4/25/2017. VSEPR Theory. Two Electron Groups. Shapes of Molecules. Two Electron Groups with Double Bonds. Three Electron Groups.

Transcription

1 Chapter 10 Lecture Chapter 10 Bonding and Properties of Solids and Liquids 10.3 Shapes of Molecules and Ions (VSEPR Theory) Learning Goal Predict the three-dimensional structure of a molecule or a polyatomic ion and classify it as polar or nonpolar. Fifth Edition VSEPR Theory In the valence-shell electron-pair repulsion theory (VSEPR), the electron groups around a central atom are arranged as far apart from each other as possible have the least amount of repulsion of the negatively charged electrons have a geometry around the central atom that determines the molecular shape 2 Shapes of Molecules Two Electron Groups The three-dimensional shape of a molecule is the result of bonded groups and lone pairs of electrons around the central atom is predicted using the VSEPR (valence-shell electron-pair repulsion) theory In BeCl 2 two electron groups are bonded to the central atom Be (exception to the octet rule) repulsion is minimized when the two electron groups are opposite each other at 180 the shape of the BeCl 2 molecule is linear 3 4 Two Electron Groups with Double Bonds In CO 2 two electron groups are bonded to C (electrons in a double bond count as one group) minimal repulsion occurs when the two electron groups are opposite each other (180 ) the shape of the CO 2 molecule is linear Three Electron Groups In BF 3 three electron groups surround the central atom B (B is an exception to the octet rule) minimal repulsion occurs when the three electron groups are at angles of 120 the shape of the BF 3 molecule is trigonal planar 5 6 1

2 Two Electron Groups and a Lone Pair Four Electron Groups In SO 2 three electron groups, two bonded groups and one lone pair, surround the S atom repulsion is minimized with three electron groups at angles of 120, a trigonal planar arrangement. with two O atoms bonded to S and one lone pair of electrons, the shape of the SO 2 molecule is bent (120 ) In a molecule of CH 4 four electron groups are bonded to a C atom repulsion is minimized by placing the four electron groups at angles of 109, a tetrahedral arrangement the shape with four bonded atoms is tetrahedral 7 8 Three Bonding Atoms and One Lone Pair In a molecule of NH 3 four electron groups, three bonding groups, and one lone pair, surround a N atom repulsion is minimized with four electron groups at angles of 109, which is a tetrahedral arrangement of electron groups with three bonded atoms and one lone pair of electrons, the shape is trigonal pyramidal Two Bonding Atoms and Two Lone Pairs In a molecule of H 2 O, four electron groups, two groups bonded to H atoms and two lone pairs, surround the O atom four electron groups minimize repulsion in a tetrahedral arrangement the shape of the H 2 O molecule with two bonded atoms is bent(109 ) 9 10 Shapes with Two or Three Electron Groups Shapes with Four Electron Groups

3 Predicting Molecular Shape (VSEPR Theory) The shape of a molecule of N 2 O (N N O) is 1) linear 2) trigonal planar 3) bent (120 ) State the number of electron groups, lone pairs, bonded atoms, and use VSEPR theory to determine the shape of the following molecules or ions: 1) tetrahedral 2) pyramidal 3) bent A. PF 3 B. H 2 S Chapter 10 Lecture Chapter 10 Bonding and Properties of Solids and Liquids 10.4 Electronegativity and Bond Polarity Fifth Edition C. CCl 4 D. PO Learning Goal Use electronegativity to determine the polarity of a bond. Electronegativity Electronegativity is the relative ability of atoms to attract shared electrons is higher for nonmetals, with fluorine as the highest with a value of 4.0 is lower for metals, with cesium and francium as the lowest with a value of 0.7 increases from left to right going across a period on the periodic table decreases going down a group on the periodic table Some Electronegativity Values for Group A Elements

4 Nonpolar Covalent Bonds Using the periodic table, predict the order of increasing electronegativity for the elements O, K, and C. A nonpolar covalent bond occurs between nonmetal atoms consists of an equal (or almost equal) sharing of electrons has a zero (or close to zero) electronegativity difference of 0.0 to 0.4 Examples: Atoms Electronegativity Type of Bond Difference N N = 0.0 Nonpolar covalent Cl Br = 0.2 Nonpolar covalent H Si = 0.3 Nonpolar covalent Polar Covalent Bonds A polar covalent bond occurs between nonmetal atoms consists of atoms that share electrons unequally has an electronegativity difference range of 0.5 to 1.7 Examples: Atoms Electronegativity Type of Bond Difference O Cl = 0.5 Polar covalent Cl C = 0.5 Polar covalent O S = 1.0 Polar covalent Comparing Nonpolar and Polar Covalent Bonds Dipoles and Bond Polarity Bonds become more polar as the difference in electronegativity increases. A polar covalent bond that has a separation of charges is called a dipole. The positive and negative ends are represented by the Greek letter delta with a + or charge. Arrows can also be used to represent dipoles. Ionic Bonds An ionic bond occurs between metal and nonmetal ions is a result of electron transfer has a large electronegativity difference (1.8 or more) Examples: Atoms Electronegativity Type of Bond Difference Cl K = 2.2 Ionic N Na = 2.1 Ionic S Cs = 1.8 Ionic 24 4

5 Electronegativity and Bond Types Predicting Bond Types Use electronegativity differences to classify each of the following bonds as nonpolar covalent (NP), polar covalent (P), or ionic (I): A bond between A. K and N B. N and O C. Cl and Cl D. H and Cl 27 Chapter 10 Lecture Chapter 10 Bonding and Properties of Solids and Liquids 10.5 Polarity of Molecules Learning Goal Use the three-dimensional structure of a molecule to classify it as polar or nonpolar. Fifth Edition Nonpolar Molecules A nonpolar molecule may contain identical atoms (nonpolar bonds) may have a symmetrical arrangement of polar bonds that cancel dipoles Polar Molecules A polar molecule contains polar bonds has a separation of positive and negative charge called a dipole indicated by a dipole arrow has dipoles that do not cancel

6 Determining Molecular Polarity The polarity of a molecule is determined from its electron-dot formula shape polarity of the bonds dipole cancellation Identify each of the following molecules as (P) polar or (NP) nonpolar: A. PBr 3 B. HBr C. Br 2 D. SiBr