Ionic vs. Covalent Compounds

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1 Ionic vs. Covalent Compounds 7

2 Electron Dot Diagrams American Chemist, G. N. Lewis (1916), developed a system of representing the valence electrons with dots Electron Dot Structures - Valence electrons are represented as dots, the nucleus and inner electrons are represented by the symbol for the element Note: The dots only represent the outermost s and p orbital electrons. Each side of the symbol represents one orbital - one s and three p Hydrogen 1s 1 H. Lithium 2s 1 Li Beryllium 2s 2 Be Sodium 3s 1 Na Magnesium 3s 2 Mg. 8

3 Electron Dot Diagrams - Ions Sodium Ion 2s 2 2p 6 +1 : Na : Magnesium Ion 2s 2 2p 6 2+ : Mg : Fluorine 2s 2 2p 5 : F 1 Fluorine Ion 2s 2 2p 6 : F : 9

4 Covalent Bonds Single Covalent Bond - Is formed when a single pair of electrons is shared by two atoms H 2 is formed by a covalent single bond. Water H 2 O H : O : H H:H Water has two pairs of shared electrons and two pairs of unshared electrons Double Covalent Bond - Is formed when two pairs of electrons are shared by two atoms O 2 :O::O: 10

5 Covalent Bonds (con t) Triple Covalent Bond - Is formed when three pairs of electrons are shared by two atoms N 2 :N::N: Bond Energy - When a covalent bond forms from free atoms energy is released to the surroundings. This means that the resulting energy stored in the bond is less than that stored in the free atoms. The amount of energy released is called the bond energy. This is also the amount of energy needed to break the bond. The higher the bond energy the more stable the bond. The higher the bond energy the shorter the bond... 11

6 Electron Dot Diagrams Compounds Bonds in compounds can be represented several ways. The short hand version is the chemical formula: H 2 O, CH 3 CH 2 OH (ethanol), O 2. We can also show each of the bonds. Each dash represents two electrons and one bond. Water Ethanol H H H O H H C C O H H H For Oxygen it would be O=O 12

7 Building Electron Dot Diagrams How can you tell which atoms are connected to which? 1. If the formula contains only one atom of an element and several atoms of another element, the single atom is in the center, with the other atoms bonded to the central atom. 2. If it is not obvious which atom is in the center, build the structure that seems the most symmetrical. 3. How many dots should appear in the structure? The # of dots should be equal to the total # of valence electrons. 4. Where do the dots go? Place the dots around the nonmetallic elements so that each has eight electrons - an octet structure. 13

8 Polyatomic Ions More than one atom covalently bonded with an ionic charge 14

9 Electron Dot - Polyatomic Ions Hydroxide (polyatomic ion - OH ) has 6 valence electrons from the Oxygen, 1 from the hydrogen and 1 from the negative charge for a total of eight valence electrons Hydroxide (OH ) Ammonium (NH + 4 ) [:O H] H + H N H H Ammonium has 5 valence electrons from the Nitrogen, 1 from each of the four hydrogens; take away 1 for the positive charge for a total of eight valence electrons 15

10 Covalent Compounds When an atom forms a covalent bond it only shares one pair of electrons in each bond (a double bond is two separate bonds) Coordinate Covalent Bond - is formed when one atom contributes both electrons in a covalent bond In carbon monoxide, oxygen has a stable configuration but the carbon does not As shown, the problem is solved if the oxygen donates one of its unshared pairs of electrons for bonding 16

11 Coordinate Covalent Compounds 17

12 Exceptions to the Octet Rule Molecules that contain an odd number of valence electrons cannot be drawn properly with Lewis Dot Structures Nitrogen Dioxide 18

13 Exceptions to the Octet Rule Some structures have fewer than eight valence electrons in the central atom. BF 3 which has Boron as the central atom. Boron has three valence electrons and can bond in the gaseous state to three fluoride atoms resulting in six valence electrons around the central atom. Some central atoms can be surrounded by more than eight valence electrons. Examples of this are PCl 5 and SF 6 19

14 Sharing of Electrons Just because the electrons are shared, they aren t always shared equally. How the electrons are shared is determined by the electronegativity difference between the two atoms in the bond. Electronegativity difference Bond Type between the two atoms x 0.5 nonpolar covalent 0.5 < x < 1.7 polar covalent x 1.7 ionic 20

15 Non-Polar Molecules: Non-Polar Bonds Given a table of electronegativities you will be expected to determine if a bond is polar, non-polar or ionic. Among the best examples of non-polar molecular are bonds between identical atoms such as: H 2, O 2, F 2, Br 2, I 2, N 2, Cl 2 The larger the difference between the electronegativities the more unequal the sharing of the electrons. In an extreme case, Ionic Bonding, the electrons are completely transferred to the atom with the higher electronegativity. 21

16 Polar Molecules One atom of a polar bond has a partial negative characteristic, the other a partial positive characteristic One of the best examples of this is Water H has an electronegativity of 2.1 O has an electronegativity of H + H Partial Positive side O Partial Negative side Since one side of the molecule is positive and the other is negative, the molecule is called a dipole. 22

17 Non-Polar Molecules: Polar Bonds A polar bond does not always create a dipole Carbon Dioxide CO 2 O = C = O + Since there is no way to draw a line that divides the molecule into positive and negative sides, the poles of the molecule are the same charge and therefore it is not a polar molecule 23

18 Dipole-Dipole Interactions Occur when polar molecules are attracted to one another. The partial positive and partial negative atoms can attract each other. These are similar to but much weaker than ionic bonds. 24

19 Hydrogen Bonds The strongest of the dipole-dipole interactions. The partial positive H is attracted to the unshared pairs of electrons of a nearby highly electronegative atom (specifically N, O or F). Hydrogen bonds are about 5-10% as strong as covalent bonds + H + H O O H + + H 25

20 Electron Pair Repulsion VSEPR - Valence Shell Electron Pair Repulsion since electrons all have the same charge they repel; this causes the shared pair of electrons to move as far apart as possible adjusting the shape of the molecule 26

21 Tetrahedral H In 3D the angle separating the bonded electrons is CH 4 H C H This is the tetrahedron angle. H Key to shape: Four shared pairs (or regions) of electrons. No unshared pairs. 27

22 Pyramidal NH 3 In Ammonia, there is a unshared pair of H N H electrons on the N so the three pairs of shared electrons are H pushed closer together. Bond angle is 107. This is the pyramidal angle Key to shape: Four regions of electrons. Three shared pairs - one unshared pair. 28

23 Bent 105 H 2 O H H Water has two pairs of unshared electrons. These push the bonded O electrons closer still to an angle of 105 Key to shape: Four regions of electrons. Two shared pairs - Two unshared pairs 29

24 BF 3 F B F Trigonal Planer Boron Triflouride is an exception to the octet rule. Boron starts with three F valence electrons. When it forms the three bonds with Flourine it now has six electrons in three bonds (regions) around 120 the central atom. To have the maximum separation in space between the three sets of electrons they separate at an angle of 120. Key to shape: Three regions of electrons. Three shared pairs - zero unshared pairs. A single or double bond counts as a single region of electrons. 30

25 180 Linear CO 2 Key to shape: Two regions of electrons. Four shared pairs - zero unshared pairs. A single, double or triple bond counts as a single region of electrons. 31

26 Molecular Substances The melting and boiling points of most molecular compounds is less than those of ionic compounds Network solids (or network crystals) are solids in which all of the atoms are covalently bonded to each other Network solids consist of molecules that do not melt until the temperature reaches 1000 C or higher, or they decompose without melting at all 32

27 Graphite vs. Diamond Diamond is an example of a network solid. Diamond does not melt. It vaporizes to a gas at 3500 C or above In graphite the C atoms are bonded in planar sheets. The sheets are then held together by intermolecular forces which allows the sheets to easily slide past each other. 33

28 Unit Cells The basic arrangement in space of the ions in an ionic compound. Also called crystal structure. Cubic Face-Centered Body-Centered Coordination Number - The number of ions of opposite charge that surround each ion in a crystal

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