VSEPR Theory PDF

VSEPR Theory  VSEPR – Group 1: BeI , HgCl , CO 2 2 2 – Group 2: AlCl3, SO2, O3 – Group 3: CH4, OF2, PCl3 – Group 4: PCl5, SBr2, XeF2 – Group 5: SF6, ClF5, XeF4 In groups, draw the Lewis structure of your compounds,...

VSEPR Theory  VSEPR – Group 1: BeI , HgCl , CO 2 2 2 – Group 2: AlCl3, SO2, O3 – Group 3: CH4, OF2, PCl3 – Group 4: PCl5, SBr2, XeF2 – Group 5: SF6, ClF5, XeF4 In groups, draw the Lewis structure of your compounds, observe the groups attached to the central atom, and the groups of lone pairs. Create a chart and be prepared to discuss your findings Molecule Lewis Structure Groups Bonded Lone pairs to Central Atom BeF2 F Be F 2 0 HgCl2 CO2  Steps to draw Lewis Structure 1. Decide which atoms are bonded 2. Count ALL valence electrons 3. Place two electrons in each bond 4. Complete the octets of the atoms attached to the central atom by adding the electrons in pair 5. Place the remaining electrons on the central atom in pair 6. If the central atom doesn’t have 8 electrons form double, triple bonds.  VSEPR One of the important ways that molecular compounds differ from ionic compounds is their structures. Ionic bonding is nondirectional. Covalent bonds are highly directional. For a central atom in a molecule there are preferred orientations for the atoms attached to it. As a result , in polyatomic molecules atoms remain in the same orientation regardless their state.  VSEPR Why the shape of the molecule is important? Most of the physical and chemical properties depend upon the three-dimensional arrangements of their atoms. Example: Enzyme But, how can we determine the shapes of the compounds?  VSEPR ► Remember… Atoms are bound together by electron pairs called bonding pairs ► These can be single (one pair e- = single bond) or multiple (2 pair e- = double bond; 3 pair e- = triple bond) ► Some atoms in a molecule can also have pairs of electrons not involved in bonding called lone pairs or non-bonded pairs  VSEPR Three types of repulsions take place in an atom ► Lone Pair – Lone Pair (LP-LP) ► Lone Pair – Bonding Pair (LP-BP) ► Bonding Pair – Bonding Pair (BP-BP) Lone pairs occupy more space than bonding electron pairs ► Double bonds occupy more space than a single bond  VSEPR The best theoretical explanation of molecular shapes are based on the quantum mechanics. There is a theory quite simple, but remarkably efficient effective in predicting the shapes of the molecules called VSEPR VSEPR (Valence Shell Electron Pair Repulsion) VSEPR theory states that repulsion between the sets of valence-shell electrons surrounding an atom causes these sets to be oriented as far apart as possible  VSEPR ► A molecule must avoid these repulsions to remain stable. ► When repulsion cannot be avoided, the weaker repulsion (i.e. the one that causes the smallest deviation from the ideal shape) is preferred. ► Lone pair-lone pair (LP-LP) repulsion is considered to be stronger than the lone pair-bonding pair (LP-BP) repulsion, which in turn is stronger than the bonding pair-bonding pair (BP-BP) repulsion. LP-LP > LP-BP > BP-BP  Things to remember When assigning a VSEPR Shape to a molecule, we focus on the central atom and the bonding pairs or lone pairs associated with it Electron pairs are considered to exist in a domain Domains can be made up of : A lone pair A single bond A double bond A triple bond and ALL should be considered as one electron pair Example  Steps to determined VSEPR shape 1. Draw the Lewis structure of the molecule 2. Determine the central atom (the least electronegative) 3. Determine the number of bonding pairs 4. Determine the number of lone pairs 5. Consult the VSEPR chart to find the shape Molecules with the central atom with an incomplete octet Molecules that only have 2 bonding pairs on the central atom will have a LINEAR SHAPE with a bond angle of 180° Molecules that only have 3 bonding pairs on the central atom will have a TRIANGULAR (TRIGONAL) PLANAR SHAPE with bond angles of 120° Example BeF2 Example BF3  Molecules with the central atom surrounded by four bonding pairs (i.e. four atoms) If the central atom is placed at the center of a sphere, than each of the four pairs of electrons will occupy a position to be as far apart as possible. This will result in the electron pairs being at the corners of a regular tetrahedron, therefore these molecules are said to have a TETRAHEDRAL SHAPE. (Pyramid with four triangular faces and 4 corners) The angle between each bond will be 109.5° Example CCl4 Molecules with the central atom surrounded by 5 pairs of bonding pairs As with the tetrahedral molecules, the electron pairs will arrange themselves as far apart as possible. To achieve this, the atoms will arrange themselves in a TRIANGULAR (TRIGONAL) BIPYRAMIDAL SHAPE which consists of 2 three-sided pyramids sharing the same base. In this type of molecule, the 3 atoms making the base will lie in the same plane with the central atom in the middle of it. The other atoms will be positioned above and below this plane. The bond angles within the base will 120° and the bond between the other atoms and the base will be 90°. Example PF5 Molecules with the central atom surrounded by 6 pairs of bonding pairs In order for the 6 pairs of electrons to be as far apart as possible in this case, each bonding pair will be at one corner of a REGULAR OCTAGON (two square pyramids that share a common square base) The central atom is at the center of a square plane made up of 4 atoms and the other 2 atoms will be placed above and below the plane. All bond angles will be 90° Example SF6  Molecules with the central atom surrounded by 2 bonding pairs and 2 non-bonding pairs The four pairs of electrons will be arranged tetrahedrally but since only 2 pairs are bonding electrons, the surrounding atoms are at 2 corners of the tetrahedron. As a result these molecules will have a BENT OR V-SHAPE. The repulsion between the non-bonding pairs will result in a bond angle of 104.5°. For each pair of non-bonding electrons, the bond angle decreases by 2.5° Example H2O Molecules and Covalent Compounds Electronegativity and Bond Polarity Polarity of Molecules Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings 24  Electronegativity Electronegativity values: Indicate the attraction of an atom for shared electrons Increase from left to right going across a period on the periodic table Is high for the nonmetals with fluorine as the highest Is low for the metals 25  Some Electronegativity Values for Group A Elements High values Low values Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings 26  Nonpolar Covalent Bonds A nonpolar covalent bond, Occurs between nonmetals Is an equal or almost equal sharing of electrons Examples: Atoms Electronegativity Type of Bond Difference N-N 3.0 - 3.0 = 0.0 Nonpolar covalent Cl-Br 3.0 - 2.8 = 0.2 polar covalent H-Si 2.1 - 1.8 = 0.3 polar covalent 27  Polar Covalent Bonds A polar covalent bond, Occurs between nonmetals atoms Is an unequal sharing of electrons Has a moderate electronegativity difference (0 to 1.7) Examples: Atoms Electronegativity Type of Bond Difference O-Cl 3.5 - 3.0 = 0.5 Polar covalent Cl-C 3.0 - 2.5 = 0.5 Polar covalent O-S 3.5 - 2.5 = 1.0 Polar covalent 28 Comparing Nonpolar and Polar Covalent Bonds Copyright © 2005 by Pearson Education, Inc. Publishing as Benjamin Cummings 29 Range of Bond Types 30 Predicting Bond Types 31  Polarity of molecules Now that we learn about bonding types , have a look at another concept: polarity of molecules. To start with we have two types of molecules: - Non -polar molecule - Polar molecule How can we identify them?  Polarity of Molecules—Nonpolar In a nonpolar molecule - all the bonds are nonpolar, or - the polar bonds (dipoles) cancel each other out. Molecules such as H , Cl and CH are 2 2 4 nonpolar because they contain only nonpolar bonds. 33  Polarity of Molecules—Nonpolar A nonpolar molecule also occurs when polar bonds (dipoles) cancel each other because of a symmetrical arrangement. Molecules such as CO and CCl contain polar 2 4 bonds with dipoles that cancel each other out.  Polarity of Molecules—polar A polar molecule occurs when the dipoles from individual bonds do not cancel each other out. For molecules with two or more electron groups, the shape (such as bent or trigonal pyrimidal) determines whether or not the dipoles cancel.  Polarity of Molecules—polar Examples of polar molecules include HCl, H O, and NH. 2 3 HCl is linear and contains a polar bond.  Polarity of Molecules—polar H O is bent and contains two polar bonds 2 as well as two lone pairs on oxygen Guide to Determination of Polarity of a Molecule  Learning Check Determine if the molecule OF is polar or 2 nonpolar  Solution Determine if the molecule OF is polar or 2 nonpolar. Step 1 Determine if the bonds are polar or nonpolar covalent. Oxygen has an electronegativity of 3.5, and fluorine has an electronegativity of 4.0. O—F bonds are polar covalent. Step 2 If the bondsSolution are polar covalent, draw the electron-dot formula and determine if the dipoles cancel. Dipoles in O—F bonds do not cancel; the molecule is polar.