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Covalent and Metallic Bonding CHAPTER 7 Covalent and Metallic Bonding 1 © 2013 Marshall Cavendish International (Singapore) Private Limited Chapter 7 Covalent and Metallic Bonding 7.1 Covalent Bond: Sharing Electrons 7.2 Structure and Properties of Covalent Substances 7.3 Metallic Bond 2 7.1 Covalent Bond: Sharing Electrons Learning Outcomes By the end of this section, you should be able to: • describe the formation of a covalent bond by the sharing of electrons; • describe the formation of covalent bonds between non-metallic elements using‘dot and cross’diagrams; • deduce the arrangement of electrons in covalent molecules. 3 7.1 Covalent Bond: Sharing Electrons What is a Covalent Bond? • Covalent bonds are formed between atoms of non-metals. • Valence electrons are shared between two atoms. A covalent bond is the bond formed by the sharing of electrons between two atoms. • Each atom in the molecule achieves the stable electronic configuration of a noble gas. 4 7.1 Covalent Bond: Sharing Electrons Formation of a Molecule • Covalent bonds can be formed between – atoms of the same element; – atoms of different elements. • A molecule is formed when a group of two or more atoms are held together by covalent bonds. • Examples of molecules of elements: Hydrogen (H2), oxygen (O2), chlorine (Cl2) • Examples of molecules of compounds: Water (H2O), methane (CH4), carbon dioxide (CO2), ammonia (NH3) 5 URL 7.1 Covalent Bond: Sharing Electrons Example: Hydrogen (H2) 1 1 H H H H 2 2 Each hydrogen atom By sharing electrons, each has one electron in its atom has two electrons in outer shell. its outer shell. Each hydrogen atom has gained a stable duplet configuration. URL 6 7.1 Covalent Bond: Sharing Electrons Example: Hydrogen (H2) ‘Dot and cross’ Structural Molecular Model diagram formula formula H H H — H H2 H H This line represents a single covalent bond. The sharing of two electrons results in the formation of a single covalent bond. 7 7.1 Covalent Bond: Sharing Electrons Example: Chlorine (Cl2) 1 2 1 2 3 3 Cl Cl 8 8 7 Cl 4 7 Cl 4 6 5 6 5 Each chlorine atom has The two chlorine atoms combine seven outer electrons. to share a pair of electrons. Each chlorine atom has obtained an octet configuration. Note: ‘Dot and cross’diagrams sometimes only show outer shell electrons. 8 7.1 Covalent Bond: Sharing Electrons Example: Chlorine (Cl2) ‘Dot and cross’ Structural Molecular Model diagrams formula formula Cl Cl Cl Cl Cl — Cl Cl2 Cl Cl As in the hydrogen molecule, a single covalent bond is formed by sharing one pair of electrons. 9 7.1 Covalent Bond: Sharing Electrons Example: Oxygen (O2) 1 2 3 8 1 2 4 7 O O O 5 6 O 3 8 7 6 5 4 Each oxygen atom: The two oxygen atoms • has six outer electrons. combine to share two pairs of electrons. • needs two more electrons to form a stable octet configuration. Each oxygen atom has achieved an octet electronic configuration. 10 7.1 Covalent Bond: Sharing Electrons Example: Oxygen (O2) ‘Dot and cross’ Structural Molecular Model diagram formula formula O O O O O O O O O2 This represents a double covalent bond between the two oxygen atoms. The sharing of four electrons results in the formation of a double covalent bond. 11 7.1 Covalent Bond: Sharing Electrons Example: Nitrogen (N2) 3 1 4 2 1 5 3 N 7 N 2 N 6 4 N 8 7 5 8 6 Each nitrogen atom: The two nitrogen atoms • has five outer electrons. combine to share three • needs three more electrons pairs of electrons. to form a stable octet configuration. Each nitrogen atom has achieved an octet electronic configuration. 12 7.1 Covalent Bond: Sharing Electrons Example: Nitrogen (N2) ‘Dot and cross’ Structural Molecular Model diagram formula formula N N N N N N N2 N N This represents a triple covalent bond between the two nitrogen atoms. The sharing of six electrons results in the formation of a triple covalent bond. 13 7.1 Covalent Bond: Sharing Electrons Example: Water (H2O) • A water molecule is formed by: – two hydrogen atoms – one oxygen atom O • In a water molecule, each hydrogen atom is bonded H H to the oxygen atom by a single covalent bond. Two single covalent bonds are formed. 14 7.1 Covalent Bond: Sharing Electrons Example: Water (H2O) • The oxygen atom shares two 7 6 8 outer electrons with two hydrogen O 5 atoms. 1 4 • Each hydrogen atom shares its 1 2 3 outer electron with the oxygen 1 2 atom. H 2 H • The oxygen atom achieves an octet electronic configuration. • Each hydrogen atom achieves a duplet electronic configuration. 15 7.1 Covalent Bond: Sharing Electrons Example: Water (H2O) ‘Dot and cross’ Structural Molecular Model diagram formula formula O O O H2O H H H H H H The sharing of four electrons between oxygen and hydrogen results in the formation of two single covalent bonds. 16 7.1 Covalent Bond: Sharing Electrons Example: Methane (CH4) • A methane molecule is formed by: – four hydrogen atoms H H – one carbon atom C • In a methane molecule, four hydrogen atoms are bonded H H to one carbon atom by four single covalent bonds. 17 7.1 Covalent Bond: Sharing Electrons Example: Methane (CH4) • The oxygen atom shares four outer electrons with four hydrogen H H atoms. 2 1 1 2 3 2 • Each hydrogen atom shares its 1 4 C outer electron with the carbon 8 5 6 atom. 1 7 2 1 • The carbon atom achieves an H 2 H octet electronic configuration. • Each hydrogen atoms achieves a duplet electronic configuration. 18 7.1 Covalent Bond: Sharing Electrons Example: Methane (CH4) ‘Dot and cross’ Structural Molecular Model diagram formula formula H H H H C C H C H CH4 H H H H H H The sharing of four electrons between a carbon atom and four hydrogen atoms results in the formation of four single covalent bonds. 19 7.1 Covalent Bond: Sharing Electrons Example: Carbon dioxide (CO2) • A carbon dioxide molecule is formed from two oxygen atoms and one carbon atom. O C O • The carbon atom has four outer electrons. • It needs four more electrons to achieve an octet configuration. These come from two oxygen atoms 20 7.1 Covalent Bond: Sharing Electrons Example: Carbon dioxide (CO2) Each oxygen atom shares two of its outer electrons. O C O O C O 21 7.1 Covalent Bond: Sharing Electrons Example: Carbon dioxide (CO2) ‘Dot and cross’ Structural Molecular Model diagram formula formula O C O O C O O C O CO2 A molecule with two double covalent bonds is formed. 22 Chapter 7 Covalent and Metallic Bonding 7.1 Covalent Bond: Sharing Electrons 7.2 Structure and Properties of Covalent Substances 7.3 Metallic Bond 23 7.2 Structure and Properties of Covalent Substances Learning Outcomes By the end of this section, you should be able to: • relate the physical properties of covalent substances to their structure and bonding; • compare the structures of simple molecular substances and giant molecular substances to deduce their properties; • compare the bonding and structures of diamond and graphite to deduce their physical properties. 24 7.2 Structure and Properties of Covalent Substances Bonding and Structure of Simple Molecular Structures • Most covalent substances exist as simple molecules. • Atoms in the molecules are held by strong covalent bonds. • Molecules are held together by weak intermolecular forces of Br Br attraction. Br Br Br Br 25 7.2 Structure and Properties of Covalent Substances Melting and Boiling Points 1. Substances with simple molecular structures have low melting and boiling points. • Little energy is required to overcome the weak intermolecular forces holding the molecules together. • Hence, they have low melting and boiling points. • They are usually gases or volatile liquids at room temperature and pressure. 26 7.2 Structure and Properties of Covalent Substances Solubility 2. Substances with simple molecular structures are usually insoluble in water and soluble in organic solvents. Oil dissolves in dichloromethane but not in water. An organic solvent Other examples of organic solvents: Ethanol, turpentine, benzene Exceptions: Alcohol and sugar are soluble in water. 27 7.2 Structure and Properties of Covalent Substances Electrical Conductivity 3. Most substances with simple molecular structures do not conduct electricity in any state. • Covalent substances have no free mobile ions or electrons in their solid, liquid or gaseous states. • However, hydrogen chloride, sulfur dioxide and ammonia form ions when dissolved in water. Their ions would be mobile and act as charge carriers to conduct electricity. 28 7.2 Structure and Properties of Covalent Substances Giant Molecular Structures • Some substances exist as a giant network of atoms that are covalently bonded. • These substances are called giant molecular structures. • Examples of substances with giant molecular structures: graphite, diamond and silicon dioxide. diamond 29 7.2 Structure and Properties of Covalent Substances Melting and Boiling Points 1. Substances with giant molecular structures have very high melting and boiling points. • This is due to the large number of atoms being held by strong covalent bonds. • These covalent bonds must first be broken to melt or boil the substance. • A large amount of energy is required to overcome these bonds, leading to high melting and boiling points. 30 7.2 Structure and Properties of Covalent Substances Solubility 2. Substances with giant molecular structures are insoluble in both water and organic solvents. • All atoms are held together by strong covalent bonds and solvent molecules are not strong enough to break the strong covalent bonds. 31 7.2 Structure and Properties of Covalent Substances Electrical Conductivity 3.
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