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.

Short answer

(c) Electrons moving from a higher energy level to a lower energy level release light.

Step-by-step solution

Understanding the Options

The problem gives us three options regarding the movement of electrons in an atom, which results in the emission of light. We need to determine which option correctly describes the emission process.

Analyzing Option (a)

Option (a) suggests that light is emitted when electrons move from a lower to a higher energy level. However, this requires energy absorption rather than emission. Therefore, option (a) is incorrect for emission of light.

Analyzing Option (b)

Option (b) proposes that electrons are removed, forming a metal cation. This process might lead to ionization but doesn't specifically describe the emission of light.

Analyzing Option (c)

Option (c) states that light is emitted as electrons move from a higher energy level to a lower energy level. This process releases energy in the form of light or photons. This is how emission of light typically occurs.

Conclusion

After analyzing all the options, it is clear that option (c) correctly describes the emission of light due to electrons transitioning from higher to lower energy levels.

Key concepts

Photon Emission

When atoms absorb energy, their electrons can jump up to a higher energy level. However, these high-energy states are not stable. To return to stability, the electrons drop back down to their original or lower energy levels. During this transition, the electrons release the absorbed energy in the form of photons. Simply put, photon emission occurs when electrons transition down between energy levels, losing energy as light. This process is foundational to understanding how lights work in devices like sodium and mercury streetlights. As electrons move between defined energy states, they emit photons that we perceive as light.

Energy Levels

Atoms possess distinct energy levels and electrons inhabit these levels. They can be thought of as steps in a staircase. Each energy level has a particular amount of energy. To move an electron to a higher level, energy must be absorbed. When energy is absorbed, electrons can jump from one step to another higher up the staircase.

• Higher energy levels are unstable, meaning electrons do not remain there indefinitely.
• As electrons drop back down to lower energy levels, energy that was previously absorbed is released.
• This released energy can be observed as light.

Understanding these levels is crucial because they dictate how much energy is released or absorbed when electrons transition. Energy levels are what define the specific wavelengths of light emitted by atoms.

Light Emission

Light emission is a natural and fascinating process happening during electron transitions. When electrons fall from a higher energy level to a lower one, they release energy as light, or photons.

• The color of the emitted light depends on the amount of energy released.
• Different elements emit different colors because their energy level gaps are different.
• For example, sodium can produce a bright yellow light due to its specific electron transitions.

Light emission is not just responsible for the bright glow of streetlights but is also a principle in many technologies and scientific applications, like lasers and TVs. Understanding this concept not only explains everyday phenomena but also allows scientists to analyze the composition of distant stars and galaxies through spectroscopy.