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Art Ian G. Bautista, ECE, ECT always found in compounds that are composed of both metallic and nonmetallic elements Atoms of a metallic element easily give up their valence electrons to the nonmetallic atoms. In the process all the atoms acquire stable or inert gas configurations and, in addition, an electrical charge; that is, they become ions. Ex. Sodium chloride (NaCl) A sodium atom can assume the electron structure of neon (and a net single positive charge) by a transfer of its one valence 3s electron to a chlorine atom. After such a transfer, the chlorine ion has a net negative charge and an electron configuration identical to that of argon. Schematic representation of ionic bonding in sodium chloride (NaCl). The attractive bonding forces are coulombic ; that is, positive and negative ions, by virtue of their net electrical charge, attract one another. Ionic bonding is nondirectional Physical Properties: ◦ Hard ◦ Brittle ◦ Electrically insulative ◦ Thermally insulative
In covalent bonding stable electron
configurations are assumed by the
sharing of electrons between adjacent
atoms. Two atoms that are covalently
bonded will each contribute at least one
electron to the bond, and the shared
electrons may be considered to belong to
both atoms.
Ex. Methane (CH4) The carbon atom has four valence electrons, whereas each of the four hydrogen atoms has a single valence electron. Each hydrogen atom can acquire a helium electron configuration (two 1 s valence electrons) when the carbon atom shares with it one electron. The carbon now has four additional shared electrons, one from each hydrogen, for a total of eight valence electrons, and the electron structure of neon. Schematic representation of covalent bonding in a molecule of methane (CH 4 ).
Covalent bonding is found in elemental
solids such as diamond (carbon), silicon,
and germanium and other solid
compounds composed of elements that
are located on the right-hand side of the
periodic table, such as gallium arsenide
(GaAs), indium antimonide (InSb), and
silicon carbide (SiC).
The number of covalent bonds that is possible for a particular atom is determined by the number of valence electrons. For N’ valence electrons, an atom can covalently bond with at most 8 – N’ other atoms. For example, N’ = 7 for chlorine, and 8 – N’ = 1, which means that one Cl atom can bond to only one other atom, as in Cl 2. Similarly, for carbon, N’= 4, and each carbon atom has 8 - 4, or four, electrons to share. Electrically and Thermally insulative
Secondary, van der Waals, or physical bonds are weak in comparison to the primary or chemical ones; bonding energies are typically on the order of only 10 kJ/mol (0.1 eV/atom). Secondary bonding exists between virtually all atoms or molecules, but its presence may be obscured if any of the three primary bonding types is present. Secondary bonding is evidenced for the inert gases, which have stable electron structures, and, in addition, between molecules in molecular structures that are covalently bonded. Secondary bonding forces arise from atomic or molecular dipoles. In essence, an electric dipole exists whenever there is some separation of positive and negative portions of an atom or molecule. The bonding results from the coulombic attraction between the positive end of one dipole and the negative region of an adjacent one. Dipole - A pair of equal yet opposite electrical charges that are separated by a small distance. Schematic illustration of Van der Waals bonding between two dipoles.
Hydrogen bonding, a special type of
secondary bonding, is found to exist
between some molecules that have
hydrogen as one of the constituents.
Seatwork:
Explain the anomalous behavior of
water when it freezes. That is, why is
there volume expansion upon
solidification?
