s2.2 the covalent model

• what is an ionic bond

  the electrostatic force of attraction between oppositely charged ions
• what is a covalent bond

  the electrostatic force of attraction between the shared pair of electrons and the nuclei of the atoms that are bonded
• what is electronegativity

  a measure of the attraction of a bonded atom for the pair of electrons in a covalent bond
• How can electronegativity values determine whether a bond is ionic or covalent?​

  the bigger the difference in electronegativity values, the more ionic the compound

  difference of less than 1.8 = covalent

  difference of more than 1.8 = ionic
• why are some covalent bonds polar?

  because of a difference in electronegativity of the bonded atoms so the electron distribution is unsymmetrical and therefore polar
• how can bond polarity be shown?

  partial charges (little d things)

  dipoles (separate arrows)

  vectors (arrows on the line)
• what makes a molecule non-polar?

  the bonds must be of equal polarity and arranged symmetrically so the dipoles cancel each other out
• what makes a molecule polar?

  the bonds are of different polarity or are arranged unsymmetrically, meaning the electron distribution is uneven, so the dipoles do not cancel each other out
• what are cis-trans isomers?

  molecules which have the same basic structure but different geometries
• what are cis isomers?

  when both methyl substituents are on the same side of the carbon chain
• what are trans isomers?

  when the methyl substituents are on opposite sides of the carbon chain
• properties of simple molecular substances

  low melting and boiling point

  electrical insulator

  polar molecules are soluble in polar solvents

  non-polar molecules are soluble in non-polar solvents
• intramolecular vs intermolecular forces


  intra - hold atoms together within a molecule, strong

  inter - forces between molecules, weak
• relative strength of bonds

  ionic/covalent (intra) - 1000

  H bonds (inter) - 50

  permanent dipole-dipole interactions (inter) - 10

  london forces (inter) - 1
• types of intermolecular forces

  van der Waal's forces:

  london forces - instantaneous dipole-dipole interactions

  permanent dipole-dipole interactions

  Hydrogen bonds
• how are dipoles formed?

  electrons continuously move around in clouds of negative charge

  at any moment there may be more electron density in one area of the atom or molecule than another. this forms an instantaneous dipole

  an induced dipole forms when an instantaneous dipole in one atom or molecule influences the electron density in neighbouring atoms or molecules
• what are london forces?

  attractive forces between temporary dipoles in neighbouring molecules. they exist between all molecules whether polar or non-polar
• explain the trend in boiling points going down groups

  boiling points increase going down groups because they have a larger molecular size, so an increase in number of electrons.

  more electrons means a larger electron cloud which is less attracted to the nuclei

  this increases polarisability and therefore the strength of the london forces, meaning the boiling point increases because more energy is needed to break the intermolecular forces
• what is polarisability?

  how easily the electron distribution is distorted by an electric field. an increase in polarisability results in an increase in dispersion forces
• what are permanent dipole-dipole interactions?

  weak attractive forces between permanent dipoles in neighbouring polar molecules
• why do permanent dipole-dipole interactions increase melting/boiling point?

  they are stronger than london forces, so have stronger intermolecular forces compared to a molecules with just london forces, meaning more energy is needed to break the forces so the melting/boiling point is increased
• what are hydrogen bonds?

  dipole-dipole attraction between an electron deficient H atom on one molecule and a lone pair of electrons on a highly electronegative atom on a different molecule. it is the strongest intermolecular force
• what molecules have H bonds?

  molecules containing H bonded to N, O or F