Which one of the following atmospheric gases is the most important for absorbing heat radiation coming from the Sun as well as reradiated from the Earth surface, thereby contributing significantly to the greenhouse effect?

Introduction / Context:
This question lies at the interface of chemistry, environmental science and general knowledge. It asks which gas in the atmosphere plays a particularly important role in absorbing heat radiation from the Sun and from the Earth surface, a key part of the greenhouse effect. Understanding which gases are greenhouse gases and how they interact with infrared radiation is essential for appreciating global warming, climate change and related environmental issues that appear frequently in competitive examinations and public discussions.

Given Data / Assumptions:
The options include carbon dioxide, oxygen, carbon monoxide and nitrogen, all of which are common atmospheric gases in varying amounts.
We assume typical atmospheric composition where nitrogen and oxygen are the major components by volume, while carbon dioxide is present in smaller but significant amounts.
We focus on the ability of these gases to absorb infrared radiation and contribute to greenhouse warming.
Water vapour is an important greenhouse gas but is not listed among the options, so it is not considered here.

Concept / Approach:
Greenhouse gases are those that can absorb and emit infrared radiation due to vibrational transitions in their molecules. Molecules with more than two atoms or with a significant dipole moment often have vibrational modes that interact strongly with infrared light. Carbon dioxide has bending and stretching vibrations that fall in the infrared region relevant for heat radiation emitted by the Earth. In contrast, diatomic molecules like nitrogen and oxygen, which are symmetric and non polar, have much weaker interactions with infrared radiation at ground state conditions. Carbon monoxide is a trace gas and, while it can absorb some infrared radiation, its concentration and impact are much less significant than those of carbon dioxide in the context of the general greenhouse effect.

Step-by-Step Solution:
Step 1: Recall the main greenhouse gases in the atmosphere. These include water vapour, carbon dioxide, methane, nitrous oxide and ozone, with carbon dioxide being one of the most discussed due to human emissions. Step 2: Compare the molecular structures. Carbon dioxide is a linear triatomic molecule with vibrational modes that can interact with infrared radiation. This makes it efficient at absorbing and reemitting heat energy. Step 3: Consider oxygen and nitrogen, which are diatomic molecules (O2 and N2). These have no permanent dipole moment and only limited interaction with infrared radiation at the relevant wavelengths, so they are not major greenhouse gases. Step 4: Carbon monoxide, although it is a toxic gas and weak greenhouse gas, is present in much lower concentrations than carbon dioxide and does not dominate the greenhouse effect compared to CO2. Step 5: Therefore, among the given options, carbon dioxide is clearly the most important gas for absorption of heat radiated from the Sun and reradiated from the Earth, and it is the principal greenhouse gas listed.

Verification / Alternative check:
If you consult standard environmental science or atmospheric chemistry texts, they describe carbon dioxide as one of the primary greenhouse gases responsible for trapping heat in the atmosphere. Historical climate data and modern climate models show a strong correlation between rising atmospheric CO2 levels and increases in global average temperature. Nitrogen and oxygen, despite being abundant, are not classified as major greenhouse gases because of their limited infrared absorption. Carbon monoxide is recognised for its role in urban air pollution and smog formation rather than for a dominant greenhouse effect. These independent sources confirm the central role of carbon dioxide in absorbing heat radiation.

Why Other Options Are Wrong:
Oxygen is incorrect because O2 is a homonuclear diatomic molecule with very limited infrared activity, and it does not significantly contribute to the greenhouse effect under normal atmospheric conditions.
Carbon monoxide is incorrect because, although it absorbs some infrared radiation, it is present in much smaller concentrations and is more important as a toxic pollutant than as a major greenhouse gas compared with carbon dioxide.
Nitrogen is incorrect because N2, like O2, is a homonuclear diatomic molecule that interacts only weakly with infrared radiation and therefore does not play a leading role in greenhouse warming.

Common Pitfalls:
Students sometimes assume that because nitrogen and oxygen are the most abundant atmospheric gases, they must also be the most important for every atmospheric process, including heat absorption. This is not true for the greenhouse effect, which depends on the molecular ability to absorb infrared radiation rather than on sheer abundance alone. It is also easy to confuse carbon monoxide with carbon dioxide because of similar names, but their environmental roles are different. Remembering that greenhouse behaviour depends on molecular vibrations and dipole changes helps to pick the correct gas in such questions.

Final Answer:
The gas that most importantly absorbs heat radiated from the Sun and from the Earth among the given options is Carbon dioxide.