Carbon is known as the fundamental building block of life. It possesses the ability to bond with nearly every element, and it can bond with itself in a wide variety of ways. It is the fourth most abundant element by mass in the universe, the tenth most abundant in Earth's crust, and the second most abundant in humans (behind hydrogen). In its pure form, carbon is a fairly resistive conductor, though it is a major component of many insulators. Carbon is the sole component of a diamond, as well as graphite. The unstable isotope carbon-14 can be used diagnose and treat a variety of diseases, the most prevalent of which is cancer. We can use molecules containing carbon to both create and destroy nearly anything we can imagine.
With recent focus on climate change, much of the attention of the public has turned to carbon. We're concerned with how much of it our cars and factories and livestock are releasing into the atmosphere, but there is little focus on exactly how important this element is to us. It's easy to oversimplify things and make carbon the enemy, but greenhouse gases are just a few of the many compounds that carbon makes up. Without it, life as we know it couldn't exist, nor could many of our fuels and other necessities.
The key to carbon's ubiquity in biological lifeforms and molecules is its ability to bond with other atoms in a wide variety of ways. Carbon is tetravalent, which means it is capable of creating four covalent bonds with other atoms. Carbon's atomic number is 6, which means it has six protons in its nucleus. This means that, under normal circumstances, its outer electron shell has four electrons and four empty spaces. This makes carbon the ideal candidate for bonds of varying strength with a variety of other elements. The bonds that carbon creates with itself and other atoms make it the backbone of all sugars,fatty acids, and amino acids, which are the most fundamental compounds in biology. No other element can form bonds in the same way that carbon can.
Carbon's capability for creating bonds also makes it a major component of most of our fuels. Heptane and octane, the main components of gasoline, are based on rings of seven and eight carbon atoms respectively. Alcohols, such as ethanol, are all built on at least one carbon atom, combined with hydrogen atoms and a hydroxyl group, which is just a single oxygen and a hydrogen. Methane (natural gas) is a molecule consisting of a single carbon atom and four hydrogen atoms. All of these fuels work by breaking the strong covalent bonds and releasing the energy that they store.
Carbon's tetravalent property also means that it can be combined with itself in a variety of ways. The resulting variable molecular geometry means that a diamond can form from exactly the same material as graphite, through a different process. Combined in other ways, it can form carbon nanotubes - the strongest molecular structures modern science has created, fullerines - shells of carbon whose properties include superconductivity and high strength, and lonsdaleite - sometimes called hexagonal diamond, a unique diamond structure that has been found to form when graphite in a meteor meets the extreme heat of entering Earth's atmosphere.
The isotope carbon-14 is radioactive and occurs in nature. It decays to nitrogen-14 with a half-life of 5,000 years, and carbon's abundance means that it can be used to fairly accurately date nearly every biological artifact from the last 60,000 years or so. This makes it instrumental in archaeology for cataloging historic and late-prehistoric human history. Carbon-14's radioactivity and subsequent beta decay also make it useful in medicine for locating malignant tumors. Since cancerous cells consume more resources than other cells, injecting a small amount of carbon-14 into the patient's blood stream will make cancer stand out to a Geiger counter.
Carbon is a fundamental part of every biological process, every plastic, and everything that was ever part of a living thing. It's nearly impossible to imagine a world without carbon. We certainly wouldn't exist, and neither would most of the things that we use in our daily lives. Carbon truly is the fundamental building block of life.