How can one sodium ion surrounded by six chloride ions or vice versa be consistent with the simplest empirical formula NaCl? The answer is that each of those six chloride ions sits at the center of its own octahedron, whose vertices are defined by six neighboring sodium ions. This might seem to correspond to Na 6 Cl 6 , but note that the central sodium ion shown in the diagram can claim only a one-sixth share of each of its chloride ion neighbors.
Therefore, the formula NaCl is not just the simplest formula, but correctly reflects the stoichiometry of the compound. Sodium chloride, like virtually all salts, is a more energetically favored configuration of sodium and chlorine than the elements individually. Since ionic salts have a lower energetic configuration than their individual elements, reactions forming ionic solids tend to release energy.
For example, when sodium and chlorine react to form sodium chloride:. The release of kJ of energy shows that the formation of solid sodium chloride is exothermic. Due to the Second Law of Thermodynamics, the released energy spreads out into the environment and is therefore unavailable to drive the reverse reaction. This irreversibility is the main reason that sodium chloride is more stable than its component elements. This large magnitude arises from the strength of the coulombic force between ions of opposite charge.
This energy is one definition of lattice energy: the energy released when an ionic solid is formed from gaseous ions binding together. The exothermicity of such reactions results in the stability of ionic solids. Anne Marie Helmenstine, Ph. Chemistry Expert. Helmenstine holds a Ph. She has taught science courses at the high school, college, and graduate levels. Facebook Facebook Twitter Twitter. Updated March 02, Ionic Compound Properties Ionic compounds form when atoms connect to one another by ionic bonds.
An ionic bond is the strongest type of chemical bond, which leads to characteristic properties. One atom in the bond has a partial positive charge, while the other atom has a partial negative charge.
This electronegativity difference makes the bond polar, so some compounds are polar. But, polar compounds often dissolve in water. This makes ionic compounds good electrolytes. Due to the strength of the ionic bond, ionic compounds have high melting and boiling points and high enthalpies of fusion and vaporization.
Featured Video. Graphite is generally insoluble in any solvent due to the difficulty of solvating a very large molecule. Diamond and Graphite: Two Allotropes of Carbon : These two allotropes of carbon are covalent network solids which differ in the bonding geometry of the carbon atoms.
In diamond, the bonding occurs in the tetrahedral geometry, while in graphite the carbons bond with each other in the trigonal planar arrangement. This difference accounts for the drastically different appearance and properties of these two forms of carbon.
Diamond is also an allotrope of carbon. The diamond unit cell is face-centered cubic and contains eight carbon atoms. Boron nitride BN is similar to carbon because it exists as a diamond-like cubic polymorph as well as in a hexagonal form similar to graphite.
Hexagonal boron nitride : Hexagonal boron nitride, a two-dimensional material, is similar in structure to graphite. Cubic boron nitride is the second-hardest material after diamond, and it is used in industrial abrasives and cutting tools.
Cubic boron nitride : Cubic boron nitride adopts a crystal structure, which can be constructed by replacing every two carbon atoms in diamond with one boron atom and one nitrogen atom. Cubic boron nitride is the second-hardest material, after diamond. Recent interest in boron nitride has centered on its carbon-like ability to form nanotubes and related nanostructures. Silicon carbide SiC is also known as carborundum. Its structure is very much like that of diamond, with every other carbon replaced by silicon.
Silicon carbide exists in about crystalline forms. It is used mostly in its synthetic form because it is extremely rare in nature. It is found in a certain type of meteorite that is thought to originate outside of our solar system. Structurally, silicon carbide is very complex; at least 70 crystalline forms have been identified. Its extreme hardness and ease of synthesis have led to a diversity of applications — in cutting tools and abrasives, high-temperature semiconductors and other high-temperature applications, the manufacturing of specialty steels and jewelry, and many more.
Tungsten carbide WC is probably the most widely encountered covalent solid, owing to its use in carbide cutting tools and as the material used to make the rotating balls in ball-point pens. In many of its applications, it is embedded in a softer matrix of cobalt or coated with titanium compounds. Silicon Carbide : Silicon carbide is an extremely rare mineral, and in nature is is mostly found in a certain type of meteorite.
Recall that a molecule is defined as a discrete aggregate of atoms bound together sufficiently tightly by directed covalent forces to allow it to retain its individuality when the substance is dissolved, melted, or vaporized. The two words italicized in the preceding sentence are important. Covalent bonding implies that the forces acting between atoms within the molecule intra molecular are much stronger than those acting between molecules inter molecular , The directional property of covalent bonding gives each molecule a distinctive shape which affects a number of its properties.
Liquids and solids composed of molecules are held together by van der Waals or intermolecular forces, and many of their properties reflect this weak binding. Molecular solids tend to be soft or deformable, have low melting points, and are often sufficiently volatile to evaporate directly into the gas phase.
This latter property often gives such solids a distinctive odor. Thus, many corresponding substances are either liquid water or gaseous oxygen at room temperature. Molecular solids also have relatively low density and hardness.
The elements involved are light, and the intermolecular bonds are relatively long and are therefore weak. Because of the charge neutrality of the constituent molecules, and because of the long distance between them, molecular solids are electrical insulators. Because dispersion forces and the other van der Waals forces increase with the number of atoms, large molecules are generally less volatile, and have higher melting points than smaller ones.
Also, as one moves down a column in the periodic table, the outer electrons are more loosely bound to the nucleus, increasing the polarisability of the atom, and thus its propensity to van der Waals-type interactions. This effect is particularly apparent in the increase in boiling points of the successively heavier noble gas elements. Interactive: Charged and Neutral Atoms : There are two kinds of attractive forces shown in this model: Coulomb forces the attraction between ions and Van der Waals forces an additional attractive force between all atoms.
What kinds of patterns tend to form with charged and neutral atoms? How does changing the Van der Waals attraction or charging the atoms affect the melting and boiling point of the substance? When white phosphorus is converted to the covalent red phosphorus, the density increases to 2. Both red and black phosphorus forms are significantly harder than white phosphorus. Although white phosphorus is an insulator, the black allotrope, which consists of layers extending over the whole crystal, does conduct electricity.
Similarly, yellow arsenic is a molecular solid composed of As 4 units; it is metastable and gradually transforms into gray arsenic upon heating or illumination. Certain forms of sulfur and selenium are each composed of S 8 or Se 8 units, and are molecular solids at ambient conditions.
However, they can convert into covalent allotropes having atomic chains extending all through the crystal. The vast majority of molecular solids can be attributed to organic compounds containing carbon and hydrogen, such as hydrocarbons C n H m. Spherical molecules consisting of different number of carbon atoms, called fullerenes, are another important class.
Less numerous, yet distinctive molecular solids are halogens e. Its solid form is an insulator because all valence electrons of carbon atoms are involved into the covalent bonds within the individual carbon molecules. The lattice is formed because the ions attract each other and form a regular pattern with oppositely charged ions next to each other. Remember that the lattice arrangement is giant - for example, a single grain of salt may contain 1.
The lattice arrangement continues in three dimensions. This is why solid ionic compounds form crystals with regular shapes.
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