Answer: 4.
and 
Explanation:
a) The given reaction is 
As the mass on both reactant and product side must be equal:


As the atomic number on both reactant and product side must be equal:



b) 
Total mass on reactant side = total mass on product side
15 =15 + x
x = 0
Total atomic number on reactant side = total atomic number on product side
8 = 7 + y
y = 1

Answer:
gravity equation.
Explanation:
use this gravity equation
F= G(m1*m2/d²)
<span>Each atom in an ionic compound tends either to empty or complete the shell to 8 electrons. So when the number of electrons on the external shell is near 8 , the atom tends to attract electrons to complete the shell giving a positive ion called anion
Conversely when the number of electrons is small the atom gives these electrons to form a positive ion called a cation</span>