If the first ionization enthalpy of Sodium () is , what is the exact value of the electron gain enth — Periodic Table and Periodicity Chemistry Question

💡 Solution & Explanation

The ionization of $Na$ is $Na \rightarrow Na^+ + e^-$ ($\Delta H = +5.1 \text{ eV}$). The electron gain of $Na^+$ is the exact reverse reaction: $Na^+ + e^- \rightarrow Na$. According to Hess's law, $\Delta_{eg}H = -5.1 \text{ eV}$.