CH131_3B

CH131_3B - Covalent Bonding Ionic compounds: tend to be...

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Covalent Bonding Ionic compounds: tend to be brittle, high melting points, solids Most substances are not ionic compounds; they have different properties which reflect diff. kinds of chemical bonding => covalent bonding Basic idea => atoms achieve “octet” configuration by sharing e ’s (note: H only needs “duet” configuration) Consider the simplest molecule first (use Lewis symbols): H 2 . H + H H H . . . - H atoms share e ’s to achieve inert gas valence structure (He) - example of covalent bond We indicate shared e pair by a short solid line, i.e. H–H
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Consider the diatomic molecule Cl 2 (Group VII, Cl 7 val. e ’s) . . . . . .. . . . Cl + Cl Cl Cl . . . . . . . By sharing 1 pair of e ’s, “octet rule” satisfied, covalent bond formed . . . . . . Cl – Cl unshared e pairs called lone pairs 6 lone pairs & one shared pair (covalent bond) on Cl 2 Examples of other covalent compounds in Lewis structure notation . . . . H – Cl H – O H – N – H . . H H 3, 2, 1 lone e pairs Lewis structures show how bonds connect atoms in molecules, not 3D structures! (later)
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Sharing a single pair of e ’s is called a single bond. However, atoms can share more than a single pair of e ’s! For example, consider N 2 (N Group V) 10 valence e ’s . . . .. Lewis structures: N + N N – N . . . . . . . “octet rule” not obeyed But if we let 3 pairs of e ’s to be shared…. . N N => N N “octet rule” satisfied when 3 paris of e ’s shared triple bond Summary: Lewis structures of covalent bonding => short lines (1, 2 or 3) represent shared pairs & lone pairs indicated by dots
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. . .. Diatomics so far…consider a triatomic (3 atom molecule) CO 2 C (Group IV) O (Group VI) so total # valence e ’s = 16 (4 + 2x6) Octets can be achieved for all 3 atoms by C sharing 2 e pairs with each O . . . . .. . . . . . O + C + O O = C = O . . “Octet rule” satisfied for each atom by forming 2 double bonds between C and O atoms (16 val. e ’s)
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structures 1. Sum # valence e ’s from all atoms (use Group #). include ion charge, i.e. add charge for anions (negative); subtract charge for cations (positive) 2. Write Lewis symbol for atoms showing which atoms are bonded to which => skeleton structure Note: H & F (nearly) always terminal: Draw central atom first, i.e. C Cl 4 C O 3 3. Draw single bond between each pair of bonded atoms. 4. Complete octet of atoms bonded to central atoms (terminal atoms) Remember H needs only duet (2 e ’s) 5. Place leftover e ’s on central atom (even if more than octet). 6.
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CH131_3B - Covalent Bonding Ionic compounds: tend to be...

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