Intramolecular Forces and Intermolecular Forces

Page 1 intramolecular between stronger within 3 stronger force chemical physical Intermolecular 4 shorter

Page 2 electrostatic (x2) molecules charge greater ionic ions(x2) electronegativities metal Strongly non-metals formula unit

examples

NaCl lattice

Molecular Substances Molecular substances are discrete molecules with atoms held together by __________________ bonds Covalent bonding exists when atoms ____________ electrons _________________ atoms are involved in covalent bonds Example: CO2   There are two types of covalent bonding: _________________ covalent bonding and _____________ covalent bonding; as such, the type of intramolecular force in a molecule is either __________________ or ______________ covalent bonding (note: non-polar covalent bonding is often referred to as just covalent bonding)

Molecules can be polar or non-polar Molecules can be polar or non-polar. To determine if a molecule is polar or non-polar one must consider: the _________________ of the bonds AND the _________________ of the molecule The type of intermolecular force within a sample of molecules depends on whether it is a _________ or ______________ molecule

Polar and Non-polar Bonds Recall: for polar bonds between non-metals, the electronegativity difference is __________________ for non-polar bonds between non-metals, the electronegativity difference is _________________ Shapes of Molecules Recall: steps to determine shape of molecule: draw the ___________________________ determine VSEPR notation use the ____________________________ to determine the shape

Polar and Non-polar Molecules Recall: for a molecule to be polar is must have: 1. At least one ____________ bond 2. A shape that gives the molecule at least _________ distinct “sides” Example: NH3 vs. CO2

Intermolecular Forces for Molecular Substances The type of intermolecular force within a sample of molecules depends on whether it is a polar or non-polar molecule; so the first step in determining the type of intermolecular force is to determine if the molecule is polar or non-polar.

London Dispersion Forces (LDF) ALL molecules (covalent compounds) are held together in whole or in part by LDF LDF are weak forces that occur between two different atoms of two different molecules They are created by momentary dipoles.

Normal condition – in a non-polar species there is a symmetrical charge distribution Instantaneous condition – interactions between electron clouds produce a displacement of electronic charge creating an instantaneous dipole, with charges d+ and d- Induced dipole – the instantaneous dipole on the left induces a charge separation on the right, resulting in a dipole-dipole interaction d+ d- d+ d- d+ d-

The overall force of attraction between molecules held together by LDF depends on the number of electrons. involved and the size of the atoms; therefore, larger atoms and molecules have greater overall forces of attraction between them. This can be seen in differences in the physical properties of the substances. For example: natural gas is a gas at room temperature; it consists of non-polar CH4 molecules held together by LDF; CH4 has a melting point of -183oC octane is a liquid at room temperature; it consists of non-polar C8H18 molecules held together by LDF; C8H18 has a melting point of -57oC paraffin wax is a solid at room temperature; it consists of non-polar C20H42 molecules held together by LDF; it has a melting point of 37oC

Dipole-Dipole Forces Dipole-dipole forces exist between POLAR molecules ONLY. They occur when the slightly positive side of one molecule is attracted to the slightly negative side of another molecule. This force of attraction is the intermolecular dipole-dipole interaction.

d+ d- d+ d- d+ d- d- d+ d- d+ d- d+

These forces are stronger than LDF. Note: Substances held together by dipole-dipole forces will also be held together in part by LDF. LDF will just play a less significant role. For example: HCl  

Hydrogen Bonding Hydrogen bonds are a special type of intermolecular force. It is a very strong dipole-dipole force that occurs between hydrogen and another very electronegative atom usually O, N or F. For example: H20

LDF (dipole-dipole forces) and hydrogen bonds are collectively known as van der Waal’s forces after the Dutch scientist who first suggested their importance.

Metallic Bonds These are special “bonds” created between two ___________ atoms in a sample of a metal. The metal nuclei and core electrons are held together by LDF and the ______________ electrons of each atom leave their “original” atom and come together to create a “sea of electrons” found above the nuclei. The nuclei are arranged in rows and columns (like a lattice) and create very strong attractions; this can be seen in the _________________ melting points of most metals. The strength of the metallic bond increases with an ________________ in valence electrons.