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Chapter 8: Chemical Bonding Three general types of chemical bonds: Ionic Covalent Metallic Ionic bond Electrostatic force between particles of opposite charge Generally involves metal and a nonmetal Covalent bond Results from sharing of e- between atoms Generally involves two nonmetals Metallic bond Found in metals, e.g. Fe/Al/Cu Bonding e- are free to ‘move’ How do we discuss the arrangement of electrons when atoms form compounds? 158 Lewis symbols: the octet rule Valence: capacity of an element to form chemical bonds Valence e-: electrons which take part in chemical bonding Reside in outermost (or valence) shell, e.g., C: [He]2s22p2 4 valence e- Cl: [Ne]3s23p5 7 valence e- Representing valence electrons: Lewis symbols Lewis symbol: symbol for element plus a dot for each valence e-, e.g., Be: [He]2s2 N: [He]2s22p3 S: [Ne]3s23p4 159 Electron dots are placed in 4 "regions" around the symbol Each region can accommodate an e- pair (USE HUND'S RULE!!!) e.g. draw Lewis symbols for Mg Sc3+ B Note: The number of valence e- for the s and p-block elements is given by the element's group number! The octet rule Atoms tend to gain, lose or share e- to get to the nearest noble gas configuration Noble gases: all (except He) have s2p6 valence shells (8 e-) Hence: atoms tend to be surrounded by 8 valence e- - this is the reason that group 1 atoms form +1 ions, group 6 atoms form -2 ions, etc 160 How does the octet rule influence the number of bonds that an atom will form in a compound? Let us look at ionic and covalent bonding in term of the octet rule…. Ionic Bonding What happens in a reaction if the atoms involved differ greatly in attraction for e-? e.g. Na(s) + ½Cl2(g) NaCl(s) Na: low I1; Cl: very negative EA Write the above reaction with electron-dot symbols: e- is transferred from Na to Cl; do both species have an octet of e- ? Ionic bonds result from a transfer of electrons from the metal to the nonmetal such that both obtain an octet 161 e.g. Which ionic compound is expected to form from: Rb and O Ba and I 162