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Carbon is one of the most abundant and widely distributed elements in nature. In fact, it ranks as the fourth most abundant element in the universe, following Hydrogen (H), Helium (He), and Oxygen (O). Carbon exists in a gaseous state at high temperatures and, when burned in the air, it combines with oxygen to form carbon dioxide (CO₂). The most common types of carbon compounds in the environment are gases, including carbon dioxide (CO₂) and methane (CH₄).

https://pubchem.ncbi.nlm.nih.gov/element/6

Its atomic number is 6. If the atomic number is 6, it means it also has 6 protons (and also 6 electrons).

[Note 1] The number of protons in an atom is equal to its atomic number.

Atoms are electrically neutral, meaning that typically the number of protons and electrons in an atom is the same. Therefore, carbon would have six electrons orbiting its atomic nucleus. The electron configuration of nitrogen is represented as 1s2 2s2 2p2. 

This means that in the first electron shell of carbon, there are two electrons, and in the second electron shell, there are four electrons. The electrons in the second electron shell would occupy both the s orbital (spherical shape) and the p orbitals (dumbbell shape). Atoms are electrically neutral because the number of protons (positively charged) and electrons (negatively charged) is equal. The electron configuration of carbon is represented as 1s2 2s2 2p2. 

This notation tells us how the electrons are distributed among the electron shells and orbitals. 

The first electron shell (n = 1) can only hold a maximum of 2 electrons, and they are found in the 1s orbital. 

The second electron shell (n = 2) can hold a maximum of 8 electrons. In the case of carbon, there are 4 electrons in the second shell: 
  A. The first two electrons are in the 2s orbital (2s2). 
  B. The next two electrons are in two separate 2p orbitals (2p2). 
     Each p orbital can hold up to 2 electrons. 

Therefore, the correct electron configuration of carbon is 1s2 2s2 2p2, and you are correct that the electrons in the second electron shell (n = 2) occupy both the s orbital (spherical shape) and the three p orbitals (dumbbell shape) that are oriented along the x, y, and z axes.

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Carbon as a structure of organic compounds

Carbon can form covalent bonds by sharing its valence electrons with other atoms. Covalent bonds occur when two atoms share electrons to achieve a more stable electron configuration. Each carbon atom can share one, two, three, or even four electrons with other atoms, depending on the number of bonds it forms (As mentioned earlier, the outermost electron shell is the second shell (n = 2), and it contains four electrons in total).

As a single carbon atom can bond with up to four other atoms, it can form chains or rings by combining with other carbon atoms, as well as various elements such as hydrogen, oxygen, nitrogen, halogens, phosphorus, metals, etc., to create a wide variety of compounds. Carbon has the ability to form strong covalent bonds with other carbon atoms, as well as with other elements. This allows for the formation of a vast array of complex molecules with diverse properties and functions. The most representative substances among these carbon compounds are organic compounds. Organic compounds constitute the building blocks of living organisms and are essential for their existence. They include carbohydrates, proteins, lipids (fats), nucleic acids (DNA and RNA), and many others are organic compounds.

The presence of carbon-hydrogen (C-H) bonds in organic compounds provides them with significant stability and versatility. In fact, carbon-hydrogen (C-H) bonds are a standard used to distinguish between inorganic and organic carbon, which means the presence of carbon-hydrogen (C-H) bonds is a fundamental characteristic of organic compounds.

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Inorganic carbon

As mentioned above, if carbon-hydrogen (C-H) bonds are a standard used to distinguish between inorganic and organic carbon, then it is worth noting that inorganic carbon might not have carbon-hydrogen (C-H) bonds.

Is that correct? No, it’s not!!

Inorganic compounds can indeed have carbon-hydrogen (C-H) bonds. For example, compounds like methane (CH₄) or acetic acid (CH₃COOH) contain both carbon and hydrogen atoms bonded together. These compounds are considered inorganic despite containing C-H bonds. Actually, the presence of C-H bonds alone is not sufficient to differentiate between organic and inorganic compounds.

Therefore, to distinguish between inorganic and organic carbon, the more appropriate criterion is whether the compound is derived from living organisms or not

That is, inorganic carbon is regarded as carbon extracted from ores and minerals (not living organisms), while organic carbon is found in nature in plants and living things (living organisms). Additionally, the carbon present in oxides of carbon is an example of inorganic carbon.

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Why is organic carbon a constituent of living organisms?

The reason organic carbon become constituents of living organisms is because of plant photosynthesis.

The process of plant photosynthesis is the primary mechanism by which organic carbon becomes a constituent of living organisms on Earth. Plants absorb carbon dioxide (CO₂) and water (H₂O) and create organic matter, such as carbohydrates, through photosynthesis using light.

6CO2 + 6H2O + sunlight → C6H12O6 + 6O2

Glucose, the product of photosynthesis, is a type of organic carbon that serves as the primary energy source for plants and is used to build other organic compounds essential for plant growth and development. Once plants produce glucose through photosynthesis, it becomes the foundation of the food chain. Animals, which cannot produce their own nutrients, consume the organic matter created by plants to build their bodies and use it as energy to sustain life. Ultimately, the carbon in the organic matter that makes up our bodies originates from the carbon dioxide in the air.

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Total Carbon in Soil

Now, my interest is in the carbon content of soil. The total carbon in soil is the sum of organic carbon and inorganic carbon.

Total carbon in soil = Organic carbon in soil +  Inorganic carbon in soil 

Organic carbon is the carbon present in the soil in the form of organic matter derived from living or once-living organisms [Note: carbon-hydrogen (C-H) bonds are not a standard used to distinguish between inorganic and organic soil]. Organic matter in soil comes from decaying plant and animal material, microbial biomass, and other organic inputs. It is crucial for soil fertility and plays a significant role in soil health and nutrient cycling. On the other hand, inorganic carbon consists of carbonates and other forms of carbon that are not part of organic matter. Carbonates, such as calcium carbonate (CaCO₃), are common in many soils and result from the weathering of minerals containing carbonates or from the deposition of carbonate-rich materials in the soil. Understanding both organic and inorganic carbon is crucial for studying soil carbon dynamics, fertility, and its role in the global carbon cycle.

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Soil Organic Carbon (SOC)

The uppermost layer of soil (we usually call Topsoil) is indeed rich in organic matter, and it contains a significant amount of organic carbon. This organic matter comes from the accumulation of decaying plant and animal material, along with microbial activity, which contributes to the formation of humus; a stable, dark, organic material in soil.

That is, topsoil contains a significant amount of Soil Organic Carbon (SOC). SOC refers to the carbon content in the soil that is stored in the form of organic matter. The topsoil is where most organic matter accumulates due to the decomposition of plant and animal residues, microbial activity, and other organic inputs. SOC improves soil structure, water retention, nutrient cycling, and the overall biological activity of the soil. High levels of SOC also contribute to carbon sequestration, where carbon dioxide (CO₂) from the atmosphere is stored in the soil, helping to mitigate climate change. For sustainable land management and agricultural practices, it’s essential to preserve and enhance topsoil SOC by promoting organic matter inputs through practices such as cover cropping, reduced tillage, crop rotation, and compost application.

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What happens when the soil pH is higher (more alkaline)?

In soils with a pH greater than 7.2, inorganic carbon, specifically in the form of carbonates, can become more predominant than organic carbon. Soil pH is a measure of the soil’s acidity or alkalinity, and it affects the chemical reactions and the availability of nutrients in the soil. When soil pH is higher (more alkaline), certain chemical reactions occur that promote the formation of carbonates. Carbonates, such as calcium carbonate (CaCO₃), are inorganic compounds that result from the combination of carbon dioxide (CO₂) with calcium and other cations present in the soil. These carbonates are relatively stable and can persist in the soil for extended periods. At higher pH levels, the conditions may become less favorable for the preservation and accumulation of organic matter. Soil organisms that decompose organic material, such as bacteria and fungi, might be less active under alkaline conditions. As a result, organic matter decomposition rates may slow down, and the incorporation of organic carbon into the soil might be reduced.

It’s important to note that soil pH is just one of many factors that influence the distribution and balance between inorganic and organic carbon in soils. Other factors, such as climate, vegetation, land use, and soil management practices, also play significant roles in determining the organic and inorganic carbon content of soil.

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Reference

□ Chemistry of Carbon (Z=6)
□ Difference Between Inorganic and Organic Carbon
□ Franzluebbers, A. J. (2021). Soil organic carbon sequestration calculated from depth distribution. Soil Science Society of America Journal, 85(1), 158-171.

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