Sodium/natrium is a metal from first column group so it should have one 1+ charge. Phosphate ion has 3- charge. That is why there 3 natrium ion for 1 phosphate ion when this molecule is dissolved in water. The ion formula would be:
(Na)

(PO

) ==> 3 Na

   +  PO
 
        
             
        
        
        
If you look at the periodic table of elements, you can see that atomic number for phosphorus is 15. It means that it has 15 electrons and 15 protons total. Now you can write configuration for P which is:  1s22s22p63s23p3. or [Ne] 3s2<span> 3p</span><span>3 </span><span>
From here, you can see that it has 5 valence electrons (s2+p3).
In the periodic table of elements the number of protons+ number of neutrons is determined as atomic mass. Atomic mass of the P is 30. 
number of neutrons = atomic mass-atomic number 
number of neutrons = 30-15
number of neutrons= 15 </span>
        
             
        
        
        
Neutral atoms get smaller as you move across the periodic table from (left to right) because the atom increases in electrons. The more electrons, the bigger the effective nuclear charge (electron and proton attraction) and so basically the atom shrinks.
        
             
        
        
        
I maybe wrong but i believe 36