To determine which of the given species has the largest size, we will analyze the sizes of sodium (Na), sodium cation (Na⁺), and sodium anion (Na⁻).
### Step 1: Understand the concept of atomic and ionic sizes
- Atomic size refers to the distance from the nucleus to the outermost electron shell.
- Ionic size varies depending on whether the ion is a cation (positively charged) or an anion (negatively charged).
### Step 2: Compare the sizes
- **Neutral Sodium (Na)**: Sodium has an atomic number of 11, meaning it has 11 protons and 11 electrons. Its electron configuration is 2, 8, 1.
- **Sodium Cation (Na⁺)**: When sodium loses one electron to form Na⁺, its electron configuration becomes 2, 8. The removal of an electron reduces the electron-electron repulsion and decreases the size because there are fewer electrons in the outer shell.
- **Sodium Anion (Na⁻)**: When sodium gains an electron to form Na⁻, its electron configuration becomes 2, 8, 2. The addition of an electron increases electron-electron repulsion and increases the size because there are more electrons in the outer shell.
### Step 3: Analyze the effective nuclear charge
- In Na, there are 11 protons attracting 11 electrons.
- In Na⁺, there are still 11 protons, but now only 10 electrons, which means the effective nuclear charge felt by the remaining electrons is higher, pulling them closer to the nucleus and reducing the size.
- In Na⁻, there are 11 protons attracting 12 electrons, which means the effective nuclear charge is less per electron, allowing the electrons to be held less tightly, resulting in a larger size.
### Step 4: Conclusion
- Comparing the sizes:
- Na⁺ < Na < Na⁻
- Therefore, the largest size among the given options is for the sodium anion (Na⁻).
### Final Answer
The species with the largest size is **C. Na⁻**.
---
