1. The problem statement, all variables and given/known data Using the Semi-empirical Mass Formula show that for fixed odd atomic mass number, A, the most stable isobar has a neutron to proton ratio given by N/Z = 1 + aCA2/3/(2aA) 2. Relevant equations B(A,Z) = aV A − aS A2/3 − aCZ2A-1/3 − aA(Z − N)2/A + pairing term 3. The attempt at a solution I computed dB/dZ = 0 because the most stable means the one with the maximum binding energy. I get the formula but missing the factor of a half. The fact A is odd means that we don't have a pairing term. My question is where does the half come from? What am I doing wrong?