Question

Which electron configuration notation describes an atom in an excited state? a. \([\mathrm{Ar}] 4 \mathrm{s}^{2} 3 \mathrm{d}^{10} 4 \mathrm{p}^{2}\) b. \([\mathrm{Ne}] 3 \mathrm{s}^{2} 3 \mathrm{p}^{5}\) c. \([\mathrm{Kr}] 5 s^{2} 4 \mathrm{d}^{1}\) d. \([\mathrm{Ar}] 4 \mathrm{s}^{2} 3 \mathrm{d}^{8} 4 \mathrm{p}^{1}\)

Short answer

The electron configuration that describes an atom in an excited state is option d. \( [\mathrm{Ar}] 4 \mathrm{s}^{2} 3 \mathrm{d}^{8} 4 \mathrm{p}^{1} \).

Step-by-step solution

Identify ground state electron configurations

First, let's write down the ground state electron configurations for each noble gas mentioned in the possible answers: a. [\(\text{Ar}\)] = \([1s^2 2s^2 2p^6 3s^2 3p^6\)] b. [\(\text{Ne}\)] = \([1s^2 2s^2 2p^6 \)] c. [\(\text{Kr}\)] = \([1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6\)] d. [\(\text{Ar}\)] = \([1s^2 2s^2 2p^6 3s^2 3p^6\)]

Compare given electron configurations to the ground state

To determine which atom is in an excited state, compare the given electron configurations to the ground state configurations identified before: a. [\(\text{Ar} 4s^2 3d^{10} 4p^2\)] = [\(\text{Ar}\)] + \(4s^2 3d^{10} 4p^2\) = \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^2\) b. [\(\text{Ne} 3s^2 3p^5\)] = [\(\text{Ne}\)] + \(3s^2 3p^5\) = \(1s^2 2s^2 2p^6 3s^2 3p^5\) c. [\(\text{Kr} 5s^2 4d^{1}\)] = [\(\text{Kr}\)] + \(5s^2 4d^{1}\) = \(1s^2 2s^2 2p^6 3s^2 3p^6 3d^{10} 4s^2 4p^6 5s^2 4d^{1}\) d. [\(\text{Ar} 4s^2 3d^{8} 4p^1\)] = [\(\text{Ar}\)] + \(4s^2 3d^{8} 4p^1\) = \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{8} 4p^1\)

Identify the excited state

Now, let's find the excited state by looking for an electron that has jumped up to a higher energy level than it should be in: a. The last electron in this configuration is in the \(4p\) orbital which is normal for this atom in a ground state. b. The last electron is in the \(3p\) orbital which is normal for this atom in a ground state. c. The last electron is in the \(4d\) orbital which is normal for this atom in a ground state. d. The last electron in this configuration is in the \(4p\) orbital. However, it should be in the \(3d\) orbital to make it a ground state configuration. The electron configuration that describes an atom in an excited state is option d. \( [\mathrm{Ar}] 4 \mathrm{s}^{2} 3 \mathrm{d}^{8} 4 \mathrm{p}^{1} \).

Key concepts

Electron Configuration Notation

Understanding the electron configuration notation is fundamental in the study of atomic structure and chemistry. This notation provides a roadmap of the arrangement of electrons within the atoms, mapped out according to their respective energy levels and orbitals. An electron configuration is written using the principal quantum number (n), which indicates the energy level and the type of orbital (such as s, p, d, or f), followed by a superscript that denotes the number of electrons in those orbitals.

For example, the notation \(2p^4\) would indicate that there are four electrons in the p orbital of the second energy level. The arrangement follows the Aufbau principle, where electrons are filled from the lowest to the highest energy orbitals, observing Pauli's exclusion principle and Hund's rule for subshell filling with multiple orbitals, like the p, d, and f orbitals.

Ground State Electron Configuration

The ground state electron configuration refers to the most stable, lowest energy arrangement of electrons in an atom. This configuration is achieved when electrons are arranged in a way that fills the orbitals with lower energy levels before the higher ones. Electrons occupy the available s, p, d, and f orbitals in a sequence guided by the Aufbau principle, which leads to the most energetically favorable electron configuration.

In a ground state electron configuration, all electrons are in the lowest possible energy levels. This also means that if an atom exhibits a configuration that does not correlate with the expected pattern of electronic arrangement, such as having electrons in higher energy levels when there are vacant spots in lower ones, it is not in its ground state, but rather in an excited state—as was identified in option d of our exercise.

Noble Gas Electron Configuration

When simplifying electron configurations, noble gas electron configuration notation becomes particularly useful. This shorthand method uses the electron configuration of the noble gases as a starting point to describe the configuration of other elements. Noble gases are elements found in the furthest right column of the periodic table and are known for having their outermost electron shells completely filled, which gives them extraordinary stability.

To write electron configurations in this shorthand, you start with the noble gas that precedes the element in question and then add on the remaining electron configuration. For example, the noble gas configuration for chlorine, which has 17 electrons, would be written as \( [Ne] 3s^2 3p^5 \), with [Ne] representing the complete electron configuration for neon, the noble gas that precedes chlorine. This provides an efficient and concise way to represent electron configurations, especially for elements with many electrons.