Electronegativity, bond polarity and intermolecular forces

• Covalent Bond


  The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms. They determine the identity and chemical reactions of molecules.
• Electronegativity


  The ability of an atom to attract the pair of shared electrons in a covalent bond.
• Pauling Scale


  Pauling values indicate the size of an atoms electronegativity.

  In compounds the difference in electronegativity between the elements is an indication of the bonding between them. Compounds of elements with large differences in electronegativity are usually ionic. Compounds of elements closer in electronegativity (non-metal elements) usually have covalent bonds. Noble gases aren't included as they don't tend to form compounds.
• Pauling scale trends


  Electronegativity increasing across periods and decreases down groups. The more protons a nucleus has, the stronger the attraction with its electron. It has a greater electronegativity with a shared pair of electrons than one with less.
• Pauling scale range


  Least electronegative 0.07 (Fr) most electronegative 4.0 (F)
• Pure Covalent Bond


  Electronegativity difference of 0.0 to 0.4.
• Ionic Bond

  Large electronegativity difference, >1.8
• Atoms attraction for a shared pair of electrons

  Increases across periods when the electron pair is in the same outer shell with similar shielding while more protons are added to the nucleus and so atomic radii increases.

  Decreases down groups when the electron pair is in shells further from the nucleus and when more shielded from the nucleus so atomic radii increase as atom expands.
• Dipole


  Shared pair of electrons where the electron pair is not shared equally between two bonded atoms. It's a separation of opposite charges.
• Permanent Dipole


  Occurs across a covalent bond between two atoms with different electronegativities give rise to a small charge difference so that the atoms have opposite partial charges.
• Difference in electronegativity


  The more electronegative attracts the shared pair of electrons more strongly to form the permanent dipole. The more electronegative atom will have gained control over the electrons and will be more ionic. The greater the difference in electronegativity between atoms forming a covalent bond, the greater the polarity of the covalent bond (larger partial charges) and the stronger the attraction between the molecules' permanent dipoles.
• Polarity


  A lack of electrical symmetry in a molecule. Charge differences on opposite ends of a structure.
• Polar covalent bond


  A covalent bond in which electrons are not shared equally and has a permanent dipole.
• Non-polar covalent bond


  Covalent bonds between atoms with the same or similar electronegativity has no permanent dipoles, no partial charges.
• Electronegativity Trend Across Periods

  Increases due to more protons in nucleus.
• Electronegativity Trend Down Groups

  Decreases due to increased atomic radius.
• Fluorine


  Most electronegative element, value of 4.0.
• Atomic Radius


  Distance from nucleus to outer electron shell.
• Shielding Effect


  Inner electrons reduce nuclear attraction on outer electrons.
• Non-Metal Elements


  Typically form covalent bonds with similar electronegativities.
• Metal and Non-Metal Compounds

  Usually ionic due to large electronegativity difference.
• Electronegativity Gap


  Difference in electronegativity indicating bond type.
• Delta Symbol (δ)


  Indicates partial charges in polar covalent bonds.
• Polar molecule


  Has an overall permanent dipole due to a non-symmetrical arrangement of polar covalent bonds (bonds with permanent dipoles arising through a difference in electronegativities between atoms) so that the bond dipoles do not cancel out.
• Non-polar molecule


  Has no overall permanent dipole and either has a symmetrical arrangement of polar covalent bonds (whose permanent dipoles cancel out) or contains non-polar covalent bonds. They have the same or similar electronegativities.
• Non-Polar Bond


  Covalent bond with no permanent dipole.
• Polar Bond


  Covalent bond with a permanent dipole.
• Symmetrical Arrangement


  Even distribution of bonds in all directions.
• Non-Symmetrical Arrangement


  Uneven distribution of bonds, leading to polarity.
• Intermolecular forces


  Weak attractions between different molecules. Mainly responsible for the physical properties such as melting and boiling points.
• Bonds


  Strong attractions
• Forces


  Weak attractions
• Permanent Dipole-Dipole Forces


  Weak intermolecular attractions between the permanent dipoles of different polar molecules. The strength of permanent dipole-dipole forces increase with the size of the electronegativity difference between the atoms within the polar molecule. Only present in polar molecules
• What are London Forces?

  Weak intermolecular attractions between induced dipoles of different molecules.
• What causes London Forces?

  The random movement of electrons causes temporary dipoles to form in molecules.
• How do temporary dipoles lead to attractions?

  An instantaneous temporary dipole is formed where slightly negative charges repel neighbouring electrons, inducing a temporary dipole in one molecule leading to induced dipole-dipole attractions.
• What is the relationship between the number of electrons in a molecule and London Forces?

  The more electrons a molecule has, the stronger the London Forces are, the greater the induced dipole attractions and the stronger the attractive forces between molecules.
• Are London Forces present in polar and non-polar molecules?


  Yes, London Forces are present in both polar and non-polar molecules.
• What is the strength of London Forces compared to other types of intermolecular attractions?


  London Forces are the weakest type of intermolecular attraction.
• Induced Dipole-Dipole Attraction


  Attraction between temporary dipoles in neighboring molecules.