Question

Light is given off by a sodium- or mercury-containing streetlight when the atoms are excited in some way. The light you see arises for which of these reasons? (a) Electrons moving from a given quantum level to one of higher \(n\). (b) Electrons being removed from the atom, thereby creating a metal cation. (c) Electrons moving from a given quantum level to one of lower \(n\). (d) Electrons whizzing about the nucleus in an absolute frenzy.

Short answer

(c) Electrons moving from a given quantum level to one of lower \(n\).

Step-by-step solution

Understand the Problem

We are asked to determine the reason a streetlight emits light, specifically in the context of how the electrons within the atoms of sodium or mercury inside the streetlights behave.

Recall Atomic Transitions

Light emission in atoms occurs when electrons transition between energy levels. An electron moving from a higher energy level (higher principal quantum number, denoted as \(n\)) to a lower energy level emits light. This process releases energy in the form of photons, resulting in the emission of light.

Analyze Each Option

Let's examine each option based on the knowledge of atomic transitions:- (a) Electrons moving from a given quantum level to a higher \(n\) requires energy absorption, not emission.- (b) Removing electrons to create cations involves ionization, not light emission.- (c) Electrons moving from a higher to a lower quantum level emits light.- (d) The term 'whizzing about' implies random motion with no consistent energy change, unlikely to result in light emission.

Choose the Correct Option

Based on the understanding of how electron transitions result in light emission, option (c) is correct because it describes electrons moving from higher to lower energy levels, which releases energy as photons.

Key concepts

Energy Levels

Energy levels are foundational concepts in atomic theory. They represent the fixed distances from the nucleus where electrons can exist within an atom. These levels are quantized, meaning that electrons can only inhabit specific, discrete levels, not the spaces in between.
When we use the term 'higher energy level,' we're referring to an electron position farther from the nucleus. In contrast, a 'lower energy level' is closer to the nucleus. The principal quantum number, represented by \(n\), is used to label these energy levels. The higher the \(n\) value, the higher the energy and the further the level is from the nucleus.
Changes in energy levels are key to many atomic behaviors. When an electron changes levels, it either absorbs or releases energy in quantized amounts, resulting in photon emission or absorption.

Photon Emission

Photon emission happens when an electron in an atom moves from a higher to a lower energy level. This process is what makes things like streetlights illuminate.
When electrons return to a lower energy state, the energy difference between the two levels is released as a photon. A photon is a fundamental particle of light, carrying the energy associated with the transition.
For example, when electrons in sodium or mercury are in an excited state due to an external energy source, they eventually fall back to lower energy levels. This falling back releases energy in the form of visible light, which is why we see the glow from streetlights.

• It's all about the energy difference: larger differences mean higher energy photons, which can lead to different colors of light.
• The process is spontaneous; it occurs naturally as electrons seek their lowest energy configuration.

Quantum Mechanics

Quantum Mechanics is the branch of physics that deals with the behavior of particles at very small scales, such as atoms and subatomic particles. It provides the framework for understanding atomic transitions.
In quantum mechanics, particles such as electrons exhibit both particle-like and wave-like properties. This duality is why energy levels in atoms are discrete; electrons behave as standing waves that can only exist at certain frequencies and wavelengths around the nucleus.
Key principles of quantum mechanics include:

• Quantization: Energy is not continuous but comes in small packets called quanta.
• Probability: We cannot precisely predict where an electron is but can determine the probability of finding it in certain regions.
• Superposition: Electrons can exist in multiple states simultaneously until measured.

Understanding these principles helps explain why electrons in atoms move between specific energy levels, a cornerstone in explaining phenomena like light emission in streetlights.