A stable isotope has just<em> the right number of neutrons for the number of protons </em>(the <em>n:p ratio</em>) to hold the nucleus together against the repulsions of the protons.
A radioactive isotope has either too few or too many neutrons for the nucleus to be stable,
The nucleus will then emit <em>alpha, beta, or gamma radiation</em> in an attempt to become more stable.
<u>Answer:</u> The freezing point of solution is 5.35°C
<u>Explanation:</u>
The equation used to calculate depression in freezing point follows:

To calculate the depression in freezing point, we use the equation:

Or,

where,
Freezing point of pure solution = 5.5°C
i = Vant hoff factor = 1 (For non-electrolytes)
= molal freezing point elevation constant = 4.90°C/m
= Given mass of solute (naphthalene) = 2.60 g
= Molar mass of solute (naphthalene) = 128.2 g/mol
= Mass of solvent (benzene) = 675 g
Putting values in above equation, we get:

Hence, the freezing point of solution is 5.35°C