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CH 301 Chapter%2 - CH 301 Chapter 2 part 1 CHEMICAL BONDING In all compounds the atoms are held together by bonds IONIC bond outermost electrons

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CH 301 Chapter 2 part 1: CHEMICAL BONDING In all compounds the atoms are held together by bonds. IONIC bond: outermost electrons are often transferred from one atom to the other, resulting in IONS. The ions are held together by an electrostatic force. COVALENT bond: outermost electrons from each atom are shared somehow. METALLIC bond: array of cations are held together by a ‘sea’ of electrons. METALS lose electrons to form CATIONS. NON-METALS gain electrons to form ANIONS. FORMING IONIC BONDS (Fig 2.1) An example of a simple ionic compound is sodium chloride; imagine it forms this way: 1. Sodium atoms get ionized: Na --> Na + + e- 2. Chlorine atoms gain an electron: Cl + e- --> Cl - 3. The two kinds of ions come together to form a crystal: Na + + Cl - --> NaCl Step 1 requires an INPUT of energy, Step 2 RELEASES energy - but not as much as Step 1. Step 3 RELEASES a LOT of energy -so the OVERALL process releases energy. STRUCTURE OF IONIC COMPOUNDS (Fig 2.2 note below) Its NOT pairs of oppositely charged ions- but a regularly repeating crystalline LATTICE of ions. Something similar occurs for ionic compounds containing polyatomic ions. (See Fig 2.6) INTERACTIONS BETWEEN IONS In an IONIC compound ALL the positive ions attract ALL of the negative ions. The Coulomb potential energy between one positive ion and one negative one is given by: E = z 1 z 2 e 2 / (4 Πε 0 r 12 ) e = charge on an electron (in Coulombs); ε 0 = vaccum permittivity (another constant) z 1 = charge number for ion 1(for example a +2 ion has z = +2) r 12 = distance between ion 1 and ion 2 But all ions interact. How do we model this? We consider all of the attractions and repulsions: Attractive interactions lower the potential energy of the solid, so must be negative, Repulsive interactions raise the energy of the solid, so these are positive. Different ionic lattices have slightly different structures (more in a later chapter on this) These factors are summarized as follows: E = -A |z 1 z 2 | N A e 2 / (4 Πε 0 d) (Fig 2.3) d= average distance between ions N A = Avogadro’s constant A = the Madelung constant for this crystal NOTE: A depends on the structure of the ionic crystal, its a constant - can be looked up! (e.g., A NaCl = 1.78) YOU DON’T NEED TO MEMORIZE THE WHOLE EQUATION! NOTE HOW E RELATES TO z1, z2 and d:
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This note was uploaded on 09/09/2009 for the course CH 301 taught by Professor Fakhreddine/lyon during the Spring '07 term at University of Texas at Austin.

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CH 301 Chapter%2 - CH 301 Chapter 2 part 1 CHEMICAL BONDING In all compounds the atoms are held together by bonds IONIC bond outermost electrons

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