Their bond energy is very low and bond lengths are comparatively large Their

Their bond energy is very low and bond lengths are

This preview shows page 44 - 46 out of 79 pages.

Their bond energy is very low and bond lengths are comparatively large. Their attraction is non-directional in nature hence very weak. Thermal energy required to break the bond is therefore less. This bonding, for helium and argon is also called “van der Waal’s” bonding. Dipole Bonds The arrangement of electrons and positive nuclei results in a positively charged field at one end of the molecule and a negative field at the other. As a result of this permanent dipoles are produced. They attract each other and a bond is formed which is known as dipole bond. A typical example of this is the formation of hydrogen chloride (HCl). Fig. 9 below shows the formation of the hydrogen chloride polar molecule. HCl is a polar molecule as the region near the hydrogen nucleus is highly positive and the opposite region near the chlorine nucleus is negative. The negative region of chlorine atom is attracted to the adjacent positive region of the hydrogen atom. Other examples are hydrogen fluoride, sulphur dioxide, hydrogen bromide and hydrogen cyanide. Dipole bonds are weaker than ionic bonds but stronger than dispersion bonds. Hydrogen Bond This is a special case of inter-molecular attraction produced between certain covalently bonded hydrogen atoms and a pair of electrons of another atom. There is a positive field adjacent to the hydrogen and a negative field around the electron pair. This force is very important in water, plastics and in biological molecules such as DNA. The hydrogen bond keeps water liquid at room temperature. Water molecules consist of one oxygen atom and two hydrogen atoms bonded using a covalent bond. Water molecules have a positive charge near the hydrogen because of a concentration of electrons. This causes a negative charge at the other side of the molecule, and this distribution of charge causes a weak bond between water molecules so making the vaporizing of water very easy.
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~ Page 45 of 79 ~ Ice is hydrogen bonded. Each hydrogen atom gives up its charge to the nearest oxygen atom which then acquires a negative charge. The positively charged hydrogen ion acts as a bond between nearby oxygen ions and bond polymer chains together to give solid polymer. The hydrogen bonds keep the molecules well apart, which is why ice has a lower density than water. Without hydrogen bonding ice would boil at -80 C. Hydrogen bonds are directional in character. They are found in various tissues such as blood, skin and bones in animal life. Strength of Bonds Bond Energy (GPa) Example of Bond Covalent 1,000 Diamond Ionic 30 - 100 Salt and Ceramics Metallic 30 - 150 Metals Hydrogen 8 Ice Van der Waal’s 2 Polythene Table 2 PROPERTIES OF IONIC, COVALENT COMPOUNDS AND METALLIC SOLIDS IONIC COMPOUNDS COVALENT COMPOUNDS METALLIC SOLIDS 1 Ionic solids are crystalline in nature and therefore they have high hardness.
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