"Research Carbon Weng Says Carbon" (IV): What is carbon?

Why are there carbon atoms, carbon molecules and carbon elements, and what are the differences between them? Why are graphite and diamond with different properties and morphologies all carbon? Why are there carbon 12 and carbon 14 in carbon atoms, and what is the difference between them? What on earth is carbon?

In fact, people with junior high school level should be able to answer, but some people may have forgotten it. Let's take a look at it together.

With the development of civilization for thousands of years, human beings have continuously expanded their knowledge from both macro and micro aspects, and their understanding of the material world has been able to span 42 orders of magnitude, that is, the range of space currently observed in the macro has reached 1026-1027 (26-27 zeros after 1) meters, microscopically, the size of particles such as quarks and gluons that make up protons and neutrons is only 10-16 meters (16 zeros after the decimal point). People now know that the whole universe is composed of different substances, and there are more than tens of millions of kinds of substances, but there are not many elements that combine to form these substances. It is known that there are only more than 90 kinds in nature, and there are only 118 kinds in total. Carbon is one of these elements, but it is also a very special one.

An atom is made up of a nucleus and different numbers of electrons rotating around it with a unit negative charge. Electrons are small particles that move at high speed outside the nucleus, which is composed of positively charged protons and neutrons that are not charged but have a certain mass. These are the knowledge we have gained from junior high school textbooks. Because different atoms have different numbers of protons, neutrons and electrons, their properties vary greatly. Elements are the same kind of atoms with the same atomic number or the same nuclear charge number (I. e. proton number). Carbon element is the general name of all carbon atoms, and carbon is usually the abbreviation of carbon element.

Chemistry is the study of substances and their changes and applications. In chemistry, different English letters are used to represent different elements. When one letter is used, it is capitalized, and when two letters are used, it is capitalized and lowercase. They are called chemical symbols (or element symbols). For example, hydrogen is represented by "H", oxygen is represented by "O", iron is represented by "Fe", etc., and the chemical symbol for carbon is "C". Atoms are the smallest particles in chemical changes. From a macroscopic point of view, substances are composed of different elements or compounds, and from a microscopic point of view, they are composed of molecules, atoms or ions. Later, it was further discovered that the properties of various elements will change periodically with the increase of their atomic number (nuclear charge number), which is the result of periodic changes in the arrangement of electrons outside the nucleus of the element. In this way, more than one hundred elements can be arranged and classified to form a periodic table of chemical elements.

Figure 1. Schematic diagram of the nucleus of a carbon atom and the six electrons rotating around it

Figure 2. Several combinations of electronic clouds (blue part) in carbon atoms

The atomic number of the element carbon is 6, that is, there are 6 electrons rotating around it outside the nucleus of the carbon atom (Figure 1). Last year, Ukrainian scientists successfully captured images of the interior of carbon atoms, showing several combinations of electronic clouds (blue parts) in carbon atoms (Figure 2). Carbon element is located in the fourth main group (IVA) group in the periodic table. In the IVA group composed of C, Si, Ge, Sn and Pb elements, it has the lowest atomic number and the least nuclear charge number in this group, I .e. the least number of outer electrons outside the nucleus. We will talk about later that all the magical properties of carbon are due to their being in this special position.

Carbon atoms have a strong chemical bonding ability. Among all the elements in the periodic table, only carbon is an atom that can form more valence bonds, and only carbon is an element that can form more structures. Pure substances composed of the same element are called simple substances, and simple substances composed of different properties of the same element are called allotropes (or allotropes). With the different arrangement of carbon atoms, its composition structure is not the same. Only by carbon atoms can be composed of completely different structures and properties of dark gray graphite and brilliant diamond and other atomic crystals, but also by dozens or hundreds of thousands of carbon atoms to form carbon 60(C60) such as fullerene, carbon nanotubes or graphene molecules. The carbon 60 can further form molecular crystals. Only carbon can form an endless number of allotropes, which is unique. We will continue to talk about these in the future. Although the physical properties of various allotropes are extremely different, their chemical properties are all the same because of the same nuclear charge number and the structure of the extranuclear electron layer. For example, in the air, they can be fully burned to generate carbon dioxide like coal, while releasing a large amount of heat.

Since electrons, protons and neutrons all have a certain mass, the number of nuclear charges of different atoms will be different. The absolute weight of atoms is extremely small and difficult to weigh directly. Therefore, the weight of the lightest hydrogen atom in an atom is 1 to determine the relative atomic weight of other elements. Later, the average atomic weight of oxygen atoms existing in nature is equal to 16.0000 as a benchmark. Considering the abundance of carbon in nature and its strong ability to combine with other elements, 12C (carbon 12, its atomic mass number, I .e. carbon atom with the sum of proton number and neutron number of 12) has been uniformly set as the scale of unified atomic weight since 1961 in order to avoid confusion, and its relative atomic weight has been accurately set as 12.0000, all other atoms and molecules refer to it to determine their respective masses, thereby making the physicochemical data more precise.

It is found that atoms with the same number of protons in nature sometimes have different numbers of neutrons, so several atoms of the same element with the same number of protons but different numbers of neutrons are called isotopes. More than 300 natural isotopes have been discovered. There are more than 1200 kinds of artificial isotopes, and 13 kinds of isotopes of carbon atoms have been found, ranging from 68C (8 on the upper left represents the atomic mass number and 6 on the lower left represents the proton number) to 620C, of which 612C (carbon 12) and 613C are stable isotopes, with average relative abundances of 98.9 and 1.108 respectively. The rest of the isotopes are radioactive isotopes, of which 614C (carbon 14) has a half-life of 5730 years, and people can feel its existence. The rest of the isotopes have a very short half-life, such as 616C only 0.74 seconds, 617C only 20 milliseconds, or 0.02 seconds, so they are of little significance. Carbon 12 and carbon 14 in the atmosphere remain constant at a certain ratio, that is, only 13.56 parts per million of carbon 14 per gram of carbon. Using the half-life of carbon 14, one can estimate the age of carbon-containing materials, such as trees, bones and fossils containing biological molecules, the so-called carbon 14 monitoring method, also known as carbon dating (carbon dating). The atomic weight of the elements listed in the periodic table is the average calculated as the percentage of the atomic weight of each element that is consistent with the natural isotopic abundance, so the atomic weight of carbon is not 12.000 but 12.001.

Everything in the universe is composed of various elementary particles and nearly a hundred elements and their formed compounds. Among these elements, the most special and magical element is carbon. Carbon atoms form an infinite number of elements and compounds. There is no element like carbon, but as a single element can form many substances with completely different structures and properties. The properties of various types of carbon elements can include the properties of almost all substances on the earth, and some are even completely opposite properties. For example: the hardest (diamond)-the softest (graphite); insulator (diamond)-semiconductor (graphite)-good conductor (graphene); thermal insulator (graphite layer)-good thermal conductor (diamond); full light absorption (graphite)-full light transmission (diamond) and so on. Carbon plays an important role in the evolutionary system of the universe. It is an indispensable element for the sun and other stars to emit light. It is also a key component of various organic life on the earth. Carbon and its compounds are closely related to human beings and go deep into almost every corner of our life. It is everywhere, and people can not be separated from it all the time. The research and development of new carbon materials are changing with each passing day, It is also changing and improving our quality of life. Carbon is so important, so magical, so where does it come from? Please look at the carbon (V): carbon comes from?

This article is from Wang Maozhang Science Net blog.

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