Brackets?

[Edward Suarez 1I]

Hi friends, I have a question about Lewis structure:

for 3.39, 6th edition:
a) NH4Cl has brackets around the NH4 section, but none around Cl. Why is that? How do you know which one to put around a bracket, and how do you know when you need a bracket in the first place?
b) for K3P3-, there isnt any bonds at all. How do you know when you don't need bonds and can simply draw the K+ around the P3-? Is it because it is an ionic bond? Also, just in general, for a question like this would I just add 3 electrons (Because of negative 3 charge) to the amount of electrons P can hold?

Thank you

[Chem_Mod]

1. You have to put a bracket around NH4+ because it is formed from multiple atoms while Cl- is just one atom so just put in the formal charge and draw in the valence electrons.
2. We only denote covalent bonds with a line but we do not for ionic bonds. In this instance, remember that ionic bond means complete electron transfer, so P3- means that P actually gained three extra electrons. So to answer your question, yes, P gained 3 electrons to become P3- and obtained an octet for itself.

Hope that helps!