

I realized there is at least one sticky preconception that the students have which I had never noticed before. Whereas I thought the idea of a covalent bond exiting between equal and opposite charges was straight forward, the students really want to add the ion charges together. I have no idea why.  When probed, students simply replied that they thought they should add charges.

A similar thing happened when balancing ions.  For example, with iron(II) phosphide the chemical formula would be $Fe_3 P_2$ .  Students would have a hard time because they somehow wanted to had a +1 to the iron ion, so that it would have a total of 3 like the phosphorus ion.  They were missing the idea that adding atoms to the atom ratio was a multiplier of the charge, not an addition.

I checked in halfway through the class with the voting question above, using Plickers.  The students sorted it out pretty well using Peer Instruction.  Once they were back to their individual practice though, the problems continued.  On Wednesday we will have a quiz and I’m curious to see how they will do.