What is the order of binding energy per nucleon?

Nuclear fission is possible because the binding energy per nucleon ...

The binding energy per nucleon of ${}_3^7Li$ and ${}_2^4He$ nuclei are $5.60\,MeV$ and $7.06\,MeV$ respectively. In the nuclear reaction ${}_3^7Li + {}_1^1H \to 2{}_2^4He + Q$, the value of energy $Q$ released is.........$MeV$.

$M_p$ denotes the mass of a proton and $M_n$ that of a neutron. $A$ given nucleus,of binding energy $B$,contains $Z$ protons and $N$ neutrons. The mass $M(N, Z)$ of the nucleus is given by ($c$ is the velocity of light):

Assertion : The binding energy per nucleon,for nuclei with atomic mass number $A > 100$,decreases with $A$.
Reason : The nuclear forces are weak for heavier nuclei.

What is mass defect? Write down its formula.