Problem 107 - Chemistry The Science in Context

Question

Which of the following electron configurations represent an excited state? a. \([\mathrm{He}] 2 s^{1} 2 p^{5}\) b. \([\mathrm{Kr}] 4 d^{10} 5 s^{2} 5 p^{1}\) c. \([\mathrm{Ar}] 3 d^{10} 4 s^{2} 4 p^{5}\) d. \([\mathrm{Ne}] 3 s^{2} 3 p^{2} 4 s^{1}\)

Step-by-Step Solution

Answer

a. \([\mathrm{He}] 2 s^{1} 2 p^{5}\) b. \([\mathrm{Kr}] 4 d^{10} 5 s^{2} 5 p^{1}\) c. \([\mathrm{Ar}] 3 d^{10} 4 s^{2} 4 p^{5}\) d. \([\mathrm{Ne}] 3 s^{2} 3 p^{2} 4 s^{1}\) Answer: a and d represent excited states.

Step 1: Identify the ground state electron configurations for each element

To determine if the given electron configurations represent excited states, we first need to find the ground state electron configurations for the respective elements. To do this, we'll use the periodic table and follow the aufbau principle. a. \([\mathrm{He}] 2 s^{1} 2 p^{5}\) -> Ground state of Fluorine: \([\mathrm{He}] 2 s^{2} 2 p^{5}\) b. \([\mathrm{Kr}] 4 d^{10} 5 s^{2} 5 p^{1}\) -> Ground state of Indium: \([\mathrm{Kr}] 4 d^{10} 5 s^{2} 5 p^{1}\) c. \([\mathrm{Ar}] 3 d^{10} 4 s^{2} 4 p^{5}\) -> Ground state of Bromine: \([\mathrm{Ar}] 3 d^{10} 4 s^{2} 4 p^{5}\) d. \([\mathrm{Ne}] 3 s^{2} 3 p^{2} 4 s^{1}\) -> Ground state of Silicon: \([\mathrm{Ne}] 3 s^{2} 3 p^{2}\) and Ground state of Potassium: \([\mathrm{Ne}] 3 s^{2} 3 p^{6} 4 s^{1}\)

Step 2: Compare the given electron configurations to their ground state configurations

Now, we will compare each given electron configuration to its respective ground state configuration to see if it represents an excited state. a. \([\mathrm{He}] 2 s^{1} 2 p^{5}\) vs \([\mathrm{He}] 2 s^{2} 2 p^{5}\) -> One electron is missing from the 2s subshell, so this represents an excited state. b. \([\mathrm{Kr}] 4 d^{10} 5 s^{2} 5 p^{1}\) vs \([\mathrm{Kr}] 4 d^{10} 5 s^{2} 5 p^{1}\) -> These configurations are the same, so this is not an excited state. c. \([\mathrm{Ar}] 3 d^{10} 4 s^{2} 4 p^{5}\) vs \([\mathrm{Ar}] 3 d^{10} 4 s^{2} 4 p^{5}\) -> These configurations are the same, so this is not an excited state. d. \([\mathrm{Ne}] 3 s^{2} 3 p^{2} 4 s^{1}\) vs \([\mathrm{Ne}] 3 s^{2} 3 p^{2}\) and \([\mathrm{Ne}] 3 s^{2} 3 p^{6} 4 s^{1}\) -> An electron is promoted from the 3p subshell to the 4s subshell, so this represents an excited state.

Step 3: Identify the answer

Based on our comparison, we can conclude that the electron configurations a. \([\mathrm{He}] 2 s^{1} 2 p^{5}\) and d. \([\mathrm{Ne}] 3 s^{2} 3 p^{2} 4 s^{1}\) represent excited states.

Key Concepts

Ground State Electron Configuration

The concept of ground state electron configuration is fundamental in understanding how electrons are arranged around an atom's nucleus. Electrons occupy the lowest available energy levels to achieve the most stable arrangement. This is known as the atom's ground state. For example, the ground state electron configuration of fluorine is

• ext{He}] 2s² 2p⁵

When considering any given electron configuration, comparing it to the ground state helps identify whether electrons are in their lowest energy state or if they have moved to higher energy levels, indicating an excited state.
By understanding the ground state configuration, you can determine stability and reactivity. Atoms in their ground state tend to be more stable.

Aufbau Principle

The Aufbau Principle is a key guiding rule for determining the electron configuration of an atom in its ground state. It dictates that electrons fill atomic orbitals in order of increasing energy levels. Essentially, the electrons occupy the lowest energy orbitals first. This helps predict the distribution of electrons in an atom.
A simple way to remember this is through the order of filling:

• s-orbitals are filled before p-orbitals
• d-orbitals follow
• f-orbitals are filled last

The Aufbau principle supports efficient and stable electron arrangement. It allows us to make quick predictions about the electron configuration of atoms up to a given atomic number. When you see an atom not adhering to this principle, it often indicates an electron in an excited state.

Excited State Identification

Identifying an excited state involves noticing deviations from the ground state electron configuration. These deviations occur when one or more electrons occupy higher energy levels than usual. For instance, when an electron is promoted from a lower to a higher energy orbital, like from 2s to 2p or 3p to 4s, it indicates an excited state.
Take the example of an electron configuration

• ext{Ne}] 3s² 3p² 4s¹

Here, the presence of an electron in the 4s orbital instead of completing the 3p subshell suggests excitation. Electrons gain energy, possibly from external sources, to reach such excited states. Such configurations are less stable than their ground state counterparts.
By contrasting given configurations with their respective ground state configurations, it's possible to spot these excited states regularly. This understanding is essential not only in chemistry but also in physics and materials science, where electron excitation plays a significant role.