The correct option is **B. ionization energy**. ### Explanation of the Correct Answer **Ionization Energy** is defined as the amount of energy required to remove an electron from an atom or ion in its gaseous state. In this case, we are discussing the removal of one electron from the 3S orbital of a gaseous sodium atom (Na). The value given, approximately 496 kJ/mol, represents the energy needed to overcome the attraction between the negatively charged electron and the positively charged nucleus of the sodium atom. #### Step-by-Step Breakdown: 1. **Understanding the Sodium Atom**: Sodium (Na) has an atomic number of 11, meaning it has 11 protons and, in its neutral state, 11 electrons. The electron configuration of sodium is 1s² 2s² 2p⁶ 3s¹, indicating that there is one electron in the outermost shell (3S orbital). 2. **Removing an Electron**: When we talk about ionization energy, we are specifically referring to the process of converting a neutral atom into a positively charged ion (cation) by removing one of its electrons. For sodium, this process can be represented as: \[ \text{Na(g)} \rightarrow \text{Na}^+(g) + e^- \] The energy required for this process is the ionization energy. 3. **Energy Requirement**: The value of 496 kJ/mol indicates that it takes this amount of energy to remove one mole of electrons from one mole of sodium atoms in the gas phase. This energy is necessary to overcome the electrostatic forces holding the electron in the atom. ### Why the Other Options are Incorrect - **A. Electron Affinity**: Electron affinity refers to the energy change that occurs when an electron is added to a neutral atom in the gas phase. For sodium, this would involve adding an electron to form Na⁻, which is not what is being described in the question. Therefore, this option is incorrect. - **C. Activation Energy**: Activation energy is the minimum energy required to initiate a chemical reaction. It is not related to the removal of electrons from atoms but rather to the energy needed to break bonds or initiate reactions. Thus, this option does not apply to the context of removing an electron from an atom. - **D. Electronegativity**: Electronegativity is a measure of an atom's ability to attract and hold onto electrons when it is part of a compound. It does not refer to the energy required to remove an electron from an atom. Therefore, this option is also incorrect. ### Summary of Key Points - **Ionization Energy** is the energy required to remove an electron from a gaseous atom. - For sodium, the ionization energy is approximately 496 kJ/mol, indicating the energy needed to remove the outermost electron. - Other terms like electron affinity, activation energy, and electronegativity refer to different concepts and are not applicable in this context. ### Revision Summary - Ionization energy is the energy needed to remove an electron from an atom. - Sodium's ionization energy is about 496 kJ/mol. - Electron affinity, activation energy, and electronegativity are distinct concepts unrelated to the removal of electrons. - Understanding these definitions is crucial for mastering concepts in atomic structure and chemical reactivity.