Chap 8 - Bonding

Bonding Terms Chemical Bond – forces that hold atoms together Bond energy – energy required to break bond Bond length – distance between nuclei of 2 atoms; distance where energy is at a minimum Ionic Bond – bond between metal and nonmetal; positive and negative ions; transfer of electrons - gained and lost Forms ionic compound Covalent Bond– bond between 2 nonmetals; share electrons Polar covalent – unequal sharing of electrons; partial positive and negative charges assigned Bond polarity Nonpolar covalent – equal sharing of electrons

Determining Bond Type Type of element Metal vs nonmetal Monatomic vs polyatomic Electronegativity difference What is electronegativity? Subtract electronegativity values for 2 elements Ranges: Ionic bond = Polar covalent bond = Nonpolar covalent bond = p357 has chart with values

Dipoles What is a dipole? Molecule with area of positive charge and area of negative charge Referred to as dipole moment Show with arrow pointing in direction of the overall negative area Presence of a polar bond does not mean it has a dipole moment. Some cancel each other out Can also see in molecule modeling

Why do atoms form bonds? Seek lowest possible energy state or become more stable Want to pair up electrons Fill their outer energy level Single bond – share 1 pair electrons Double bond – share 2 pairs electrons Triple bond – share 3 pairs of electrons Bond energy increases as number of shared pairs increases Bond length shortens as number of shared pairs increase

Enthalpy of reaction ∆H = sum of energies required to break old bonds (positive) plus the sum of energies released when new bonds formed (negative) ∆H = ∑n*D(broken) -∑n*D(formed) energy required – energy released Where D = bond energy per mole – always a positive sign; and n = number of moles of that bond type *** Answers determined this way should be very similar to those found using ∆H° (standard enthalpy of formation values)

Lewis Structure Review Also known as electron dot diagram Show valance electron arrangement in atom Elements want to be like Noble Gases – Octet Rule – 8 electrons. A few violators Shared pairs vs. lone pairs Help dictate polarity or dipoles Show as stick figure Resonance – more than one possible Lewis Structure Represent with arrows between structures Identify shape - VSEPR See charts