Molecules and sharing electrons

Chemisorption occurs when the attractive potential well is large so that upon adsorption a strong chemical bond to a surface is fonued. Chemisorption involves changes to both the molecule and surface electronic states. For example, when oxygen adsorbs onto a metal surface, a partially ionic bond is created as charge transfers from the substrate to the oxygen atom. Other chemisorbed species interact in a more covalent maimer by sharing electrons, but this still involves perturbations to the electronic system. 

The protons come from the water molecules that hydrate these metal cations in solution (Fig. 10.19). The water molecules act as Lewis bases and share electrons with the metal cations. This partial loss of electrons weakens the O -H bonds and allows one or more hydrogen ions to be lost from the water molecules. Small, highly charged cations exert the greatest pull on the electrons and so form the most acidic solutions. 

In the HC1 molecule, the shared electrons are attracted more by the chlorine atom (electronegativity value 3.0) than by the hydrogen atom (electronegativity value 2.1). But the total transfer of electrons from hydrogen to chlorine does not happen because the electronegativity difference between hydrogen and chlorine is only 0.9, less than the 1.9 needed to form an ionic bond. 

A covalently bonded molecule. By sharing electrons, one carbon and four hydrogen atoms complete their outermost shells to form methane. In carbon, the outermost shell contains four electrons whereas in hydrogen the outermost shell contains only one electron. 

In water, four valence electrons form two lone pair orbitals that have been determined (Pople, 1951) to point above and below the plane formed by the three nuclei (H—O—H) of the molecule. The shared electrons with the protons give the molecule two positive charges, and the lone pair electrons give the molecule two negative charges. The result is a molecule with four charges and a permanent electric dipole (McCelland, 1963) of 1.84 Debye. 

A covalent bond occurs when two atoms, both in need of electrons to become stable, share electrons that are usually from their outermost energy shells. Instead of one atom giving an electron to another atom, the atoms overlap and share electrons still bound to its nucleus. When a solid formed with covalent bonds melts or freezes, the strength of the covalent bonds that form the molecules are overcome by the strength of the intermolecular forces. 

Two types of electrons are responsible for the absorption of ultraviolet and visible radiation by organic molecules (1) shared electrons that participate directly in bond formation and are thus associated with more than one atom and (2) unshared outer electrons that are largely localized about such atoms as oxygen, the halogens, sulfur, and nitrogen (Skoog et al., 1992). 

Recall that atoms can form stable units called molecules by sharing electrons. This is called intramolecular (within the molecule) bonding. In this chapter we will consider the properties of the condensed states of matter (liquids and solids) and the forces that cause the aggregation of the components of a substance to form a liquid or a solid. These forces may involve covalent or ionic bonding, or they may involve weaker interactions usually called intermolecular forces (because they occur between, rather than within, molecules). 

The electronic structure of molecules of covalent compounds involving the principal groups of the periodic table can usually be written by counting up the number of valence electrons in the molecule and then distributing the valence electrons as unshared electron pairs and shared electron pairs in such a way that each atom achieves a noble-gas structure. 

As we said above, bonds aren t always purely ionic or purely covalent. The best models for ionic bonds include some electron sharing. And covalent bonds don t always share electrons completely evenly. Any difference in electronegativity between bonding nuclei means that the electrons will be attracted more to one nucleus than the other. However, a bond could be considered purely covalent if it is between two identical nuclei, as in hydrogen gas, H2, or oxygen gas, O2. An extreme example of equality in electron sharing is the metallic bond. In a metal, all the atoms are identical and share electrons so readily that the metal can be thought of as one big molecule. 

Molecules like H2, O2, N2, CK, and Br2 have zero dipole moments, that is, are non-polar. The two identical atoms of each of these molecules have, of course, the same electronegativity and share electron equally e is zcto and hence p is zero, too. 

Lewis and valenceA Lewis structure shows the valence electrons in a molecule. Two shared electrons form a structures single bond, with correspondingly more for multiple bonds. Some atoms may also have nonbonding electrons (lone-pairs). Valence structures show the bonds simply as lines. 

Figure 4.9 The redox process in compound formation. A, In forming the ionic compound MgO, each Mg atom transfers two electrons to each O atom. (Note that atoms become smaller when they lose electrons and larger when they gain electrons.) The resulting Mg and ions aggregate with many others to form an ionic solid. B, In the reactants H2 and CI2, the electron pairs are shared equally (indicated by even electron density shading). In the covalent product HCl, Cl attracts the shared electrons more strongly than H does. In effect, the H electron shifts toward Cl, as shown by higher electron density (red) near the Cl end of the molecule and lower electron density (blue) near the H end.

All these various bonds, whether single or multiple, are called covalent bonds. However, it is possible even in this simple approach to recognize the difference between non-polar covalent bonding and polar covalent bonding. In a homonuclear diatomic molecule, the shared electron pair or pairs must be shared equally and thus there is no polarity in the system nucleus-elec-trons-nucleus. In a heteronuclear diatomic molecule, however, one of the... 

As mentioned in Section 1.3, elemental hydrogen consists of molecules made up of 2 H atoms held together by a chemical bond consisting of two shared electrons. Just as it is useful to show electrons in atoms with a Lewis symbol, it is helpful to visualize molecules and the electrons in them with electron-dot formnlas or Lewis formulas as shown for the H2 molecule in Figure 3.4. 

Molecules that share electrons are generally smaller, have lower melting and boiling points, are insoluble in water, and do not conduct electricity. The s and p orbitals of nearby atoms overlap to form a mixed orbital. 

The strength and type of bonding between two atoms depend on the tendency of the participating atoms to donate, attract and share electrons. The variation of the electronic energy with the bond length r between the nuclei is schematically shown in Fig. 3.4 for the OH (hetero-nuclear) and O2 (homo-nuclear) molecules. The curve with the lowest energy corresponds to the ground state while the other curves represent different electronic states,... 

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