UNIT 4 IMF’S & OCTET RULE EXCEPTIONS

Intermolecular forces are not bonding forces. They do not result in the formation of chemical compounds. They are, however, extremely important factors in the structure and properties of chemical compounds There are 3 primary forms of intermolecular forces: – Hydrogen Bonding – Dipole – Dipole Forces – Dispersion Forces (sometimes referred to as London Dispersion Forces ) INTERMOLECULAR FORCES (IMF’S)

The general relative strength of the IMF’s: – Hydrogen bonding – Dipole – Dipole Forces – Dispersion Forces Relative strength, however, can be misleading. Because not all of these forces are present in every compound, the presence of only one may have a significant affect on structure, melting point, boiling point, and other chemical and physical properties. RELATIVE STRENGTH OF IMF’S

Dipole – Dipole Forces result from the attraction of the negative end of one polar molecule with the positive end of an adjacent polar molecule. The positive end and negative ends are the result of electron densities in the molecules. DIPOLE – DIPOLE FORCES

Despite the name, hydrogen bonding is NOT a chemical bond. Hydrogen bonding is a powerful dipole – dipole force between highly polar molecules. Hydrogen bonding is the electrostatic attraction present in molecules containing highly polarized hydrogen atoms. The attraction is between the hydrogen of one molecule with atoms of nitrogen, oxygen, and fluorine atoms found in adjacent polar molecules. The naming represents the strength of the forces that are present when hydrogen bonding is present. Examples where hydrogen bonding exists; – H 2 O, NH 3, HF HYDROGEN BONDING

DIAGRAMS OF HYDROGEN BONDING

Dispersion Forces are temporary dipole – dipole forces that result during the random movement of electrons within adjacent molecules. Most dispersion forces naturally occur as a result of the momentary location of the electrons present in the molecules. Because like charges repel, the electrons of one molecule can induce a polarity in an adjacent molecule by influencing the location of the electrons in the neighboring molecule. Dispersion Forces are the only intermolecular force present in non-polar compounds. Even so, these forces can be significant. The strength of Dispersion Forces is directly related to the number of electrons present in the compound. DISPERSION FORCES

IMAGES FOR DISPERSION FORCES

The higher the number of bonds that exist between two atoms in a compound, generally the stronger the bond and the shorter the distance between bonded atoms. Triple bonds are generally shorter and stronger (contain higher bond energy) than double bonds. Double bonds are generally shorter and stronger than single bonds. RELATIVE STRENGTH OF MULTIPLE COVALENT BONDS

Types of bonding, polarity of bonds, and the number of electrons involved in bonding compounds can affect: Structure Melting points & boiling points Surface tension, viscosity, & capillary action IMPLICATIONS OF BOND TYPES AND RELATIVE STRENGTHS

The extremely high polarity of ionic compounds leads to the formation of crystal lattices to provide stability. This can also be true in covalent compounds where molecular polarity is high. An example of this is sugar. By their nature (the high degree of stability) crystal lattices generally require more energy to experience phase changes. Lattices also tend to dramatically reduce the migration of electrons. As a result ionic compounds cannot conduct electricity effectively unless the lattice is broken down through melting or solution IMPLICATIONS OF BOND TYPES AND RELATIVE STRENGTHS ON STRUCTURE

The strength of the intermolecular forces or, in the case of ionic compounds, the presence of a lattice can dramatically affect melting points or boiling points. The energy provided to support the phase change from a solid to a liquid must first overcome the intermolecular forces or lattice energy. The higher these forces are, the higher the temperature must be to produce melting The energy provided to support the phase change from liquid to gas similarly must overcome these same attractive forces leading to higher boiling points. IMPLICATIONS OF BOND TYPES AND RELATIVE STRENGTHS ON MELTING POINT & BOILING POINT

The strength of the intermolecular forces generally affects surface tension, viscosity, and capillary action of molecular compounds. All of these properties relate to how much attraction exists between adjacent molecules. Therefore, all three generally increase with the increase in the intermolecular forces. There are more properties that impact each of these properties, so while there is a direct association to intermolecular forces, these alone will not be determinate. IMPLICATIONS OF BOND TYPES AND RELATIVE STRENGTHS ON SURFACE TENSION, VISCOSITY, & CAPILLARY ACTION

There are three categories of exceptions to the octet rule in the formation of molecular compounds: – Odd number of electrons (Radicals) – Less than 8 valence electrons on the central atom (Reduced Octet) – More than 8 valence electrons on the central atom (Expanded Octet) EXCEPTIONS TO OCTET RULE IN COVALENT COMPOUNDS

When there is an odd number of total valence electrons, the central atom will not have a completed pair. Radicals are highly unstable and will readily react with other radicals or ionize through the acceptance of an electron to complete the octet. RADICALS

Beryllium and Boron are elements that will form molecular compounds with less than eight valence electrons on the central atom. The nature of the bonds in these instances is highly controversial. While exhibiting characteristics of covalent compounds, there are also some indications that the bonds are ionic. As of today, the debate is in the favor of molecular compounds that exhibit ionic bond characteristics. The shape of reduced octets involving beryllium will be linear while the shape of reduced octets involving boron will be trigonal planar. REDUCED OCTETS

Elements with available d orbitals can form molecular compounds with more than 8 valence electrons around the central atom. While not common, this also means that noble gases from argon on can form molecular compounds. One of the best known is XeF 4. The VSEPR shapes with expanded octets have either trigonal bipyramidal or octahedral electron geometries. EXPANDED OCTETS

As a part of VSEPR theory, there is a theory that the electron orbitals of the elements are hybridized. The following table equates the type of hybridization to the electron geometry and VSEPR shapes: HYBRIDIZATION OF ORBITALS STRUCTURES Electron GeometryVSEPR ShapeHybridization LinearLinear (non-polar)sp Trigonal PlanarTrigonal planar(non-polar) Bent w/ one lone pair (polar) sp 2 TetrahedralTetrahedral (non-polar) Trigonal Pyramidal (polar) Bent w/ two lone pairs (polar) sp 3