235 U --->x + 4 He

Question

^{235 U --->x + 4} He

 

Not good in chem at all.  Please someone explain!

Answer 1

I think what this is referring to is a type of radioactive decay called alpha decay (or alpha emission) = nucleus of an isotope X emits a helium-4 nucleus to produce a new isotope Y. The equation is balanced as follows:

^A_ZX = ⁴₂He + ^(A-4)_(Z-2)X

 

²³⁵U --> ⁴He + ²³¹Th

 

Uranium-235 decays to Thorium-231

Answer 2

To find the nuclear species X, you must meet 2 conditions:

 

(1) The charge on the left hand and right hand sides of the equation must be the same.

 

(2) The mass number (number of protons plus number of neutrons) on the left and right hand sides must be the same.

 

²³⁵U → X + ⁴He

 

To meet (1), you have 92 protons in ²³⁵U (atomic number is 92).  ⁴He has 2 protons.  Therefore if Z is the atomic number of X, Z must satisfy

 

92 = Z + 2 ⇒ Z = 90

 

To meet (2), you have a mass number of 235 on the left.  On the right you have the mass number of X, which we may call A, and you have a mass number of 4 for ⁴He.  Therefore, A must satisfy

 

235 = A + 4 ⇒ A = 231

 

The mass number of X is 231, and the atomic number of X is 90.  Then X is designated as

 

²³¹₉₀X (The 90 should be right below the 231, but that is awkward to type in this window.)  The species X is a nucleus with 90 protons and 231-90 = 141 neutrons.