Problem 108 - Chemistry The Science in Context

Question

Which of the following electron configurations represent an excited state? a. \([\mathrm{Ne}] 3 s^{2} 3 p^{1}\) b. \([\mathrm{Ar}] 3 d^{10} 4 s^{1} 4 p^{2}\) c. \([\mathrm{Kr}] 4 d^{10} 5 s^{1} 5 p^{1}\) d. \([\mathrm{Ne}] 3 s^{2} 3 p^{6} 4 s^{1}\)

Step-by-Step Solution

Answer

a. \([\mathrm{Ne}] 3 s^{2} 3 p^{1}\) b. \([\mathrm{Ar}] 3 d^{10} 4 s^{1} 4 p^{2}\) c. \([\mathrm{Kr}] 4 d^{10} 5 s^{1} 5 p^{1}\) d. \([\mathrm{Ne}] 3 s^{2} 3 p^{6} 4 s^{1}\) Answer: The electron configurations that represent an excited state are option b, c, and d.

Step 1: Identify the Noble Gas Core

For each electron configuration, begin by identifying the noble gas core in brackets. This represents the electron configuration of the noble gas with the same atomic number. a. \([\mathrm{Ne}] 3 s^{2} 3 p^{1}\): Ne = 10 electrons b. \([\mathrm{Ar}] 3 d^{10} 4 s^{1} 4 p^{2}\): Ar = 18 electrons c. \([\mathrm{Kr}] 4 d^{10} 5 s^{1} 5 p^{1}\): Kr = 36 electrons d. \([\mathrm{Ne}] 3 s^{2} 3 p^{6} 4 s^{1}\): Ne = 10 electrons

Step 2: Determine Total Electrons

Next, add the electrons in each following subshell to the noble gas core. a. 10 + 2 + 1 = 13 electrons b. 18 + 10 + 1 + 2 = 31 electrons c. 36 + 10 + 1 + 1 = 48 electrons d. 10 + 2 + 6 + 1 = 19 electrons

Step 3: Compare with Aufbau Principle

Compare each electron configuration with the expected distribution of electrons based on the Aufbau principle. a. Electron configuration for 13 electrons (Aluminum): \([\mathrm{Ne}] 3 s^{2} 3 p^{1}\) b. Electron configuration for 31 electrons (Gallium): \([\mathrm{Ar}] 3 d^{10} 4 s^{2} 4 p^{1}\) c. Electron configuration for 48 electrons (Cadmium): \([\mathrm{Kr}] 4 d^{10} 5 s^{2}\) d. Electron configuration for 19 electrons (Potassium): \([\mathrm{Ar}] 4 s^{1}\)

Step 4: Identify Excited States

Compare the given electron configurations with those from step 3. If they do not match, then the electron configuration is in an excited state. a. Given: \([\mathrm{Ne}] 3 s^{2} 3 p^{1}\) / Expected: \([\mathrm{Ne}] 3 s^{2} 3 p^{1}\) - Not in an excited state b. Given: \([\mathrm{Ar}] 3 d^{10} 4 s^{1} 4 p^{2}\) / Expected: \([\mathrm{Ar}] 3 d^{10} 4 s^{2} 4 p^{1}\) - In an excited state c. Given: \([\mathrm{Kr}] 4 d^{10} 5 s^{1} 5 p^{1}\) / Expected: \([\mathrm{Kr}] 4 d^{10} 5 s^{2}\) - In an excited state d. Given: \([\mathrm{Ne}] 3 s^{2} 3 p^{6} 4 s^{1}\) / Expected: \([\mathrm{Ar}] 4 s^{1}\) - In an excited state Therefore, the electron configurations that represent an excited state are option b, c, and d.

Key Concepts

Aufbau principle

The Aufbau principle is a fundamental concept in chemistry that helps us understand how electrons are distributed in an atom's orbitals. "Aufbau" is a German word meaning "building up", and this principle suggests that electrons fill atomic orbitals from the lowest to the highest energy levels. This order is crucial for determining the ground state of an atom's electron configuration.

In practice, when an atom gains electrons, they occupy the lowest available energy state first. This process continues in a step-by-step manner along increasing energy levels. Specifically, electrons fill the s, p, d, and f sub-levels in a particular sequence to create the most stable electron configuration.

• s-orbitals fill before p-orbitals.
• p-orbitals fill before d-orbitals.
• d-orbitals fill before f-orbitals.

However, there are a few exceptions due to electron-electron interactions and stability considerations, especially as the number of electrons increases. Recognizing these exceptions is key when identifying an excited state, where an electron has absorbed energy and moved to a higher energy level than its position suggested by the Aufbau principle.

electron configuration

Electron configuration describes the distribution of electrons among the atomic orbitals of an atom. It follows a predictable pattern that reflects the Aufbau principle.

• Electrons are primarily located in specific orbitals denoted as s, p, d, and f.
• Each orbital can hold a limited number of electrons: 2 for s, 6 for p, 10 for d, and 14 for f.

An essential method for writing these configurations includes denoting each subshell and the number of electrons within it, using superscripts.

For example, the electron configuration for a sodium atom is written as: \[1s^2 \, 2s^2 \, 2p^6 \, 3s^1\]Using shorthand notation, it would be: \[[Ne] \, 3s^1\]Configurations play a vital role in understanding an atom's chemical properties, reactivity, and even an excited state. An excited state is when an electron moves into a higher energy level subshell, differing from the ground state configuration. This typically deviates from what the Aufbau principle predicts.

noble gas core

To simplify electron configurations, especially for elements with many electrons, chemists often use noble gas core notation. This shorthand method involves representing the inner electrons of an atom with the symbol of the nearest noble gas element that precedes the element in the periodic table.

For example, consider the configuration of Chlorine:

• Full configuration: \[1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^5\]
• Noble gas core notation: \[[Ne] \, 3s^2 \, 3p^5\]

The noble gas core acts as a reference point and represents a stable, complete set of s and p orbitals. This stability is what makes noble gases inert and serves as a convenient benchmark for outer electron configuration.

Using noble gas notation not only simplifies the way we describe electron distributions for elements beyond the first couple of rows in the periodic table, but it also makes it easier to identify unusual configurations, such as those found in excited states. An electron configuration that diverts from the expected noble gas core is often an indicator of an excited electron state.