To understand molecular structure and bond angles

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.

A2, AB, AB2, AB3, AB4, AB5, AB6

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, H2O - bent

Section 12.4

Structure of Molecules

A. Molecular Structure

• Linear structure – atoms in a line

– Carbon dioxide, CO2

Section 12.4

Structure of Molecules

A. Molecular Structure

• Trigonal planar – atoms in a triangle

– Boron Trifluoride, BF3

Section 12.4

Structure of Molecules

A. Molecular Structure

• Tetrahedral structure

– Methane, CH4

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 balloons 3 balloons 4 balloons 5 balloons 6 balloons

Section 12.4

Structure of Molecules

B. The VSEPR Model

– 180o - linear

Two Pairs of Electrons

• BeCl2

Section 12.4

Structure of Molecules

B. The VSEPR Model

– 120o – trigonal planar

Three Pairs of Electrons

• BF3

Section 12.4

Structure of Molecules

B. The VSEPR Model

– 109.5o – tetrahedral

Four Pairs of Electrons

• CH4

Section 12.4

Structure of Molecules

B. The VSEPR Model

Predicting Molecular Structure using the VSEPR model1.Draw the Lewis structure2.Count the pairs of electrons and arrange to minimize repulsions3.Determine the positions of the atoms4.Name the molecular structure according to the position of the atoms not the lone electron pairs

e.g. NH3 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

H2O

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 120o