Bombarding plutonium-239 with an alpha particle creates an isotope of curium. Balance the equation. 239
94
Pu +
4
2
He →
a
b
Cm +
c
d
n

Identify the values for the unknown letters.

a is

b is

c is

d is

the answer is c.the valence of the atoms combine .

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.