Number of neutrons = mass number - atomic number = 40 - 19 = 21 answer
Most of the mass of an atom is located in the (nucleus), and most of the volume is taken up by the (electron clouds).
The ones in the parenthesis are your answers.
Hope this helps!
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.
