Write the electron configuration for each of the following ions, and determine which ones possess noble-gas configurations: (a) \(\mathrm{Sr}^{2+},(\mathbf{b}) \mathrm{Ti}^{2+},(\mathbf{c}) \mathrm{Se}^{2-},(\mathbf{d}) \mathrm{Ni}^{2+},(\mathbf{e}) \mathrm{Br}^{-}\) \((\mathbf{f}) \mathrm{Mn}^{3+}\)

To find the electron configuration for a given ion, we need to first have its neutral atom's electron configuration. We can easily do this by looking at the atomic number on the periodic table. Then, we'll add or remove the corresponding number of electrons according to the ion's charge. Here are the electron configurations for the given ions: (a) Sr²⁺: The neutral Sr atom's electron configuration is [Kr] 5s². Since Sr²⁺ has lost two electrons, its electron configuration is [Kr]. (b) Ti²⁺: The neutral Ti atom's electron configuration is [Ar] 4s²3d². If Ti²⁺ has lost two electrons, its electron configuration is [Ar] 3d². (c) Se²⁻: The neutral Se atom's electron configuration is [Ar] 4s²3d¹⁰4p⁴. Since Se²⁻ has gained two electrons, its electron configuration is [Ar] 4s²3d¹⁰4p⁶. (d) Ni²⁺: The neutral Ni atom's electron configuration is [Ar] 4s²3d⁸. If Ni²⁺ has lost two electrons, its electron configuration is [Ar] 3d⁶. (e) Br⁻: The neutral Br atom's electron configuration is [Ar] 4s²3d¹⁰4p⁵. Since Br⁻ has gained one electron, its electron configuration is [Ar] 4s²3d¹⁰4p⁶. (f) Mn³⁺: The neutral Mn atom's electron configuration is [Ar] 4s²3d⁵. If Mn³⁺ has lost three electrons, its electron configuration is [Ar] 3d⁴.

Now, to identify which ones have the noble-gas configurations, we need to recall those configurations: He: 1s² Ne: [He] 2s²2p⁶ Ar: [Ne] 3s²3p⁶ Kr: [Ar] 4s²3d¹⁰4p⁶ Xe: [Kr] 5s²4d¹⁰5p⁶ Rn: [Xe] 6s²4f¹⁴5d¹⁰6p⁶ Now, we'll compare our ions' electron configurations to the noble gasses': (a) Sr²⁺: [Kr] => noble-gas configuration (Kr) (c) Se²⁻: [Ar] 4s²3d¹⁰4p⁶ => noble-gas configuration (Kr) (e) Br⁻: [Ar] 4s²3d¹⁰4p⁶ => noble-gas configuration (Kr) The ions Sr²⁺, Se²⁻ and Br⁻ possess noble-gas configurations. The other ions do not have noble-gas configurations.