Question

Write out the electron configuration for each of the following nonmetal ions: (a) \(\mathrm{Br}^{-}\) (b) \(\mathrm{O}^{2-}\) (c) \(\mathrm{Se}^{2-}\) (d) \(\mathrm{N}^{3-}\)

Short answer

(a) [Ar] 3d⁶ 4s² 4p⁶, (b) 1s² 2s² 2p⁶, (c) [Ar] 3d¹⁰ 4s² 4p⁶, (d) 1s² 2s² 2p⁶.

Step-by-step solution

Understanding Electron Configuration

Electron configuration is a way to represent the distribution of electrons in an atom's electron shells and subshells. Electrons fill orbitals starting from the lowest energy level to the highest (Aufbau principle). To write the electron configuration of an ion, we first write the configuration for the neutral atom and then add or remove electrons depending on the charge of the ion.

Br⁻ Ion

The neutral bromine (Br) atom has an atomic number of 35, which means it has 35 electrons. Its electron configuration is: \[ \mathrm{Br}: [\mathrm{Ar}] \, 3d^{10} \, 4s^2 \, 4p^5 \].As Br⁻, it gains 1 electron to become \[ \mathrm{Br}^{-}: [\mathrm{Ar}] \, 3d^{10} \, 4s^2 \, 4p^6 \],which has the same configuration as Krypton (noble gas configuration).

O²⁻ Ion

Oxygen (O) has an atomic number of 8, meaning it has 8 electrons. The neutral atom's configuration is: \[ \mathrm{O}: 1s^2 \, 2s^2 \, 2p^4 \].For O²⁻, it gains 2 electrons becoming \[ \mathrm{O}^{2-}: 1s^2 \, 2s^2 \, 2p^6 \],which is the same as the electron configuration of Neon (noble gas configuration).

Se²⁻ Ion

Selenium (Se) has an atomic number of 34. The neutral atom has an electron configuration of: \[ \mathrm{Se}: [\mathrm{Ar}] \, 3d^{10} \, 4s^2 \, 4p^4 \].When it gains two electrons to become Se²⁻, the configuration becomes \[ \mathrm{Se}^{2-}: [\mathrm{Ar}] \, 3d^{10} \, 4s^2 \, 4p^6 \].Again, this resembles the configuration of Krypton.

N³⁻ Ion

Nitrogen (N) has an atomic number of 7, so its neutral atom configuration is: \[ \mathrm{N}: 1s^2 \, 2s^2 \, 2p^3 \].The N³⁻ ion gains 3 electrons, resulting in the configuration \[ \mathrm{N}^{3-}: 1s^2 \, 2s^2 \, 2p^6 \], which matches the electron configuration of Neon.

Key concepts

Nonmetal Ions

Nonmetals are elements that tend to gain electrons during chemical reactions, forming negative ions, known as anions. These negatively charged ions are important in forming ionic compounds. In chemistry, ions like Br⁻, O²⁻, Se²⁻, and N³⁻ are examples of nonmetal ions.
When a nonmetal atom gains electrons, the added electrons join the existing electron configuration, filling up available electron shells to settle at lower energy levels.
For instance, when a bromine atom gains an electron, it results in a Br⁻ ion. This ion has an extra electron compared to the neutral atom, leading to a filled outer shell typical of a noble gas.
Nonmetal ions often achieve this stable electron arrangement — similar to that of noble gases — by gaining enough electrons to fill their outermost shells.

Aufbau Principle

The Aufbau principle is a key concept in understanding how electron configurations are determined. It states that electrons are added one by one to atomic orbitals, starting from the lowest to higher energy levels. This principle helps explain the pattern of filling electron shells and subshells.
According to the Aufbau principle, electrons fill orbitals in a specific order:

• The "s" subshells fill before the "p," "d," and "f" subshells within each shell, according to increasing energy.
• The sequence generally follows the order seen on the periodic table, which acts as a good guide for predicting electron configurations.

For instance, when writing the electron configuration for Br⁻, we start with the configuration for the neutral bromine atom and then apply the principle to accommodate the gained electron. The electrons will fill up the orbitals in the order of increasing energy, conforming to this principle.

Noble Gas Configuration

Noble gas configuration refers to the fully filled set of electron shells found in noble gases, the group 18 elements in the periodic table.
Noble gases like Helium, Neon, and Krypton have particularly stable electron configurations, as they possess a full valence shell. This is why nonmetal ions adopt these configurations; achieving a noble gas configuration offers increased stability.
For example, when bromine gains one more electron to form Br⁻, it adopts the electron configuration of Krypton, which is \[ ext{[Ar]} \, 3d^{10} \, 4s^2 \, 4p^6 \].
L Similarly, O²⁻ and N³⁻ adopt the electron configuration of Neon, \[ 1s^2 \, 2s^2 \, 2p^6 \]. Nonmetal ions striving for noble gas configuration showcase the drive towards stability in atomic structures.

Electron Shells and Subshells

Electron shells and subshells are the essential units used to understand how electrons are arranged in an atom.
An electron shell is at a specific distance from the nucleus and contains one or more electron subshells. These subshells are labeled s, p, d, and f, representing different shapes and energy levels.

• The s subshell can hold up to 2 electrons.
• The p subshell can hold up to 6 electrons.
• The d subshell is capable of holding 10 electrons.
• The f subshell can accommodate up to 14 electrons.

Understanding the capacity of each shell helps in writing the electron configurations of nonmetal ions. For instance, in the oxygen ion, O²⁻, electrons fill the 2s and 2p subshells to achieve the stable \[ 1s^2 \, 2s^2 \, 2p^6 \] configuration. Correctly determining the distribution among shells and subshells is crucial for developing a comprehension of atomic behavior.