Sharing of Electrons. Covalent Bonding Chapter 8. Ch. 8 Vocabulary OBJECTIVES. Exothermic Reaction Structural Formula (Ch. 8.3) Polar Covalent Bond

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1 Ch. 8 Vocabulary 2 Covalent Bonding Chapter 8 Covalent bond Molecule Lewis Structure Sigma bond Pi bond Bond Dissociation Energy Endothermic Reaction Exothermic Reaction Structural Formula (Ch. 8.3) Polar Covalent Bond BJECTIVES Draw Lewis Structures of Covalent compounds Name covalent compounds Write formulas for covalent compounds Describe characteristics of covalent molecules Review electronegativity and compare they types of bonds of different molecules using the electronegativity scale 3 Covalent Bonding Remember the ctet Rule: Similar arrangement of valence electrons Electron arrangements determines chemical properties Presents a model of chemical stability RULE: Atoms become stable by having 8 electrons in their outer energy level (2 for smaller atoms) When they have gotten 8 electrons they have achieved NBLE GAS CNFIGURATIN (NGC) ne way to get NGC is by transferring electrons as in an ionic bond. 4 Covalent Bonding The other way to achieve Noble Gas Configuration: Sharing Electrons Take the case of Water (H 2 ) Hydrogen can t transfer its electron, otherwise it would be just a proton and not a noble gas configuration. It also can t gain one. Hydrogen and oxygen can t both gain electrons. Sharing of Electrons In the case of NaCl, chlorine has a much stronger affinity for electrons and sodium holds its valence electron very weakly. In the case of H 2, both hydrogen and oxygen have similar affinities. In other words, the attraction for electrons is not strong enough. So what do they do? 5 6 1

2 Molecular Elements Molecules can vary greatly in size Can be just two atoms (C) to thousands or millions of atoms (DNA) :C: Two or more atoms of the same element can form a covalent bond this is called a molecular element. 7 Molecular Elements H H NN = F F They are: Hydrogen, Nitrogen, xygen, Fluorine, Chlorine, Bromine, and Iodine Their Formulas are: H 2, N 2, 2, F 2, Cl 2, Br 2, and I 2 All are gases except Br 2 (liquid) and I 2 (Solid) Diatomic Molecules Seven non metal elements are found naturally as molecular elements of two identical atoms Cl Cl Br Br I I 8 Molecular Elements Allotropes Molecular Elements Allotropes of Red Phosphorus White Black Examples: 9 10 Electron Dot Structures (Lewis Symbols) Remember (Chapter 5) electron dot structures (Lewis symbols). The number of electrons available for bonding are indicated by unpaired dots. Place the electrons one four sides of a square around the element symbol, e.g. C Lewis Structures quick method Pair up the single electrons in each atom to form a bond. Electron Dot Structures (Lewis Symbols)

3 Chlorine forms a covalent bond with itself Cl 2 Cl Cl How will two chlorine atoms react? Cl Cl octet Cl Cl The octet is achieved by each atom sharing the electron pair in the middle Cl Cl Lone pair It is called a SINGLE BND also called a sigma (σ) bond Lone Pairs are non bonding electrons or unshared electrons The Cl 2 molecule has how many lone pairs? 6 Single bonds are abbreviated with a dash Cl Cl This is the chlorine molecule, Cl 2 Contrast this with ionic bonding next slide

4 NaCl This is the formation of an ionic bond. + - Na Cl electron transfer and the formation of ions Covalent Bonding Cl 2 This is the formation of a covalent bond. Cl Cl sharing of a pair of electrons and the formation of molecules Molecular Lewis Structures ther Single Covalent Bonds 2 xygen is also one of the diatomic molecules Bonding pairs? Lone pairs? Each atom has two unpaired electrons xygen atoms are highly electronegative. So both atoms want to gain two electrons

5 Both electron pairs are shared. 6 valence electrons plus 2 shared electrons = full octet = two bonding pairs, making a double bond (a sigma (σ) and a pi (π) bond) For convenience, the double bond can be shown as two dashes. How many bonding pairs in 2? How many lone pairs in 2? = This is the oxygen molecule, 2 σ and π Bonds Single bond (Cl 2 ) has one σ bond Bond directly between the atoms Double bond ( 2 and C 2 H 4 ) have two bonds one σ and one π σ bond goes directly between the atoms π bond goes above the axis

6 σ and π Bonds in a Molecule Strength of Covalent Bonds The energy required to break a covalent bond is called the bond dissociation energy. As number of bonds increase, length gets shorter and bond energy increases. So, H H bonds are fairly easy to break and makes hydrogen gas reactive. Nitrogen has a triple bond, which takes a lot of energy to break and is pretty much inert Strength of Covalent Bonds Exothermic reactions release energy. They occur when the bonds of the products are more stable than the bonds of the reactants. In other words, energy is released forming the new bonds in the products. Example: CH4 + 2 H2 + C2 + energy Endothermic reactions absorb energy. The occur when bonds of the reactants are more stable than the bonds of the products. In other words, energy has to be entered to make the products. Example: 2H2(l) + energy 2H2(g) + 2(g) Chapter 8.2 Formulas and Names Rules for Naming Binary Inorganic Compounds Write out the name of the first nonmetal (Left most or bottom most first) Follow it by the name of the second nonmetal and end in ide Add a prefix to the name of each element to denote how many are present. Example: S 2 is sulfur dioxide because sulfur is below oxygen in the same group Prefixes for Molecular Compounds Number of Atoms Prefix 1 Mono- 2 Di- 3 Tri- 4 Tetra- 5 Penta- 6 Hexa- 7 Hepta- 8 cta- 9 Nona- 10 Deca- 35 Formulas and Names Rules for Naming Binary Inorganic Compounds mit mono if there is only one atom of the first element If o o or a o vowels appear next to each other, the first of the pair is omitted for easier pronunciation. Example: N is Nitrogen Monoxide rather than mononitrogen monooxide 36 6

7 Practice Formulas and Names Formula N Name More Practice Formula or Name CCl 4 Name or Formula N 2 N 2 C Diarsenic Trioxide N 2 5 Sulfur hexafluoride P Common Names Some molecules are so common they have common names. What is the common name of dihydrogen monoxide? Another common compound is NH 3. What is this commonly called? Common Names of Compounds Acids HCl Hydrochloric Acid H 2 S 4 Sulfuric Acid H 3 P 4 Phosphoric Acid HN 3 Nitric Acid HC 2 H 3 2 Acetic Acid (vinegar) Common Names of Compounds Bases NaH Sodium Hydroxide KH Potassium Hydroxide NH 3 Ammonia rganic Compounds rganic Compounds Made up of carbon Carbon is able to bond with other carbon atoms to form long chains, rings, sheets, and larger networks Carbon forms 4 covalent bonds The simplest with 4 hydrogens to form methane (CH 4 )

8 rganic Compounds Hydrocarbons compounds containing just hydrogen and carbon The first was methane (CH 4 ) The names of others change as more carbons are added. rganic Compounds Formula Name #of Carbons CH 4 Methane 1 C 2 H 6 Ethane 2 C 3 H 8 Propane 3 C 4 H 10 Butane 4 C 5 H 12 Pentane 5 C 6 H 14 Hexane 6 C 7 H 16 Heptane 7 C 8 H 18 ctane 8 C 9 H 20 Nonane 9 C 10 H 22 Decane Ch. 8.3 Molecular Structures Structural Formulas use letter symbols and bond symbols to show relative positions of atoms. You have already done some like H H, =, N N, HCl, and CCl4 What happens when there are more than two types of atoms and/or lone pairs are involved? Apply to C 2 Central atom: C Total number valence electrons: 16 (4 from C and 6 each from ) Draw skeleton structure: C Place electrons on s form octet All atoms have octet? Draw final structure Try S 2 (neutral molecule) Central atom S Total number valence electrons 18 Draw skeleton structure S Place electrons on s form octet Place remaining electrons on S All atoms have octet? No? need to form multiple bonds Draw final structure Try Cl 4 (anion) Central atom Total valence electrons Skeleton structure

9 Cl 4 (anion) Place remaining electrons on s Place remaining electrons on Cl. All atoms have an octet? Ch. 8.4 Molecular Shapes The shape of a molecule determines many of its physical and chemical properties. Molecular geometry (shape) can be determined with the Valence Shell Electron Pair Repulsion model, or VSEPR model which minimizes the repulsion of shared and unshared atoms around the central atom Molecular Shapes Electron pairs repel each other and cause molecules to be in fixed positions relative to each other. Unshared electron pairs also determine the shape of a molecule. Electron pairs are located in a molecule as far apart as they can be Practice ther examples from PhET. Draw the Lewis Structures and note the shapes of the following: NF3 CS2 BH3 NH4 + S

10 Exceptions to the ctet Rule There are three classes of exceptions to the octet rule: Molecules with an odd number of electrons; Molecules in which one atom has less than an octet; Molecules in which one atom has more than an octet. dd Number of Electrons Few examples. Generally molecules such as Cl 2, N, and N 2 have an odd number of electrons. Exceptions to the ctet Rule Less than an ctet Relatively rare. Molecules with less than an octet are typical for compounds of Groups 1, 2, and 13 (3A). Most typical example is BF Exceptions to the ctet Rule More than an ctet This is the largest class of exceptions. Atoms from the 3 rd period on down can accommodate more than an octet. From the third period, the d-orbitals are low enough in energy to participate in bonding and accept the extra electron density. Example: SF 6 Ch. 8.5 and Polarity Atoms form bonds to increase their stability. They acquire an octet of electrons and get noble gas configuration. The types of bonds we looked at so for are... Ionic and Covalent and Polarity Bonding involves a sharing of electrons. Some are shared equally, slightly or none at all. Ionic bonds sharing is so unequal that it is considered a transfer of electrons Covalent Pure covalent bonds are shared equally, such as with 2 or Cl 2 a measure of the ability of an atom in a bond to attract electrons. You can make a comparison between two atoms. The difference in electronegativity (ΔEN, pronounced delta E N ) tells you the type of bond. Most bonds fall in between

11 The higher the number, the more electronegative the atom is, i.e. the more it wants the electrons. increases across a period. Why? Protons increase in the nucleus, the same energy level being filled and the pull of the nucleus increases the attraction for the valence electrons. (Higher Z* across a period) Use your table to find ΔEN for the following ionic compounds. LiF NaCl CsF KBr ΔEN = 3.0 ΔEN = 2.1 ΔEN = 3.3 ΔEN = 2.0 Which one is most ionic? Which one is least ionic? What happens when the electronegativity numbers are similar? Then the difference between them is very small. The bond is then more described as sharing between the atoms and the bond is covalent. What happens when the electronegativity is the same? ΔEN = 0 and the bond is described as pure covalent bond. Ex: The F F bond. EN for F = 4.0 and 4.0 for the other F. So, ΔEN = 0. All electrons are shared equally for the other diatomic molecules What happens when the ΔEN is greater than 0 and 0.5? The bonds are still considered covalent. The compounds they make tend to be gases or low boiling liquids at room temperature. Example: Find ΔEN for C H C EN =2.5; HEN =2.1 ; ΔEN = 0.4 The molecules CH 4, C 2 H 6, C 3 H 8 are gases at RT. So, ionic bonds are on our scale are between ΔEN 2.0 and ΔEN=3.3. (Mostly) Covalent bonds are between ΔEN=0.0 and ΔEN 0.5. (Pure Covalent bonds are ΔEN=0.0) What happens between 0.5 and 2.0?

12 Between ΔEN > 0.5 and ΔEN < 2.0, there is a partial transfer of the shared electrons to the more electronegative element. The bond that forms is called polar covalent. A polar covalent bond has some degree of ionic character. So what happens in a polar covalent bond? The electron spends time around each element, but more time around the more electronegative element. ΔEN= EN (Cl) EN(H) = = 0.9 δ+ δ H Cl Bond Polarity HCl is PLAR because it has a positive end and a negative end. (difference in electronegativity) Cl has a greater share in bonding electrons than does H. Cl has slight negative charge (- ) and H has slight positive charge (+ ) 69 Practice Calculate the EN and classify the type of bond Pair of Atoms Ca S Ba C Br Ca F H Br EN and Bond Type 70 What is the ΔEN for H bonds in water? EN = H EN = Δ EN = ΔEN=1.4 ΔEN=0.0 How does this affect the properties of water? ΔEN=0.4 ΔEN=

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