To determine the correct order of radius for the species Sodium positive (Na⁺), Sodium (Na), and Sodium negative (Na⁻), we need to analyze the effect of electron loss or gain on the atomic radius of these species.
### Step-by-Step Solution:
1. **Identify the Atomic Numbers**:
- Sodium (Na) has an atomic number of 11, meaning it has 11 protons and 11 electrons.
- Sodium positive (Na⁺) has lost one electron, so it has 10 electrons but still has 11 protons.
- Sodium negative (Na⁻) has gained one electron, so it has 12 electrons and still has 11 protons.
2. **Understand the Concept of Nuclear Charge**:
- The nuclear charge is the total charge of the nucleus, which is equal to the number of protons.
- For Na⁺, the nuclear charge is 11 (from 11 protons) acting on 10 electrons.
- For Na, the nuclear charge is 11 acting on 11 electrons.
- For Na⁻, the nuclear charge is 11 acting on 12 electrons.
3. **Analyze the Effect of Electron Count on Atomic Radius**:
- In Na⁺, with 11 protons and only 10 electrons, the effective nuclear charge is higher per electron. This means the electrons are pulled closer to the nucleus, resulting in a smaller radius.
- In Na, with 11 protons and 11 electrons, the effective nuclear charge is balanced, resulting in a moderate radius.
- In Na⁻, with 11 protons and 12 electrons, the effective nuclear charge is lower per electron. This means the outermost electron experiences less attraction from the nucleus, leading to a larger radius.
4. **Establish the Order of Radius**:
- Based on the analysis:
- Na⁺ has the smallest radius (due to higher effective nuclear charge).
- Na has a moderate radius.
- Na⁻ has the largest radius (due to lower effective nuclear charge).
- Therefore, the correct order of radius is:
**Na⁺ < Na < Na⁻**
### Final Answer:
The correct order of radius is: **Na⁺ < Na < Na⁻**
