Question

What is meant by the phrase excited state as applied to an atom or molecule? Symbolically, how is an excited state signified?

Short answer

An excited state is when an atom/molecule has higher energy than the ground state, often indicated by an asterisk (*).

Step-by-step solution

Understanding Excited State

An excited state of an atom or molecule refers to any state that is higher in energy than the ground state. The ground state is the lowest energy state of the system, and when an atom or molecule absorbs energy, one or more of its electrons are elevated to higher energy levels, leading to an excited state.

Symbolic Representation

An excited state is often symbolically represented by indicating the energy level of the electrons. If an atom or molecule is in an excited state, its electron configuration differs from that of the ground state, and an asterisk (*) is often used to signify the excited state, e.g., \( n^* \)

Key concepts

Ground State

The ground state of an atom or molecule represents the lowest energy state that the system can occupy. When electrons are in this state, they are at their most stable configuration and possess the minimum energy necessary to maintain this state.
In the ground state, every electron occupies the lowest available energy level, and this arrangement is crucial for understanding how atoms interact with light and other particles. An important detail is that atoms prefer to stay in their ground state until they absorb enough energy to move to a higher energy level, entering an excited state.
For example, in a ground state hydrogen atom, the single electron orbits in the lowest possible energy level, which is also known as the 1s orbital. No energy is being added or removed, so the electron remains stably in this level.

Electron Configuration

Electron configuration refers to the distribution of electrons among the various energy levels or orbitals within an atom. It is a critical concept for predicting the chemical behavior of elements as it dictates how atoms bond and interact with each other.
The configuration is typically written using numbers and letters that represent the principal energy levels, subshells, and the number of electrons within those subshells. For instance, a ground state oxygen atom is represented as:

• 1s²
• 2s²
• 2p⁴

When an atom is excited, the electron configuration changes because some electrons move to higher energy levels, leading to a distinct configuration that is different from the ground state. This altered configuration is crucial in explaining phenomena such as spectral lines in atomic spectra, where light emitted or absorbed by an atom shows distinct lines corresponding to these electronic transitions.

Energy Levels

Energy levels are specific regions around the nucleus of an atom where electrons are likely to be found. These levels increase in energy, moving further away from the nucleus, and are often denoted by the principal quantum number, "n".
Each principal energy level contains one or more sublevels, which further divide into orbitals where electrons reside. As you move to higher energy levels, both the energy and the number of sublevels increase. For example, the first energy level (n=1) carries only one sublevel (1s), while the second (n=2) has two sublevels (2s, 2p), and so forth.
When an atom absorbs energy, electrons in the ground state jump to these higher energy levels, leading to what is called an excited state. This process helps in explaining various physical and chemical properties of elements, and this transitions result in the emission or absorption of light at specific wavelengths characteristic of each element. Understanding these energy levels is crucial, as they lay the foundation for quantum mechanics and modern chemistry.