Answer
**Relative Atomic Mass of Chlorine (35.5):**
- Chlorine has two main isotopes: **^35Cl** (75.76% abundance) and **^37Cl** (24.24% abundance).
**Magnesium Chloride Ions:**
- **Magnesium Ion (\( \mathrm{Mg}^{2+} \)):** Positively charged because magnesium loses two electrons to achieve a stable electron configuration.
- **Chloride Ion (\( \mathrm{Cl}^- \)):** Negatively charged because chlorine gains one electron to achieve a stable electron configuration.
Solution
### 1. Relative Atomic Mass of Chlorine (35.5)
**Explanation:**
- **Isotopes of Chlorine:** Chlorine naturally occurs mainly in two isotopic forms:
- **^35Cl:** Has an atomic mass of approximately 34.969 amu.
- **^37Cl:** Has an atomic mass of approximately 36.966 amu.
- **Relative Atomic Mass Calculation:** The relative atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, based on their abundance.
\[
\text{Relative Atomic Mass} = \frac{(\% \text{ abundance of } ^{35}\text{Cl} \times 34.969) + (\% \text{ abundance of } ^{37}\text{Cl} \times 36.966)}{100}
\]
- **Given Relative Atomic Mass:** Chlorine's relative atomic mass is **35.5**. This value suggests that the abundances of the two isotopes are nearly equal but slightly favor the lighter isotope (^35Cl).
**Conclusion:**
A relative atomic mass of 35.5 indicates that **^35Cl** is slightly more abundant than **^37Cl** in nature. Specifically, roughly **75.76%** of chlorine atoms are **^35Cl**, and **24.24%** are **^37Cl**, calculating based on the average:
\[
35.5 = (0.7576 \times 35) + (0.2424 \times 37) \approx 35.5
\]
### 2. Magnesium Chloride Ions
Magnesium chloride (\( \mathrm{MgCl}_2 \)) dissociates into magnesium ions (\( \mathrm{Mg}^{2+} \)) and chloride ions (\( \mathrm{Cl}^{-} \)) in solution.
#### i. Why is a Magnesium Ion Positively Charged?
**Explanation:**
- **Electron Configuration of Magnesium (Mg):**
- Atomic number: 12
- Neutral atoms have 12 protons and 12 electrons.
- Electron configuration: \( 1s^2 2s^2 2p^6 3s^2 \)
- **Ion Formation:**
- To achieve a stable electron configuration similar to the noble gas neon (\( \text{Ne} \)), magnesium **loses two electrons** from its outermost energy level (3s orbital).
\[
\mathrm{Mg} \rightarrow \mathrm{Mg}^{2+} + 2e^-
\]
- **Charge Result:**
- Losing two electrons (each with a negative charge) results in a net positive charge of **+2**.
**Conclusion:**
A magnesium ion is **positively charged** (\( \mathrm{Mg}^{2+} \)) because the magnesium atom loses two electrons to achieve a stable electron configuration, resulting in a net positive charge.
#### ii. Why Does a Chloride Ion Have a Single Negative Charge?
**Explanation:**
- **Electron Configuration of Chlorine (Cl):**
- Atomic number: 17
- Neutral atoms have 17 protons and 17 electrons.
- Electron configuration: \( 1s^2 2s^2 2p^6 3s^2 3p^5 \)
- **Ion Formation:**
- To achieve a stable electron configuration similar to the noble gas argon (\( \text{Ar} \)), chlorine **gains one electron** to fill its 3p orbital.
\[
\mathrm{Cl} + e^- \rightarrow \mathrm{Cl}^-
\]
- **Charge Result:**
- Gaining one electron (each with a negative charge) results in a net negative charge of **-1**.
**Conclusion:**
A chloride ion has a **single negative charge** (\( \mathrm{Cl}^- \)) because the chlorine atom gains one electron to complete its valence shell, resulting in a net negative charge.
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