Molecular Geometry: The VSEPR Model

Lewis Symbols

-A Lewis symbol for an element is composed of a chemical symbol surrounded by dots that are used to represent valence electrons. An example of a Lewis symbol is shown below (Figure 3) with the element carbon, which has the electron configuration of 1s22s22p2: -This Lewis symbol shows that carbon has four valence electrons in its outer orbital, and these four electrons play a major role in bonding of carbon molecules. Lewis symbols differ slightly for ions. When forming a Lewis symbol for an ion, the chemical symbol is surrounded by dots that are used to represent valence electrons, and the whole structure is placed in square brackets with a superscript representing the charge of the ion

Lewis Theory

-Between 1916 and 1919, Gilbert Newton Lewis, Walther Kossel, and Irving Langmuir came up with a theory to explain chemical bonding. This theory would be later called Lewis Theory , and it is based on the following principles: 1. Valence electrons, or the electrons in the outermost electron shell, have an essential role in chemical bonding. 2. Ionic bonds are formed between atoms when electrons are transferred from one atom to another. Ionic bond is a bond between metals and nonmetals . 3. Covalent bonds are formed between atoms when pairs of electrons are shared between atoms. A covalent bond is between two nonmetals. 4. Electrons are transferred/shared so that each atom may reach a more stable electron configuration (e.g., the noble gas configuration, which contains eight valence electrons). This is called the octet rule

Constructing Lewis Structures for Neutral Molecules

-Electron dot diagrams (also known as Lewis Dot Structures) are commonly used to convey the structural formula of atomic and molecular bonds. In particular, they indicate the chemical bonding and lone electrons that exist within a compound 1. Determine the total number of valence electrons in the molecule. Valence electrons are represented by dots placed around the chemical symbol (group number) 2. Up to two electrons are placed on each side of the elemental symbol, for a total maximum of up to eight. 3. Place a single electron on each side of the elemental symbol before pairing them up. 4. Using a single straight, line draw out the single bonds and initial framework, called the skeleton, of the molecule. 5.Complete the octets around the non-central atoms (i.e., the terminal atoms) by using the lone-pairs of electrons. Compare the number of electrons currently depicted to the number needed in the central atom. 6. If there are extra lone-pair electrons and the octet rule is not filled for the central atom, use the extra electrons to form double or triple bonds around the central atom -Note: that if you are working with a compound that has a positive or negative charge association, then the charge needs to be included in the total electron count.

Valence Shell Electron Pair Repulsion (VSEPR)

-Molecular geometry is the three-dimensional manner in which the atoms of a molecule project into space -The shape of a molecule is important because it affects such physical properties as solubility, melting point, and boiling point. The shapes of molecules can be predicted from their Lewis structures by using a model developed about 30 years ago, known as the Valence Shell Electron Pair Repulsion (VSEPR) theory. -The VSEPR concept revolves around the idea that the electrons in a molecule repel each other and will try to get as far away from each other as possible. The electrons are "attached" to a central atom in the molecule and can "pivot" freely on the atom's surface to move away from the other electrons. Electrons come in several flavors: -Bonding pairs —This set of two electrons is involved in a bond, so we will write the two dots between two atoms. This applies to single, double, and triple bonds. -Nonbonding pairs —Also known as a lone pair. -Single electrons —In almost every case, this single electron will be nonbonding. Almost 100% of the examples will involve pairs, but there are some examples that involve a lone electron. -The VSEPR theory assumes that each atom in a molecule will achieve a geometry that minimizes the repulsion between electrons in the valence shell of that atom. The five compounds shown in Figure 4 can be used to demonstrate how the VSEPR theory can be applied to simple molecules.

Octet Rule

-The octet rule states that elements gain or lose electrons to attain an electron configuration of the nearest noble gas. -Atoms tend to form bonds in such a way as to satisfy the octet rule, with every atom surrounded by eight electrons. Noble gases have complete outer electron shells, which make them very stable. -Other elements also seek stability, which governs their reactivity and bonding behavior. For example, elements in column 7 (the halogens) are one electron away from filled energy levels, so they are very reactive. -Fluorine, for example, has seven electrons in its outer electron shell. Fluorine readily bonds with other elements so that it can have a filled energy level, like neon

Introduction

-When two atoms approach each other and react with each other, it is their outer shells that come into contact first; therefore, the electrons in their outer shells are normally involved in any chemical reaction. These electrons are known as valence electrons. So, it is the number of valence electrons in an atom's outer shell that determine, to a large extent, how that element will react chemically. The periodic table is arranged such that the main group elements (i.e., columns 1 through 8, not the transition elements) in the same column have the same number of valence electrons in their outer shell -For example, all of the elements in column 1 have one valence electron and all elements in column 8 have eight valence electrons. However, helium is the exception to this formatting.

Tetrahedral Geometry

-a central atom is located at the center, with four substituents located at the corners of a tetrahedron. The bond angles are 109.5° when all four substituents are the same -Ex: BeF2 and BF3 are both two-dimensional molecules in which the atoms lie in the same plane. If we place the same restriction on methane (CH4), we would get a square-planar geometry in which the H-C-H bond angle is 90° -If we let this system expand into three dimensions, however, we end up with a tetrahedral molecule in which the H-C-H bond angle is 109°28'

Wedge and Dash Projection

-a means of representing a molecule in which three types of lines are used in order to represent the three-dimensional structure: (1) solid lines to represent bonds that are in the plane of the paper, (2) dashed lines to represent bonds that extend away from the viewer, and (3) wedge-shaped lines to represent bonds oriented facing the viewer.

Linear Geometry

-describes the arrangement of three or more atoms placed at an expected bond angle of 180° -Ex: consider the central atom beryllium fluoride BeF2 where there are only two places in the valence shell where electrons can be found. Repulsion between these pairs of electrons can be minimized by arranging them so that they point in opposite directions. -Thus, the VSEPR theory predicts that BeF2 should be a linear molecule, with a 180° angle between the two Be-F bonds

Lewis Dot Diagram

-handy way of picturing valence electrons. The valence electrons are written as dots surrounding the symbol for the element: one dot is place on each side first, and when all four positions are filled, the remaining dots are paired with one of the first set of dots, with a maximum of two dots placed on each side

Trigonal Bipyramid Geometry

-one atom at the center and five more atoms at the corners of a triangular dipyramid -Ex: repulsion between the five pairs of valence electrons on the phosphorus atom in phosphorus pentafluoride (PF5) can be minimized by distributing these electrons toward the corners of a trigonal bipyramid. Three of the positions in a trigonal bipyramid are labeled equatorial because they lie along the equator of the molecule. The other two are axial because they lie along an axis perpendicular to the equatorial plane. The angle between the three equatorial positions is 120°, while the angle between an axial and an equatorial position is 90°

Trigonal Planar Geometry

-one atom at the center and three atoms at the corners of a triangle, called peripheral atoms, all in one plane. -There are three places on the central atom in boron trifluoride (BF3) where valence electrons can be found. Repulsion between these electrons can be minimized by arranging them toward the corners of an equilateral triangle. -The VSEPR theory, therefore, predicts a trigonal planar geometry for the BF3 molecule, with an F-B-F bond angle of 120°

Octahedral Geometry

-six atoms are symmetrically arranged around a central atom -Ex: there are six places on the central atom in sulfur hexafluoride (SF6) where valence electrons can be found. The repulsion between these electrons can be minimized by distributing them toward the corners of an octahedron. The term octahedron literally means "eight sides," but it is the six corners, or vertices, that interest us. To imagine the geometry of an SF6 molecule, locate fluorine atoms on opposite sides of the sulfur atom along the X, Y, and Z axes of an XYZ coordinate system