Covalent molecular solids

Many physical properties of covalent molecular solids are due to intermolecular forces. The melting and boiling points of molecular substances are relatively low compared with those of ionic substances. That s why salt doesn t melt when you heat it but sugar does. Many molecular substances exist as gases or vaporize readily at room temperature. Oxygen (O2), carbon dioxide (CO2), and hydrogen sulfide (H2S) are examples of covalent gases. Hardness is also due to the intermolecular forces between individual molecules, so covalent molecules form relatively soft solids. Paraffin is a common example of a covalent solid. 

What is the difference between a covalent molecular solid and a covalent network soUd Do their physical properties differ Explain your answer. (9.5)... 

Ceramics—ionic/mixed ionic-covalent Molecular solids—van der Waals Semi-metals—mixed covalent-metallic Intermetallics—mixed metallic-ionic... 

PCI5 is even closer to the ionic-covalent borderline than is PF5, the ionic solid [PCl4]" [PCl6] melting (or subliming) to give a covalent molecular... 

The Lewis structure of the product, a white molecular solid, is shown in (32). In this reaction, the lone pair on the nitrogen atom of ammonia, H3N , can be regarded as completing boron s octet in BF3 by forming a coordinate covalent bond. 

The molecules (or atoms, for noble gases) of a molecular solid are held In place by the types of forces already discussed In this chapter dispersion forces, dipolar interactions, and/or hydrogen bonds. The atoms of a metallic solid are held in place by the delocalized bonding described in Section 10-. A network solid contains an array of covalent bonds linking every atom to its neighbors. An ionic solid contains cations and anions, attracted to one another by electrical forces as described in Section 8-. 

In sharp contrast to molecular solids, network solids have very high melting points. Compare the behavior of phosphorus and silicon, third-row neighbors in the periodic table. As listed in Table 11-2. phosphorus melts at 317 K, but silicon melts at 1683 K. Phosphorus is a molecular solid that contains individual P4 molecules, but silicon is a network solid in which covalent bonds among Si atoms connect all the atoms. The vast array of covalent bonds In a network solid makes the entire stmcture behave as one giant molecule. ... 

P4 Ofi The relatively low melting point of 25 °C indicates a molecular solid. The molecular structure shows that P4 Og is a discrete molecule. Strong covalent bonding holds the atoms in each molecule together, but each molecule is attracted to the others only by dispersion forces. In this molecular solid, little energy is required to overcome dispersion forces and allow P4 Og solid to melt. 

IC1 is a covalently bonded, molecular solid NaCl is an ionic solid. 

I2 is a molecular solid, held together by covalent bonds. 

There are several methods you can use to predict the type of bond in an unknown substance. For example, you can consider the substance s physical properties. In contrast to ionic solids, covalent (molecular) compounds typically have the following properties ... 

Condensed matter can be classified by the nature of the forces that hold it together ionic solids covalent solids metallic solids molecular solids. 

Molecular Orbital Theory a model that uses wave functions to describe the position of electrons in a molecule, assuming electrons are delocalized within the molecule Molecular Solid a solid that contains molecules at the lattice points Molecule a group of atoms that exist as a unit and are held together by covalent bonds... 

The concepts which we need for understanding the structural trends within covalently bonded solids are most easily introduced by first considering the much simpler system of diatomic molecules. They are well described within the molecular orbital (MO) framework that is based on the overlapping of atomic wave functions. This picture, therefore, makes direct contact with the properties of the individual free atoms which we discussed in the previous chapter, in particular the atomic energy levels and angular character of the valence orbitals. We will see that ubiquitous quantum mechanical concepts such as the covalent bond, overlap repulsion, hybrid orbitals, and the relative degree of covalency versus ionicity all arise naturally from solutions of the one-electron Schrodinger equation for diatomic molecules such as H2, N2, and LiH. 

Solids can be characterized as amorphous if their particles are randomly arranged or crystalline if their particles are ordered. Crystalline solids can be further characterized as ionic solids if their particles are ions, molecular solids if their particles are molecules, covalent network solids if they consist of a covalently bonded array of atoms without discrete molecules, or metallic solids if their particles are metal atoms. 

In earlier chapters, we saw examples of how the metallic or nonmetallic character of an element affects its chemistry. Metals tend to form ionic compounds with nonmetals, whereas nonmetals tend to form covalent, molecular compounds with one another. Thus, binary metallic hydrides, such as NaH and CaH2, are ionic solids with high melting points, and binary nonmetallic hydrides, such as CH4, NH3, H20, and HF, are covalent, molecular compounds that exist at room temperature as gases or volatile liquids (Section 14.5). 

Oxides exhibit similar trends. In the third row, for example, Na20 and MgO are typical high-melting, ionic solids, and P4O10, S03, and C1207 are volatile, covalent, molecular compounds (Section 14.9). The metallic or nonmetallic character of an oxide also affects its acid-base properties. Na20 and MgO are basic, for example,... 

Caro E et al (2002) Non-covalent and semi-covalent molecularly imprinted polymers for selective on-line solid-phase extraction of 4-nitrophenol from water samples. J Chromatogr A 963(1-2) 169-178... 

---