The relative atomic mass of chlorine is 35.5 . What does this tell you about the abundance of the two naturally occurring isotopes of chlorine? Magnesium chloride contains magnesium ions, \( \mathrm{Mg}^{2+} \), and chloride ions, \( \mathrm{Cl}^{-} \). i Explain why a magnesium ion is positively charged. ii Explain why a chloride ion has a single negative charge.

### 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.