Section 12.4 Structure of Molecules 1.To understand molecular structure and bond angles 2.To learn to predict molecular geometry from the number of electron pairs 3.To learn to apply the VSEPR model to molecules with double bonds Objectives

Section 12.4 Structure of Molecules Hemoglobin is the iron- containing oxygen- transport metalloprotein in the red cells of the blood in mammals and other animals. Hemoglobin transports oxygen from the lungs to the rest of the body, such as to the muscles, where it releases the oxygen load.

Section 12.4 Structure of Molecules Draw possible structures that you can think of for the molecules below. Think in three dimensions. A 2, AB, AB 2, AB 3, AB 4, AB 5, AB 6

Section 12.4 Structure of Molecules Structures for some simple molecules

Section 12.4 Structure of Molecules A. Molecular Structure Three dimensional arrangement of the atoms in a molecule –Water, H 2 O - bent

Section 12.4 Structure of Molecules A. Molecular Structure Linear structure – atoms in a line –Carbon dioxide, CO 2

Section 12.4 Structure of Molecules A. Molecular Structure Trigonal planar – atoms in a triangle –Boron Trifluoride, BF 3

Section 12.4 Structure of Molecules A. Molecular Structure Tetrahedral structure –Methane, CH 4

Section 12.4 Structure of Molecules B. The VSEPR Model Valence shell electron pair repulsion (VSEPR) model –Molecular structure is determined by minimizing repulsions between electron pairs –Pairs of electrons in bonds and “lone pairs” influence the molecular structure 2 balloons3 balloons 4 balloons 5 balloons6 balloons

Section 12.4 Structure of Molecules B. The VSEPR Model –180 o - linear Two Pairs of Electrons BeCl 2

Section 12.4 Structure of Molecules B. The VSEPR Model –120 o – trigonal planar Three Pairs of Electrons BF 3

Section 12.4 Structure of Molecules B. The VSEPR Model –109.5 o – tetrahedral Four Pairs of Electrons CH 4

Section 12.4 Structure of Molecules B. The VSEPR Model Predicting Molecular Structure using the VSEPR model 1.Draw the Lewis structure 2.Count the pairs of electrons and arrange to minimize repulsions 3.Determine the positions of the atoms 4.Name the molecular structure according to the position of the atoms not the lone electron pairs e.g. NH 3 has four pairs of electrons surrounding the N atom which form a tetrahedron. The molecule is named however as a trigonal pyramid

Section 12.4 Structure of Molecules B. The VSEPR Model molecular shapes 2 H2OH2O Predicting molecular shapes molecular shapes

Section 12.4 Structure of Molecules C. Molecules with Double Bonds When using VSEPR model to predict molecular geometry of a molecule a double bond is counted as the same as a single electron pair ethylene H-C-H angle is 120 o