They're not important when solving math problems in class because you need to get the correct and accurate answers hence don't need to change them into significant figures
Answer:
Part A:
Charge is 
Configuration is 
Part B:
Charge is 
Configuration is 
Part C:
Charge is 
Configuration is 
Explanation:
Monatomic ions:
These ions consist of only one atom. If they have more than one atom then they are poly atomic ions.
Examples of Mono Atomic ions: 
Part A:
For P:
Phosphorous (P) has 15 electrons so it require 3 more electrons to stabilize itself.
Charge is 
Full ground-state electron configuration of the mono atomic ion:

Part B:
For Mg:
Magnesium (Mg) has 12 electrons so it requires 2 electrons to lose to achieve stable configuration.
Charge is 
Full ground-state electron configuration of the mono atomic ion:

Part C:
For Se:
Selenium (Se) has 34 electrons and requires two electrons to be stable.
Charge is 
Full ground-state electron configuration of the mono atomic ion:

Answer:
1,085g of water
Explanation:
If we have the value 4520kj is because the question is related to Energy and heat capacity. In this case, the law and equation that we use is the following:
Q= m*C*Δt where;
Q in the heat, in this case: 4520kj
m is the mas
Δt= is the difference between final-initial temperature (change of temperature), in this exercise we don´t have temperatura change.
In order to determine the mass, I will have the same equation but finding m
m= Q/C*Δt without m=Q/C
So: m= 4,520J/4.18J/g°C
m= 1,0813 g
Water is not alive because it has no living organisms.