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CHEM 1100 (7)
Chapter 5


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University of Guelph
CHEM 1100
Kim Bolton

CH. 5 CHEMICAL BONDING From Poison to Seasoning • Both chlorine and sodium alone are reactive and toxic  together they are harmless (make up table salt) • Chlorine is reactive because chlorine atoms have 7 valence electrons (1 short of a stable configuration) • Sodium is reactive because sodium atoms have 1 valence electron (1 electron away from stable configuration) • Both elements are toxic because they can alter a molecule by forcefully exchanging electrons with it  sodium transfers electrons to the molecule; chlorine transfers electrons away from the molecule • When sodium and chlorine combine both atoms (cation Na+ & anion Cl- both have 8 electrons in outer electrons) are stabilized by the exchange and a chemical bond forms (sodium chloride) • Lewis theory gives insight in chemical bonding and explains why particular elements combine in particular proportions • Valence shell electron pair repulsion (VSEPR) theory predicts the shapes of molecules Chemical Bonding and Professor G.N. Lewis • Model predicted the molecules that would form from certain elements ( 9 times out of 10) • Fundamental ideas: 1. The valence electrons are most important in chemical bonding 2. Valence electrons are transferred from one atom to another to form ionic bonds, or shared between atoms to form covalent bonds 3. When valence electrons are transferred or shared, the atoms in the compound form full outer Bohr orbits and therefore gain stable electron configurations. Because a stable electron configuration usually involves 8 electrons this is known as the octet rule (exceptions are helium and hydrogen for which a stable configuration is only 2 electrons called a duet) • We use dots to represent valence electrons (1 dot = 1 valence electron) CH. 5 CHEMICAL BONDING • lewis structure of an element is its chemical symbol surrounded by dots (max 2 dots per side) to represent valence electrons • We ignore the inner or core electrons and represent only the valence electrons • Elements without an octect tend to react with other elements to form an octect, except for hydrogen which forms a duet Ionic Lewis Structures • We combine the Lewis structures of elements to form Lewis Structures for compounds • We represent ionic bonding (bonding between metal and non-metal through transfer of electrons) by moving dots from the Lewis structure of the metal to the Lewis structure of the non-metal • The metal becomes a cation and the non-metal becomes an anion • because metal and non-metal require a charge we indicate the magnitude of the charge on the upper right corner of the symbol • Indicate the anion in brackets to show the charge belongs to whole assembly including the electron dots MgF 2 Covalent Lewis Structures • In covalent bonds atoms share electrons • Represent covalent bonding in Lewis Theory by lettings atoms share their dots • The 2 electrons in the middle are shared by both atoms and count towards the octet of each CH. 5 CHEMICAL BONDING • The electrons between two atoms are called bonding electrons, while those on a single atoms are called lone pair electrons • Bonding electrons count toward the octet of both atoms • Lone pair atoms count toward the octet of the atom they are on • We can simplify our notation by representing bonding electron pairs with dashes Multiple Bonds • Atoms often share more than one electron pair to form complete octets • Oxygen exists as the diatomic molecule O 2 • The lewis structure for each isolated oxygen atom has 6 electrons • When we bring the 2 oxygen atoms together we initially have an incomplete octet • We can complete its octet by converting one of the lone pairs into a second bonding pair: • This lewis structure has 2 bonding pairs called a double bond • The 4 electrons that constitute the double bond count toward the octet of both oxygen atoms so both atoms have an octet • Double bonds are shorter (the bonding atoms are closer together) and stronger than single bonds because the bond contains twice as many electrons CH. 5 CHEMICAL BONDING • Atoms can also share three electron pairs to form complete octets • A lewis structure that has 3 bonding pairs is called a triple bond • Triple bonds are shorter and stronger than single and double bonds • Triple bond molecules are very stable and hard to break Steps for Writing Lewis Structures 1. Write the skeletal structure of the molecule. (elements that need a duet will never be a central atom and when a molecule contains several atoms of the same type those atoms will often be in terminal positions aka symmetrical) 2. Determine the total number of electrons for the molecule. (add together the valence electrons for each atom in the molecule. The final lewis structure must contain exactly this total number of electrons) 3. Place the electrons as dots to give octets to as many atoms as possible. (begin by putting a single bond between all bonded atoms. Distribute the remaining electrons beginning with terminal atoms and ending with central atom. By using single bonds, try to give each atom an octet except hydrogen when it’s a duet) 4. If the central atom has not obtained an octet, form double or triple bonds as necessary to give it an octet. (move lone pair electrons from the outer atoms into the bonding region with the central atom, thus converting them into bonding pairs and forming multiple bonds) Chemical Bonding in Ozone • Ozone (O3) is an atmospheric gas that protects life on earth from exposure to ultraviolent light • Ozone reacts with UV light as the reaction: O3 + UV light  O2 + O • The ozone molecule absorbs the UV light, causing one of the oxygen-oxygen bonds to break CH. 5 CHEMICAL BONDING • it seems that the lewis theory predicts 2 different types of bonds in the ozone molecule, one double bond and one single bond • However, real ozone molecules have 2 identical bonds (equal length and strength). Each bond is shorter than a single bond but longer than a double bond • Each bond in ozone is a bond and a half • Actual structure: • The averaging of 2 identical lewis structures is called resonance and is usually represented by drawing both structures – cal
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