Problem 75 - Chemistry and Chemical Reactivity

Question

Light is given off by a sodium- or mercury-containing streetlight when the atoms are excited. The light you see arises for which of the following reasons? (a) Electrons are moving from a given energy level to one of higher energy. (b) Electrons are being removed from the atom, thereby creating a metal cation. (c) Electrons are moving from a given energy level to one of lower energy.

Step-by-Step Solution

Answer

(c) Electrons moving to a lower energy level emit light.

Step 1: Understand the Context

Light emitted by streetlights containing sodium or mercury arises from atom excitation. When atoms are excited, their electrons interact with light, causing changes in energy levels.

Step 2: Evaluate Option (a)

Option (a) suggests that electrons move to a higher energy level. This process would actually absorb energy and not emit it, so it doesn't explain why light is emitted. Thus, electrons moving to higher energy levels are not the cause of light emission.

Step 3: Evaluate Option (b)

Option (b) proposes that electrons are removed from the atom, forming metal cations. While ionization can occur, it is not the typical process for light emission in streetlights. This process doesn't directly result in light emission.

Step 4: Evaluate Option (c)

Option (c) suggests that electrons move from a higher to a lower energy level. When this occurs, the excess energy is released in the form of light. This is the process of photon emission which matches how streetlights emit light.

Step 5: Conclusion

The emission of light from streetlights is due to electrons transitioning from higher to lower energy levels within the atoms, option (c). This release of energy in the form of light is a well-understood phenomenon in physics.

Key Concepts

Energy Levels

Atoms are composed of electrons orbiting the nucleus. These electrons occupy regions in space around the nucleus known as energy levels or shells. Each energy level corresponds to a certain amount of energy.
These energy levels are typically labeled with numbers or letters, such as "n" or "K, L, M, N". The lowest energy level is the closest to the nucleus, while higher energy levels are further away.
This concept of energy levels is essential to understanding how elements absorb or emit light.

• When an electron absorbs energy, it can jump to a higher energy level.
• Conversely, when an electron falls to a lower energy level, it releases energy in the form of light.

Understanding these concepts helps elucidate phenomena like the emitted light in sodium or mercury streetlights.

Electron Transition

An electron transition refers to the movement of an electron between energy levels. This is a key mechanism in radiation and is foundational to photon emission.
When an electron absorbs enough energy, it transitions to a higher energy level, a state known as excitation.
However, this is unstable, and the electron often returns to a lower energy state.

• During the descent to a lower energy level, the energy difference is released as a photon, which is the basic quantum of light.
• It's this transition from higher to lower energy levels that produces the visible light.

In streetlights specifically, this process is harnessed to emit the light we see, as electrons in the atoms of elements like sodium and mercury undergo these transitions.

Atomic Excitation

Atomic excitation happens when energy is supplied to an atom causing its electrons to jump to higher energy levels.
This energy can be in the form of light, heat, or electric energy. When the atoms in a streetlight are excited, it is usually due to electrical energy.
During excitation:

• Electrons absorb energy and move to a higher energy state.
• The excited state is transient, meaning electrons can't stay there permanently.

As electrons return to their original energy levels, they emit the excess energy as light. This is the reason why excited atoms release photons, resulting in visible light. In streetlights, this process is continuously repeated, producing constant illumination.