Question

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

Short answer

The electron configurations for the ions are: (a) Sr²⁺: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10}4p^6\) (noble gas configuration), (b) Ti²⁺: \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^2\), (c) Se²⁻: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\) (noble gas configuration), (d) Ni²⁺: \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^8\), (e) Br⁻: \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6\) (noble gas configuration), (f) Mn³⁺: \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^4\). The ions with noble gas configurations are Sr²⁺, Se²⁻, and Br⁻.

Step-by-step solution

(a) Electron Configuration of Sr2+ Ion

First, locate the neutral Strontium (Sr) atom on the periodic table and note its atomic number (38). Write the electron configuration of a neutral Sr atom, which is 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s². Since Sr²⁺ has lost 2 electrons, remove these electrons from the outer shell, resulting in the electron configuration of 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶, which matches the noble gas configuration of Krypton (Kr).

(b) Electron Configuration of Ti2+ Ion

Locate neutral Titanium (Ti) on the periodic table with atomic number 22. Write the electron configuration of a neutral Ti atom, which is 1s²2s²2p⁶3s²3p⁶4s²3d². Since Ti²⁺ has lost 2 electrons, remove these electrons from the outer shell, resulting in an electron configuration of 1s²2s²2p⁶3s²3p⁶3d², which does not match any noble gas configurations.

(c) Electron Configuration of Se2- Ion

Locate neutral Selenium (Se) on the periodic table with atomic number 34. Write the electron configuration of a neutral Se atom, which is 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁴. Since Se²⁻ has gained 2 electrons, add these electrons to the outer shell, resulting in an electron configuration of 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶, which matches the noble gas configuration of Krypton (Kr).

(d) Electron Configuration of Ni2+ Ion

Locate neutral Nickel (Ni) on the periodic table with atomic number 28. Write the electron configuration of a neutral Ni atom, which is 1s²2s²2p⁶3s²3p⁶4s²3d⁸. Since Ni²⁺ has lost 2 electrons, remove them from the outer shell, resulting in an electron configuration of 1s²2s²2p⁶3s²3p⁶3d⁸, which does not match any noble gas configurations.

(e) Electron Configuration of Br- Ion

Locate neutral Bromine (Br) on the periodic table with atomic number 35. Write the electron configuration of a neutral Br atom, which is 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁵. Since Br⁻ has gained 1 electron, add it to the outer shell, resulting in an electron configuration of 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶, which matches the noble gas configuration of Krypton (Kr).

(f) Electron Configuration of Mn3+ Ion

Locate neutral Manganese (Mn) on the periodic table with atomic number 25. Write the electron configuration of a neutral Mn atom, which is 1s²2s²2p⁶3s²3p⁶4s²3d⁵. Since Mn³⁺ has lost 3 electrons, remove them from the outer shell, resulting in an electron configuration of 1s²2s²2p⁶3s²3p⁶3d⁴, which does not match any noble gas configurations.

Summary

The ions that possess noble gas configurations are: Sr²⁺, Se²⁻, and Br⁻.

Key concepts

Noble Gas Configuration

In the context of ion electron configurations, noble gas configuration refers to an ion that has an electron configuration matching that of the nearest noble gas in the periodic table. Noble gases — Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn) — are elements known for their chemical inertness, largely due to their complete valence shells, which make them very stable.

When an atom gains or loses electrons and becomes an ion, it often does so to achieve a noble gas configuration, sometimes referred to as an 'octet' in the case of atoms other than Helium. For instance, when a Strontium (Sr) atom loses two electrons to form a Sr²⁺ ion, the result is an electron configuration that matches that of Krypton (Kr), thereby reaching stability like a noble gas. Similarly, a Selenium (Se) atom can gain two electrons to become a Se²⁻ ion, resulting in the same electron configuration as Krypton (Kr), illustrating this concept of noble gas-like stability.

Understanding the noble gas configuration of ions is crucial because it provides insight into the stability and reactivity of the ions. Ions strive to achieve the electron configuration of noble gases because such arrangements offer the most stable and energetically favorable state.

Periodic Table

The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number, electron configuration, and recurring chemical properties. Elements are presented in order of increasing atomic number, which is the number of protons in an atom's nucleus, and are arranged in rows, or periods, and columns, or groups, that share common characteristics.

For students learning about electron configurations, the periodic table is an essential tool. It not only indicates the atomic number but also shows how orbitals are filled with electrons. The table is divided into blocks — s, p, d, and f — that correspond to the subshells of an atom, helping predict how electrons are distributed in atoms and ions. By knowing the position of an element on the periodic table, one can determine the number of electrons in a neutral atom and then add or remove electrons to find the ion's electron configuration.

For example, in the problem given, the atomic numbers for Titanium (Ti), Selenium (Se), and Nickel (Ni) are used to derive their neutral electron configurations. Then the ionic charges are considered to adjust those configurations accordingly, either by subtracting electrons for positively charged ions or adding electrons for negatively charged ions.

Atomic Number

The atomic number is a fundamental property of an element and its atoms, indicating the number of protons in the nucleus of an atom. It defines the identity of the element; for instance, all atoms with 6 protons are carbon atoms. The atomic number is also equal to the number of electrons in a neutral atom, which makes it directly related to the atom's electron configuration.

Understanding atomic numbers is crucial for determining electron configurations, as shown in the given steps. When working out the electron configuration for ions, students start with the neutral atom's electron configuration, which is directly found using its atomic number. From there, they modify the configuration based on whether the atom has lost or gained electrons to become an ion.

For example, Manganese (Mn) has an atomic number of 25, indicating it has 25 protons and, in its neutral state, 25 electrons. If it forms Mn³⁺ by losing three electrons, its electron configuration is adjusted by removing three electrons from the outermost orbitals. As shown in the exercise, the concept of atomic number makes the determination of electron configurations and ion charge states a systematic process.