Formula for phosphorus selenide???

Question

Hi,

I need to find the empirical formula for phosphorus selenide.

I wrote P2Se3, but apparently it is wrong.

Isn't P3+ and Se2-??

What would the empirical formula be?? Thank you!

Answer 1

Hello,

A quick Google search suggests that the most common form of phosphorus selenide is P4Se4.   That would make the empirical formula PSe.   Another form seems to be P4Se.

You are completely correct about P commonly being +3 and Se -2.   However both phophorus and selenium are elements which can form complex compounds in which some of the molecular bonds are internal P-P and Se-Se bonds.   Those give empirical formulas which are different than you would expect.

A simple example would be sodium oxide   Na2O   Sodium is +1, oxygen is -2,  no problem.   But there is a stable compound sodium peroxide.  Na2O2.   Sodium is still +1  but it appears that oxygen is -1.  How is that possible?

The answer is that peroxide contains an oxygen-oxygen bond,  so the structure looks like

Na  O-O  Na

One of the bonding sites on each of the oxygens is occupied bonding to the other oxygen.   So each oxygen has a "formal" charge of -1

The same sort of thing happens with phosphorus selenide.   There is no way for you to be expected to know this.   You would have to look this up in texts or on-line.

Answer 2

the empirical formula is P2 Se

because both phosphorus and selenium have other oxidation states. and P2 Se is the simplest formula that can take place.

phosphorus's oxidation states include: 5, 3, -3

selenium's oxidation states include: 6,4,-2