# Screenshot_648.png this is the formula given for the Q value of the ALPHA decay process my doubt is ....shouldn't the rest mass energy of products be added ..and then subtracted from the initial substance here all the terms are being subtracted... i thought first we wll have to add the rest mass energy of the products and then subtract it from the reactaant

@Anupam Kanojia

The Q-value of the reaction is defined as the difference between the sum of the rest masses of the initial reactants and the sum of the masses of the final products, in energy units (usually in MeV).

Consider a typical reaction, in which the projectile a and the target A gives place to two products, B and b. This can also be expressed in the notation that we used so far, a + A → B + b, or even in a more compact notation,

The Q-value of this reaction is given by:

Q = [ma + mA – (mb + mB)]c2

So The “Q-value” of the alpha decay,

Qα is the difference of the mass of the parent and the combined mass of the daughter and the α-particle, multiplied by Qα = (mP − mD − mα) .

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