Bellringer #lewis #bonding • What type of bonds do these elements form? • NaF, KI, CO2, HF, CCl4, H2O, CaCl2 • Draw the Lewis Bonding Structures for: CCl3OH, SiS2, N2, N2F2 Electronegativity What is it? • Electronegativity is the power of an atom to attract electron density in a covalent bond Remember the definition … … look for where you get the ticks! Electronegativity Pauling’s electronegativity scale • The higher the value, the more electronegative the element • Fluorine is the most electronegative element • It has an electronegativity value of 4.0 Electronegativity Pauling’s electronegativity scale Electronegativity Pauling’s electronegativity scale H 2.1 He - Li 1.0 Be 1.5 B 2.0 C 2.5 N 3.0 O 3.5 F 4.0 Ne - Na 0.9 Mg 1.2 Al 1.5 Si 1.8 P 2.1 S 2.5 Cl 3.0 Ar - Chemical Bonding and VSEPR •6 The Shapes of Molecules • The shape of a molecule has an important bearing on its reactivity and behavior. • The shape of a molecule depends a number of factors. These include: 1. Atoms forming the bonds 2. Bond distance 3. Bond angles •7 Valence Shell Electron Pair Repulsion • Valence Shell Electron Pair Repulsion (VSEPR) theory can be used to predict the geometric shapes of molecules. • VSEPR is revolves around the principle that electrons repel each other. • One can predict the shape of a molecule by finding a pattern where electron pairs are as far from each other as possible. •8 Bonding Electrons and Lone Pairs • In a molecule some of the valence electrons are shared between atoms to form covalent bonds. These are called bonding electrons. • Other valence electrons may not be shared with other atoms. These are called non-bonding electrons or they are often referred to as lone pairs. •9 VSEPR • In all covalent molecules electrons will tend to stay as far away from each other as possible • The shape of a molecule therefore depends on: 1. the number of regions of electron density it has on its central atom, 2. whether these are bonding or non-bonding electrons. •10 Lewis Dot Structures • Lewis Dot structures are used to represent the valence electrons of atoms in covalent molecules • Dots are used to represent only the valence electrons. • Dots are written between symbols to represent bonding electrons •11 Violations of the Octet Rule Violations of the octet rule usually occur with B and elements of higher periods. Some common examples include: Be, B, P, S, and Xe. •Be: 4 •B: •BF3 6 •P: 8 OR 10 •S: 8, 10, OR 12 •SF4 VSEPR Predicting Shapes VSEPR: Predicting the shape • Once the dot structure has been established, the shape of the molecule will follow one of basic shapes depending on: 1. The number of regions of electron density around the central atom 2. The number of regions of electron density that are occupied by bonding electrons •14 VSEPR: Predicting the shape • • The number of regions of electron density around the central atom determines the electron structure The number of regions of electron density that are occupied by bonding electrons and hence other atoms determines the actual molecular shape •15 Basic Molecular shapes The most common shapes of molecules are shown at the right •16 Linear Molecules Linear molecules have only two regions of electron density. •17 Angular or Bent Angular or bent molecules have at least 3 regions of electron density, but only two are occupied •18 Triangular Plane Triangular planar molecules have three regions of electron density. All are occupied by other atoms •19 Tetrahedron Tetrahedral molecules have four regions of electron density. All are occupied by other atoms •20 Trigonal Bipyramid A few molecules have expanded valence shells around the central atom. Hence there are five pairs of valence electrons. The structure of such molecules with five pairs around one is called trigonal bipyramid. •21 Octahedron A few molecules have valence shells around the central atom that are expanded to as many as six pairs or twelve electrons. These shapes are known as octahedrons •22 Molecular Polarity Molecular Polarity depends on: 1. the relative electronegativities of the atoms in the molecule 2. The shape of the molecule 3. Molecules that have symmetrical charge distributions are usually non-polar •23 Non-polar Molecules •The electron density plot for H2. • Two identical atoms do not have an electronegativity difference The charge distribution is symmetrical. • The molecule is non-polar. •24 Polar Molecules •The electron density plot for HCl • • • • Chlorine is more electronegative than Hydrogen The electron cloud is distorted toward Chlorine The unsymmetrical cloud has a dipole moment HCl is a polar molecule. •25 Molecular Polarity To be polar a molecule must: 1. have polar bonds 2. have the polar bonds arranged in such a way that their polarity is not cancelled out 3. When the charge distribution is nonsymmetrical, the electrons are pulled to one side of the molecule 4. The molecule is said to have a dipole moment. • HF and H2O are both polar molecules. CCl4 is non-polar •26 Don’t Forget • Chapter 8 HW packet due Thursday and Friday of this week!
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