Covalent Bonds Molecule two or more atoms held together by covalent bonds single bond, or single covalent bond sharing of one pair of valence electrons double bond, or double covalent bond sharing of two pairs of valence electrons
Bond Nota8ons structural formula nota8on used to represent atoms and bonding H H, O=O molecular formula Further abbrevia8on H 2, O 2
Fig. 2-12 Name and Molecular Formula Electrondistribution Diagram Lewis Dot Structure and Structural Formula Spacefilling Model (a) Hydrogen (H 2 ) (b) Oxygen (O 2 ) (c) Water (H 2 O) (d) Methane (CH 4 )
Covalent Bonds con8nued Covalent bonds can form between atoms of the same element or atoms of different elements Molecule any two or more atoms bonded together May be same or different elements Compound combina8on of two or more different elements
Electronega8vity (χ) Bond Strength an atom s afrac8on for the electrons in a covalent bond as well as it s own More electronega8ve an atom the more strongly it pulls shared electrons toward itself Imbalances of electronega8vity may result in unequal sharing of electrons in a covalent bond
Types of Covalent Bonds nonpolar covalent bond atoms share the electrons equally polar covalent bond one atom is (considerably) more electronega8ve results in unequal sharing causes a par8al posi8ve and/or nega8ve charge for each atom or an en8re molecule
(a) Polar water molecule (b) Nonpolar methane molecule slight negative charge polar slight positive charge δ O δ- nonpolar because charges are symmetric H 2 O O δ+ H H δ+
Ionic Bonds Large differences in electronega8vi8es Atoms some8mes strip electrons from their bonding partners Example - transfer of an electron from sodium to chlorine APer which both atoms have charges Numbers of protons (+) in each atom are no longer balanced by electrons (- ) Ion A charged atom (or molecule) Has gained or lost an electron
Ions Ca8on - posi8vely charged ion Lost an electron Fewer electrons than protons = extra + Anion - nega8vely charged ion Gained an electron More electrons than protons = extra ionic bond afrac8on between an anion and a ca8on Opposites charges afract
Ionic Bonds Transfer of electron + Na Sodium atom Cl Chlorine atom Na + Sodium ion Cl Chloride ion EN = 0.9 EN = 3.0 Sodium chloride (NaCl)
Ionic Bonds Transfer of electron + Na Sodium atom Cl Chlorine atom Na + Sodium ion Cl Chloride ion EN(χ) = 0.9 EN = 3.0 Sodium chloride (NaCl)
Ionic Bonds Transfer of electron + Na Sodium atom Cl Chlorine atom Na + Sodium ion Cl Chloride ion EN(χ) = 0.9 EN(χ) = 3.0 Sodium chloride (NaCl)
Ionic Bonds Transfer of electron + Na Sodium atom Cl Chlorine atom Na + Sodium ion Cl Chloride ion EN(χ) = 0.9 EN(χ) = 3.0 Sodium chloride (NaCl)
Ionic Bonds Ionic compound Compounds formed by ionic bonds Many are salts Salts, (NaCl, table salt), are open found in nature as crystals Na + Cl
Weak Chemical Bonds covalent bonds strongest ionic bonds next strongest hydrogen bonds weakest very important Weak chemical bonds reinforce shapes of large molecules and help molecules adhere to each other
hydrogen bond Hydrogen Bonds hydrogen atom covalently bonded to one electronega8ve atom is also afracted to another electronega8ve atom In living cells, the electronega8ve partners are usually oxygen or nitrogen atoms Water (H 2 O) Ammonia (NH 3 ) δ - δ+ δ+ δ - Hydrogen bond afrac8on of H+ to an opposite charge (- ) δ+ δ+ In water, a hydrogen atom of one water molecule (δ+) δ+ will form a hydrogen bond with an unshared oxygen (δ- ) electron of a neighboring water molecule 38
If electrons are distributed asymmetrically in large molecules they can result in hot spots of posi8ve or nega8ve charge localized regions of charge Van der Waals interac8ons afrac8ons between molecules that are close together as a result of these charges Collec8vely can be strong Other Interac8ons as between molecules of a gecko s toe hairs and a wall surface
Molecular Shape and Func8on molecular shape very important to its func8on determined by the posi8ons of its atoms valence orbitals In a covalent bond the s and p orbitals may hybridize crea8ng specific molecular shapes s orbital z (a) Hybridization of orbitals Space-filling Model Water (H 2 O) Methane (CH 4 ) (b) Molecular-shape models x y Three p orbitals Ball-and-stick Model 104.5º Four hybrid orbitals Tetrahedron Hybrid-orbital Model (with ball-and-stick model superimposed) Unbonded electron pair
Molecular Shape Biological molecules recognize and interact with each other with a specificity based on molecular shape Molecules with similar shapes Natural endorphin Key Carbon Hydrogen Nitrogen Sulfur Oxygen can have similar biological effects Morphine Shape dictates func8on (a) Structures of endorphin and morphine Natural endorphin Morphine Brain cell Endorphin receptors (b) Binding to endorphin receptors
Chemical reac8ons Reac8ons the making and breaking of chemical bonds Reactants star8ng molecules of a chemical reac8on Products final molecules of a chemical reac8on 2 H 2 O 2 2 H 2 O Reactants Reaction Products
Chemical Reac8ons All chemical reac8ons are reversible products of the forward reac8on become reactants for the reverse reac8on Chemical equilibrium reached when the forward and reverse reac8on rates are equal
You should now be able to 1. Iden8fy the four major elements 2. Dis8nguish between the following pairs of terms: neutron and proton, atomic number and mass number, atomic weight and mass number 3. Dis8nguish between and discuss the biological importance of the following: nonpolar covalent bonds, polar covalent bonds, ionic bonds, hydrogen bonds, and van der Waals interac8ons