Animated Solution for Chemistry - Environmental Chemistry: Among the gases (A)-(E), the gases that cause greenhouse effect are : (A) ${\text{CO}}_2$ (B) ${\text{H}}_2\text{O}$ (C) CFCs (D) ${\text{O}}_2$ (E) ${\text{O}}_3$

Imagine our Earth suspended in space, wrapped in a delicate, invisible blanket we call the atmosphere. Every day, the Sun bathes our planet in high-energy solar radiation, primarily in the form of visible light and ultraviolet (UV) rays. This incoming energy easily pierces through the atmosphere and warms the Earth's surface.

However, the Earth doesn't just keep absorbing heat indefinitely; it must maintain a thermal balance. As the surface warms up, it radiates energy back out into space. Because the Earth is much cooler than the Sun, this outgoing energy takes the form of lower-energy, longer-wavelength infrared (IR) radiation.

This is where the magic—and the danger—happens. Not all gases in our atmosphere treat this outgoing infrared radiation the same way.

For a gas to absorb infrared radiation, its molecules must possess a dipole moment that changes as the molecule vibrates. Symmetrical, homonuclear diatomic molecules like Nitrogen ($N_2$) and Oxygen ($O_2$) do not meet this criterion. They are essentially transparent to infrared heat, allowing it to pass right through them.

On the other hand, molecules with three or more atoms, or asymmetrical diatomic molecules, can vibrate in ways that alter their charge distribution. These are our greenhouse gases. When outgoing infrared radiation hits them, they absorb the energy, vibrate vigorously, and then re-radiate that heat in all directions—including back down to the Earth's surface. This continuous cycle of trapping and re-radiating heat is the greenhouse effect.

Let's look at the gases provided in our question:

(A) Carbon Dioxide ($C{O}_2$): The most famous greenhouse gas. Though not the most abundant, its rising levels due to human activity make it the primary driver of modern climate change.

(B) Water Vapor ($H_{2}O$): Surprisingly to many, water vapor is actually the most abundant and powerful natural greenhouse gas in our atmosphere. It plays a critical role in keeping our planet habitable.

(C) Chlorofluorocarbons (CFCs): These synthetic compounds are notorious for destroying the ozone layer, but they are also incredibly potent greenhouse gases, trapping thousands of times more heat per molecule than $C{O}_2$.

(D) Oxygen ($O_2$): As discussed, this symmetrical diatomic molecule cannot absorb infrared radiation. It is not a greenhouse gas.

(E) Ozone ($O_3$): While ozone high in the stratosphere protects us from UV rays, ozone in the lower atmosphere (troposphere) acts as a powerful greenhouse gas and a harmful pollutant.

By filtering our list, we can clearly see that $C{O}_2$, $H_{2}O$, CFCs, and $O_3$ are all greenhouse gases, while $O_2$ is not.

Therefore, the correct combination is (A), (B), (C), and (E), which corresponds perfectly to option (c). Understanding these molecular properties isn't just about passing an exam; it's about understanding the very mechanics of our planet's climate system.