In order for matter or energy to be conserved, one must have a system.
a. open
b. closed
c. unique
d. stationary

Closed system.  If the system is not closed, matter or energy can escape from the system.  an example of this is if you react magnesium and hydrochloric acid in a open system.  The H₂ gas is going to escape making it look like some of the mass disappeared .  in that same reaction some in an open system will also loose heat to the surrounding which will make it look like less heat was produced.

Because electrons have negligible mass, to account for the mass of the isotope, there must be 16 neutrons, 16 neutrally charged, massive, fundamental particles present in the sulfur nucleus. and thus finally, if we have the 32s2− ion, there are 18 electrons, 16 protons, and 16 neutrons.