30 3·Mechanisms FIGURE 3.5.Schematic representation of metallic bonding.The valence electrons become disassociated with "their" atomic core and form an electron "sea" that acts as the binding medium be- tween the positively charged ions. Van der Compared to the three above-mentioned bonding mechanisms, Waals Bond van der Waals bonds!are quite weak and are therefore called secondary bonds.They involve the mutual attraction of dipoles. This needs some explanation.An atom can be represented by a positively charged core and a surrounding negatively charged electron cloud [Figure 3.6(a)].Statistically,it is conceivable that the nucleus and its electron cloud are momentarily displaced with respect to each other.This configuration constitutes an electric dipole,as schematically depicted in Figure 3.6(b).A neighboring atom senses this electric dipole and responds to it with a simi- lar charge redistribution.The two adjacent dipoles then attract each other. FiGURE 3.6.(a)An atom is represented by a (a) positively charged core and a surrounding B negatively charged electron cloud.(b)The electron cloud of atom'A'is thought to be displaced,thus forming a dipole.This in- duces a similar dipole in a second atom,'B'.Both dipoles are then mutually attracted,as proposed by van der Waals. (b) IJohannes Diederik van der Waals (1837-1923),Dutch physicist,re- ceived in 1910 the Nobel Prize in physics for his research on the math- ematical equation describing the gaseous and liquid states of matter.He postulated in 1873 weak intermolecular forces that were subsequently named after him.Compared to the three above-mentioned bonding mechanisms, van der Waals bonds1 are quite weak and are therefore called secondary bonds. They involve the mutual attraction of dipoles. This needs some explanation. An atom can be represented by a positively charged core and a surrounding negatively charged electron cloud [Figure 3.6(a)]. Statistically, it is conceivable that the nucleus and its electron cloud are momentarily displaced with respect to each other. This configuration constitutes an electric dipole, as schematically depicted in Figure 3.6(b). A neighboring atom senses this electric dipole and responds to it with a simi￾lar charge redistribution. The two adjacent dipoles then attract each other. FIGURE 3.5. Schematic representation of metallic bonding. The valence electrons become disassociated with “their” atomic core and form an electron “sea” that acts as the binding medium be￾tween the positively charged ions. 30 3 • Mechanisms 1Johannes Diederik van der Waals (1837–1923), Dutch physicist, re￾ceived in 1910 the Nobel Prize in physics for his research on the math￾ematical equation describing the gaseous and liquid states of matter. He postulated in 1873 weak intermolecular forces that were subsequently named after him. + – – – – – – – – A B (a) (b) + + FIGURE 3.6. (a) An atom is represented by a positively charged core and a surrounding negatively charged electron cloud. (b) The electron cloud of atom ‘A’ is thought to be displaced, thus forming a dipole. This in￾duces a similar dipole in a second atom,‘B’. Both dipoles are then mutually attracted, as proposed by van der Waals. Van der Waals Bond