Structure and Bonding AS Flashcards
ionic bonding
electrostatic attraction between oppositely charged ions
covalent bonding
electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms
metallic bonding
electrostatic attraction between the sea of delocalised electrons and lattice of positive ions
ions are formed when
electrons are transferred from a metal atom to a non metal
bond strength depends on
charge density of the ions. the higher the charge density the stronger the attraction between the ions and the stronger the bonds. small and highly charged ions have a high charge density
structure of ionic bonding
regular lattice structure - oppositely charged ions attracted in all directions
high melting points and boiling points of ionic bonds
- strong electrostatic attraction between oppositely charged ions
- a lot of energy needed to break these strong bonds
conduction of electricity when molten or dissolved ionic bonds
solids - ions are in the fixed positions so no mobile charge carriers present
liquids and solutions - ionic lattice is broken and ions can move
solubility of ionic bonds
depends on the relative strengths of the attraction between the ions in the lattice and the attraction between ions and polar water molecules
covalent bonding bond length
distance between the nuclei and 2 covalently bonded atoms
average bond energy / enthalpy
energy required to break a bond in every molecule in one mole of gaseous molecules
covalent bond strength depends on:
- number of electrons being shared
- bond length ( shorter bonds mean shared e’s are closer to 2 nuclei so stronger)
- additional ionic character
bond strength is reflected by…
the bond energy. the lower the bond energy the easier it is to break a bond
what is dative covalent bonding
- a covalent bond where both shared electrons are donated by the same atom
- need one electron rich atom (lone pair of electrons)
- also need one electron poor atom ( capable of accepting lone pair of electrons )
how can dative bonds be represented
by an arrow
100% covalent
- equal share of electrons
- identical electronegativity
eg cl2
100% ionic
- complete transfer of electrons
- very big difference in electronegativities
eg nacl
intermediate bonding
either covalent w some ionic character or other way around
covalent bonds with some ionic character
- ionic character arises as a result of uneven sharing of bonding pair of electrons
- occurs when the 2 atoms involved have different electron activities
cl2
- same electronegativity
- electrons shared evenly
- non polar bond
- 100% covalent
hcl
- cl is more electronegative than h
- electrons are more strongly attracted to the cl
- uneven distribution of electrons
- cl delta negative
- h delta positive
- additional character adds strength
- polar bond
electronegativity
the ability of an atom to attract the bonding pair of electrons towards itself in a covalent bond
how is a bond polar
a bond is polar if the atoms involved have different electronegativities
- the difference in charge between the delta + and the delta - ends is called a dipole
induced dipole dipole interaction
- between ALL molecules
- electron in a molecule are moving. at any instant in time there might be more e- on one side of a molecule than the other - causes a small instantaneous dipole
- dipole will induce a dipole in the neighbouring molecule and so on
- attractions between the dipoles is a weak and temporary force of attraction
- the more electrons there are in the molecules the larger the instantaneous and induced dipoles and the greater the attraction
why do melting and boiling points increase down the noble gases
- more electrons
- stronger induced d-d attractions
- more energy needed to overcome the forces
- higher melting/boiling points
permanent dipole dipole interactions
- attraction between molecules with a permanent dipole
- stronger than id-id
- exist in addition to id-id
- only occur in polar molecules
- polar molecules dissolve in polar solvents and non polar in non polar solvents
hydrogen bonding
- particularly strong intermolecular force
- the electrostatic attraction between a very delta + H atom and a lone pair of electrons of an O N or F atom in a neighbouring molecule
- only occur in molecules that contain hydrogen bonded to N O or F (very electronegative elements so very polar bonds)
- mp and bp higher than expected
what does strength of hydrogen bond depend on
the electronegativity of the atom bonded to the H so F > O > N the overall effect also depends on the number of hydrogen bonds formed
anomalous properties of water
- ice less dense due to molecules being pushed a part when water freezes, hydrogen bonds are long bonds
- unexpectedly high mp and bp due to sh bonds being the strongest intermolecular force between molecules - more energy required to break them
- unusually high surface tension due to h bonds across surface
