The allotropes of carbon include graphite, one of the softest known substances, and diamond, the hardest naturally occurring substance. It bonds readily with other small atoms, including other carbon atoms, and is capable of forming multiple stable covalent bonds with suitable multivalent atoms. Carbon is known to form almost ten million compounds, a large majority of all chemical compounds. Carbon also has the highest sublimation point of all elements. At atmospheric pressure it has no melting point, as its triple point is at 10.8±0.2 MPa and 4,600 ± 300 K (4,330 ± 300 °C; 7,820 ± 540 °F), so it sublimes at about 3,900 K (3,630 °C; 6,560 °F). Graphite is much more reactive than diamond at standard conditions, despite being more thermodynamically stable, as its delocalised pi system is much more vulnerable to attack. For example, graphite can be oxidised by hot concentrated nitric acid at standard conditions to mellitic acid, C₆(CO₂H)₆, which preserves the hexagonal units of graphite while breaking up the larger structure.

Carbon sublimes in a carbon arc, which has a temperature of about 5800 K (5,530 °C or 9,980 °F). Thus, irrespective of its allotropic form, carbon remains solid at higher temperatures than the highest-melting-point metals such as tungsten or rhenium. Although thermodynamically prone to oxidation, carbon resists oxidation more effectively than elements such as iron and copper, which are weaker reducing agents at room temperature.

Carbon is the sixth element, with a ground-state electron configuration of 1s²2s²2p², of which the four outer electrons are valence electrons. Its first four ionisation energies, 1086.5, 2352.6, 4620.5 and 6222.7 kJ/mol, are much higher than those of the heavier group-14 elements. The electronegativity of carbon is 2.5, significantly higher than the heavier group-14 elements (1.8–1.9), but close to most of the nearby nonmetals, as well as some of the second- and third-row transition metals. Carbon's covalent radii are normally taken as 77.2 pm (C−C), 66.7 pm (C=C) and 60.3 pm (C≡C), although these may vary depending on coordination number and what the carbon is bonded to. In general, covalent radius decreases with lower coordination number and higher bond order.

Carbon compounds form the basis of all known life on Earth, and the carbon–nitrogen cycle provides some of the energy produced by the Sun and other stars. Although it forms an extraordinary variety of compounds, most forms of carbon are comparatively unreactive under normal conditions. At standard temperature and pressure, it resists all but the strongest oxidizers. It does not react with sulfuric acid, hydrochloric acid, chlorine or any alkalis. At elevated temperatures, carbon reacts with oxygen to form carbon oxides and will rob oxygen from metal oxides to leave the elemental metal. This exothermic reaction is used in the iron and steel industry to smelt iron and to control the carbon content of steel:

Fe₃O₄ + 4 C_(s) → 3 Fe_(s) + 4 CO_(g)

Carbon monoxide can be recycled to smelt even more iron:

Fe₃O₄ + 4 CO_(g) → 3 Fe_(s) + 4 CO_2(g)

with sulfur to form carbon disulfide and with steam in the coal-gas reaction:

C_(s) + H₂O_(g) → CO_(g) + H_2(g).

Carbon combines with some metals at high temperatures to form metallic carbides, such as the iron carbide cementite in steel and tungsten carbide, widely used as an abrasive and for making hard tips for cutting tools.

Carbon can form very long chains of interconnecting carbon–carbon bonds, a property that is called catenation. Carbon-carbon bonds are strong and stable. Through catenation, carbon forms a countless number of compounds. A tally of unique compounds shows that more contain carbon than do not. A similar claim can be made for hydrogen because most organic compounds contain hydrogen chemically bonded to carbon or another common element like oxygen or nitrogen.

The simplest form of an organic molecule is the hydrocarbon — a large family of organic molecules that are composed of hydrogen atoms bonded to a chain of carbon atoms. A hydrocarbon backbone can be substituted by other atoms, known as heteroatoms. Common heteroatoms that appear in organic compounds include oxygen, nitrogen, sulfur, phosphorus, and the nonradioactive halogens, as well as the metals lithium and magnesium. Organic compounds containing bonds to metal are known as organometallic compounds (see below). Certain groupings of atoms, often including heteroatoms, recur in large numbers of organic compounds. These collections, known as functional groups, confer common reactivity patterns and allow for the systematic study and categorization of organic compounds. Chain length, shape and functional groups all affect the properties of organic molecules.

In most stable compounds of carbon (and nearly all stable organic compounds), carbon obeys the octet rule and is tetravalent, meaning that a carbon atom forms a total of four covalent bonds (which may include double and triple bonds). Exceptions include a small number of stabilized carbocations (three bonds, positive charge), radicals (three bonds, neutral), carbanions (three bonds, negative charge) and carbenes (two bonds, neutral), although these species are much more likely to be encountered as unstable, reactive intermediates.

Carbon occurs in all known organic life and is the basis of organic chemistry. When united with hydrogen, it forms various hydrocarbons that are important to industry as refrigerants, lubricants, solvents, as chemical feedstock for the manufacture of plastics and petrochemicals, and as fossil fuels.

When combined with oxygen and hydrogen, carbon can form many groups of important biological compounds including sugars, lignans, chitins, alcohols, fats, and aromatic esters, carotenoids and terpenes. With nitrogen it forms alkaloids, and with the addition of sulfur also it forms antibiotics, amino acids, and rubber products. With the addition of phosphorus to these other elements, it forms DNA and RNA, the chemical-code carriers of life, and adenosine triphosphate (ATP), the most important energy-transfer molecule in all living cells.

In the periodic table, the elements with eight electrons outermost form the group known as the noble gases (Group 18 [0]), the least reactive of the elements. The carbon group elements (Group 14), with four electrons, occupy a middle position. Elements to the left of Group 14 have fewer than four electrons in the valence shell and tend to lose them (with their negative charges) to become positively charged ions, represented by the symbol for the element with a superscript indicating the number and sign of the charges; such elements are called metals. The nonmetals (except boron) are in the groups to the right of Group 14; each has more than four electrons in its outermost shell and tends to acquire electrons to complete its octet, forming negatively charged ions.

Chemical reactions result from the exchange of electrons among atoms. In general, if a metal loses its few valence electrons to a nonmetal, the resulting oppositely charged ions are attracted to one another and form a bond, classified as ionic or electrovalent. Two nonmetals, neither of which can actually lose its valence electrons in chemical reaction, may nevertheless share them in pairs in such a way that what is called a covalent bond results. Metal atoms will bond to one another in a third type of bond, which releases their valence electrons in a way that allows them to conduct electricity.

All the carbon group atoms, having four valence electrons, form covalent bonds with nonmetal atoms; carbon and silicon cannot lose or gain electrons to form free ions, whereas germanium, tin, and lead do form metallic ions but only with two positive charges. Even lead, the most metallic of the carbon group atoms, cannot actually lose all four of its valence electrons, because, as each one is removed, the remainder are held more strongly by the increased positive charge. Because the distinction between covalent and ionic (electrovalent) bonds is often a matter of convenience for the chemist, and because the actual bond structure within a molecule may be quite complicated, it is often useful instead simply to count the total number of electrons an element gains or loses in bonding without regard to the nature of the bonds. This number is called the oxidation number, or oxidation state, of the element; many elements have more than one oxidation state possible, each oxidation state being found in different compounds. The oxidation state of an element is conventionally written as a Roman numeral following the name of the element in a compound—for example, lead(II) means lead in the +2 oxidation state. An alternative system of representation uses an Arabic number after the element name; thus, lead in the +2 state is written lead(+2). With the chemical symbol of the element, the oxidation state may be written as a superscript, as in Pb2+. When the compounds are ionic, the oxidation state is also the actual ionic charge. Covalent bonds generally are considered to be formed by interaction of the orbitals (in most cases, only the s, p, and d orbitals) in specific and varied ways. The most common are called sigma and pi bonds, written σ and π, respectively. The sigma bonds are symmetrical with respect to the axis of the bond, whereas the pi bonds are not. Examples of sigma and pi bonding as well as of ionic bonding can be found among the compounds of the elements of the carbon group.

The properties of the carbon group elements and those of their compounds are intermediate between properties associated with the elements of the adjacent boron and nitrogen groups. In all groups the metallic properties, resulting from the tendency to hold valence electrons more loosely, increase with atomic number. Within the carbon group, more than in any other, the change from nonmetallic to metallic character with increasing atomic number is particularly apparent. Carbon is a true nonmetal in every sense. Lead is a true metal. Silicon is almost completely nonmetallic; tin is almost completely metallic. Germanium is metallic in appearance and in a number of its other physical properties (see Table), but the properties of many of its compounds are those of derivatives of nonmetals. These changes are consequences of increase in atomic size with substantial screening of the larger nuclear charge by intervening electronic shells, as evidenced by decrease in ionization energy (energy required to remove an electron) and electronegativity power to attract electrons with increasing atomic number.

In the solid state, elemental carbon, silicon, germanium, and gray tin (defined as alpha [α] tin) exist as cubic crystals, based upon a three-dimensional arrangement of bonds. Each atom is covalently bonded to four neighbouring atoms in such a way that they form the corners of a tetrahedron (a solid consisting of four three-sided faces). A practical result is that no discrete small molecules of these elements, such as those formed by nitrogen, phosphorus, or arsenic, can be distinguished; instead, any solid particle or fragment of one of these elements, irrespective of size, is uniformly bonded throughout, and, therefore, the whole fragment can be considered as a giant molecule. Decreasing melting points, boiling points, and decreasing heat energies associated with fusion (melting), sublimation (change from solid to gas), and vaporization (change from liquid to gas) among these four elements, with increasing atomic number and atomic size, indicate a parallel weakening of the covalent bonds in this type of structure. The actual or probable arrangement of valence electrons is often impossible to determine, and, instead, relative energy states of the electrons, in the ground, or least energetic, state of the atom are considered. Thus, the same trend of nonmetallic toward metallic states is indicated by decreasing hardness and decreasing single-bond energy between atoms. Carbon crystallizes in two forms, as diamond and as graphite; diamond stands apart from all other elemental forms in the extreme stability of its crystal structure, whereas graphite has a layer structure. As may be expected, cleavage between layers of graphite is much easier to effect than rupture within a layer. The crystal structures of white beta (β) tin and elemental lead are clearly metallic structures. In a metal, the valence electrons are free to move from atom to atom, and they give the metal its electrical conductivity.

The ground-state electronic configurations of atoms of these carbon group elements show that each has four electrons in its outermost shells. As has been explained, if n represents the outermost shell (n being two for carbon, three for silicon, etc.), then these four electrons are represented by the symbols ns²np². Such a configuration suggests the importance of referring to the relatively stable noble-gas-atom configuration preceding each element in determining the properties of the element, in particular its chemical properties. The loss of four electrons by either a carbon atom or a silicon atom to give ions having a positive charge of four (or +4, written C⁴⁺ or Si⁴⁺) with the electron configurations of the preceding noble-gas atoms is precluded by the sizable ionization energies. Ions of +4 charge do not exist, nor is there any evidence that carbon or silicon ions of charge +2 can form by the loss of only two unpaired (np, or outermost) electrons. Electron loss by atoms of the heavier elements of the family is easier, but it cannot lead to ions with noble-gas-atom configurations because of the presence of underlying (i.e., d¹⁰) arrangements of electrons inside the outermost shell. It is again unlikely that the +4 ions of germanium, tin, and lead (in symbols Ge⁴⁺, Sn⁴⁺, and Pb⁴⁺) exist in known compounds, but it is true that the inertness of the ns² pair of electrons (which are, in terms of energy states, closer to the nucleus than the np² pair) increases substantially with increasing atomic number in the family and thus allows the np² electrons to be removed separately, to form at least the ions, Sn²⁺ and Pb²⁺. Oxidation states of +2 and +4 can be assigned in covalent compounds of each of these elements with elements that are more electronegative (i.e., having greater affinity for electrons).

Carbon is unique among the elements in the almost infinite capacity of its atoms to bond to each other in long chains, a process called catenation (Latin catena, chain). This characteristic reflects the strength of the bond between adjacent carbon atoms in the molecule, both in relationship to similar bonds involving other elements of the carbon family and in relationship to bonds between carbon atoms and atoms of many other elements. Only the carbon–hydrogen, carbon–fluorine, and carbon–oxygen single bonds (C―H, C―F, and C―O) are stronger than the carbon–carbon single bond (C―C), and each of these is weaker than the carbon–carbon multiple bonds (C=C or C≡C). On the other hand, the silicon–silicon single bond (Si―Si) is weaker than other single bonds involving an atom of other elements with the silicon atom. The same is undoubtedly true of the germanium–germanium and tin–tin single bonds (Ge―Ge, Sn―Sn) in relationship to single covalent bonds between atoms of these elements and atoms of other elements. Experimentally, there appears to be no practical upper limit to catenation involving carbon. This phenomenon in three dimensions produces the diamond and in two dimensions the layers in graphite. Catenation is also exhibited to a high degree by elemental silicon, germanium, and tin, but it is strictly limited in compounds of these elements; silicon may have up to 14 atoms in a chain; germanium, 9; and tin, 2 or 3 only, largely in hydrides (compounds containing hydrogen). Double and triple bonds in catenated arrangements are limited to carbon.

Catenation, via single or multiple bonds or both, combined with several other factors allows carbon to form more compounds than any other element. These factors are: (1) the stability of certain carbon bonds, in particular of the C―H bond; (2) the existence of carbon in both sp² and sp³ hybridizations; (3) the ability of carbon to form both chain and cyclic compounds (in which the chain of atoms is joined end to end to form a ring) based upon either carbon atoms alone or carbon atoms in combination with those of other nonmetals (e.g., oxygen, sulfur, nitrogen) and either upon single- or multiple-bond arrangements; and (4) the capability of many carbon compounds to exist in isomeric forms (isomers are molecules with identical numbers of the same atoms bonded in different arrangements; such molecules have quite different properties). All but a very few carbon compounds are called organic compounds, and they are discussed in the article chemical compound.

Reference has been made to some of the physical properties of the carbon group elements. Most of the variations in properties from carbon through lead parallel increase in atomic size and are comparable with those of elements in the boron, nitrogen, oxygen, and fluorine groups. The general trends are roughly those found for the adjacent boron group and nitrogen group elements. The significantly higher melting and boiling points of the carbon group elements reflect their tendency to exist as giant molecules, as opposed to the tendencies of elements in the adjacent families to exist as smaller, discrete molecules.

As is true of the lightest element in each group of elements, the physical properties of carbon differ substantially from those of the other members of its family. To a large degree, these differences reflect the substantially higher concentration of the positive charge on the carbon nucleus relative to the size of the carbon atom. That is, the nucleus of carbon holds only six electrons in two shells and, therefore, holds them close; the nucleus of lead, on the other hand, has 82 electrons distributed in six shells. The attraction between the nucleus of lead and its outermost electrons is less than in carbon, because intervening shells in lead shield the outer electrons. Structural differences between diamond and graphite produce profound differences between them in hardness, conductivity, density, heat capacity, and other properties. Inasmuch as graphite is a unique crystalline formation among the elements, its properties should not be compared directly with those of the other elements in the family.

With a given reagent, diamond is generally less reactive than graphite and, thus, requires more rigorous conditions for reaction, such as a higher temperature; the ultimate products, however, are the same. Crystalline silicon is less reactive than finely divided and, possibly, amorphous silicon. Elemental germanium resembles silicon quite closely. Tin and lead behave in general as metals and thus yield at least some ionic products in reactions that are quite different from those of the other elements. Elemental carbon is of particular importance as a high-temperature reducing agent (a reagent that donates electrons) in metallurgical processing for metal oxides, a reaction that frees the metal. For example, tin can be obtained from its ore cassiterite by reduction with carbon in the form of charcoal. Thus to cite only a few of carbon’s more important applications, carbon is used directly in the production of elemental phosphorus, arsenic, bismuth, tin, lead, zinc, and cadmium, and indirectly, as carbon monoxide, in the production of iron. Elemental silicon, in the iron–silicon alloy ferrosilicon, is also a strong reducing agent and has been used as such to liberate magnesium from its oxide.