Carbides - carbon

Carbides are a class of compounds composed of carbon and an electro positive atom

Titanium Carbide

Lattice structure of titanium carbide

KEY POINTS
Carbides are generally formed at high temperatures (> 1500 °C).
Carbides are generally quite stable and exhibit high melting points.
Carbides can be classified as salt-like, interstitial, and covalent..

In chemistry, carbides are compounds composed of carbon and less-electronegative elements (Figure 1). They are usually distinguished by their chemical bonding. They are generally prepared from metals or metal oxides at high temperatures (1500 °C or higher) by combining the metal with carbon. Carbides are used in key industrial applications, and their naming is not systematic.

Saline Carbides

Salt-like (saline) carbides are composed of the highly electropositive atoms, such as the alkali, alkali earth, and group-III metals, mixed with carbon. Aluminumforms carbides, but other elements from group XIII do not. These materials have isolated carbon centers, often described as "C4-" in the metanides, "C22-" in the acetylides, and "C34-" in the sesquicarbides. Methanides are carbides that decompose in water and generate water; aluminum carbide (Al4C3) and beryllium carbide (Be2C) are examples of this class of carbides. Acetylides are formed from alkali, alkali earth, and lanthanoid metals with the acetylide anion C22-. Lanthanoids also form carbides with the formula M2C3. Metals from group XI also form acetylides, such as copper(I) acetylide and silver acetylide. Carbides of theactinide elements, which have the structure MC2 and M2C3, are also described as salt-like derivatives of C22-. The polyatomic ion C34- is referred to as an allylenide or sesquicarbide and is found in Li4C3 and Mg2C3. The allylenide is linear andisoelectronic with CO2.

Covalent Carbides

Covalent carbides are found in carbides of silicon and boron, although all compounds of carbon exhibit some covalent characteristics. The reason these two elements form "covalent" carbides is due to their similar electronegativity and size to those of carbon. Because of this, their association is completely covalent in character. Silicon carbide has two similar crystalline forms, which are both related to the diamond structure. Boron carbide (B4C), on the other hand, has an unusual structure that includes icosahedral boron units linked by carbon atoms. In this respect boron carbide is similar to the boron-rich borides. Both silicon carbide (also known as carborundum) and boron carbide are very hard and refractory materials. Both materials have important industrial applications.

Interstitial Carbides

Interstitial carbides describe the carbides of the group-IV, -V, and VI transition metals. These carbides are metallic and refractory. They are formed so that the carbon atoms fit into octahedral interstices in a close-packed metal lattice when the metal atom's radius is greater than ~135 pm. When the metal atoms are cubic-close-packed (ccp), then filling all of the octahedral interstices with carbon achieves 1:1 stoichiometry with the rock-salt structure. When the metal atoms are hexagonal-close-packed, (hcp), since the octahedral interstices lie directly opposite each other on either side of the layer of metal atoms, filling only one of these with carbon achieves 2:1 stoichiometry. As a result of the packing, they are quite stable and have very high melting points and low electrical resistance.

Intermediate Transition Metal Carbides

In intermediate transition metal carbides, the transition-metal ion is smaller than the critical 135 pm, and the structures are not interstitial but are more complex. Multiple stoichiometries are common. For example, iron forms a number of carbides: Fe3C, Fe7C3 and Fe2C. The best known is cementite, Fe3C, which is present in steels. These carbides are more reactive than the interstitial carbides; for example, the carbides of Cr, Mn, Fe, Co, and Ni are all hydrolyzed by dilute acids and sometimes by water to yield a mixture of hydrogen and hydrocarbons. These compounds share features with both the inert interstitials and the more reactive salt-like carbides.

Metal complexes containing Cn fragments are well known. These molecular carbides often have carbon-centered clusters. Some metals such as tin and lead are not believed to form carbides.

Boron:

 

Boron is a chemical element with chemical symbol B and atomic number 5.

APPEARS IN THIS RELATED CONCEPT:

• Elemental Boron

actinide:

 

any of the 14 radioactive elements of the periodic table that are positioned under the lanthanides, with which they share similar chemistry

APPEARS IN THESE RELATED CONCEPTS:

• Lanthanides and Actinides
• The Bottom of the Periodic Table

aluminum:

 

A metallic chemical element (symbol Al) with an atomic number of 13.

APPEARS IN THIS RELATED CONCEPT:

• Aluminum

anion:

 

A negatively charged ion, as opposed to a cation

APPEARS IN THESE RELATED CONCEPTS:

• Predicting Precipitation Reactions
• Ionic Radius
• Nitrogen Compounds
• Naming Acids and Bases
• Atomic Number and Mass Number
• Electron Configuration of Cations and Anions
• Ionic Bonding and Electron Transfer
• Formulas of Ionic Compounds

carbon:

 

The chemical element (symbol C) with an atomic number of 6.

APPEARS IN THIS RELATED CONCEPT:

• Carbon is Unique

covalent:

 

Characterized by shared pairs of electrons between atoms.

APPEARS IN THESE RELATED CONCEPTS:

• Polyatomic Molecules
• Ionic Bonding Models versus Reality

critical:

 

Of the point (in temperature, reagent concentration etc.) where a nuclear or chemical reaction becomes self-sustaining.

APPEARS IN THIS RELATED CONCEPT:

• Nuclear Reactors

diamond:

 

a glimmering glass-like mineral that is an allotrope of carbon in which each atom is surrounded by four others in the form of a tetrahedron

APPEARS IN THIS RELATED CONCEPT:

• Covalent Crystals

electronegativity:

 

the tendency of an atom or molecule to attract electrons and thus form bonds

APPEARS IN THESE RELATED CONCEPTS:

• Properties of the Halogens
• Bond Polarity
• Covalent Bonds
• Carbides
• Comparison between Covalent and Ionic Compounds
• General Trends in Chemical Properties
• Electron Affinity
• Hydrogen Bonding

group:

 

A vertical column in the periodic table, which signifies the number of valence shell electrons in an element's atom.

APPEARS IN THIS RELATED CONCEPT:

• The Periodic Table

hydrogen:

 

The lightest chemical element (symbol H) with an atomic number of 1 and atomic weight of 1.00794.

APPEARS IN THIS RELATED CONCEPT:

• Carbon is Unique

inert:

 

A substance that does not react chemically.

APPEARS IN THIS RELATED CONCEPT:

• The Noble Gases (Group 18)

ion:

 

An atom or group of atoms bearing an electrical charge, such as the sodium and chlorine atoms in a salt solution.

APPEARS IN THESE RELATED CONCEPTS:

• Solutions and Heats of Hydration
• Ionic Radius
• Ion-Dipole Force
• Ions

isoelectronic:

 

(of two compounds) having the same electronic configuration although consisting of different elements

APPEARS IN THIS RELATED CONCEPT:

• Boron-Oxygen Compounds

lattice:

 

a regular spacing or arrangement of geometric points

APPEARS IN THESE RELATED CONCEPTS:

• The Kinetic Molecular Theory of Liquids and Solids
• Crystal Structure: Closest Packing

metal:

 

Any of a number of chemical elements in the periodic table that form a metallic bond with other metal atoms; generally shiny, somewhat malleable and hard, often a conductor of heat and electricity

APPEARS IN THESE RELATED CONCEPTS:

• General Properties of Metals
• Conductors
• Metallic Crystals

mixture:

 

Something that consists of diverse elements

APPEARS IN THIS RELATED CONCEPT:

• Substances and Mixtures

polyatomic:

 

Of a molecule or ion consisting of three or more atoms.

APPEARS IN THESE RELATED CONCEPTS:

• Polyatomic Molecules
• Naming Coordination Compounds

polyatomic ion:

 

a charged species (ion) composed of two or more atoms covalently bonded or of a metal complex that can be considered as acting as a single unit in the context of acid and base chemistry or in the formation of salts. Also known as a molecular ion

APPEARS IN THESE RELATED CONCEPTS:

• Naming Acids and Bases
• Lewis Structures for Polyatomic Ions

silver:

 

A lustrous, white, metallic element, atomic number 47, atomic weight 107.87, symbol Ag.

APPEARS IN THIS RELATED CONCEPT:

• Silver

stoichiometry:

 

the study and calculation of quantitative (measurable) relationships of the reactants and products in chemical reactions (chemical equations)

APPEARS IN THESE RELATED CONCEPTS:

• Weak Acid-Strong Base Titrations
• Mass-to-Mass Conversions
• Amount of Reactants and Products
• Gas Stoichiometry
• Solution Stoichiometry
• Molar Mass of Gas