The 3rd ionisation enthalpy refers to the energy required to remove the third electron after the removal of the first and second electrons from a neutral atom.
Reasoning for Mn Having the Highest 3rd Ionisation Enthalpy
Manganese (Mn) has the electronic configuration [Ar] $3d^5 4s^2$.
The removal of the first and second electrons leads to a half-filled $3d^5$ configuration, which is highly stable due to symmetrical distribution and exchange energy.
Removing the third electron from this stable half-filled $3d^5$ configuration requires a significantly higher amount of energy compared to other transition elements such as Cr, V, and Fe.
Conclusion: The transition metal having the highest 3rd ionisation enthalpy is Mn.
The portion of the line \( 4x + 5y = 20 \) in the first quadrant is trisected by the lines \( L_1 \) and \( L_2 \) passing through the origin. The tangent of an angle between the lines \( L_1 \) and \( L_2 \) is: