bonding + structure Flashcards
what is a covalent bond?
the strong electrostatic attraction between a shared pari of electrons and the nuclei of the bonded atoms
what is an ionic bond?
the electrostatic attraction between positive and negative ions
what is a dative covalent bond?
a covalent bond in which the shared pair of electrons have been provided by one atom only
what is the octet rule?
the maximum number of electrons that can pair up is equivalent to the number of electrons in the outer shell and unpaired electrons pair up
what is the electron repulsion theory?
the shape of a molecule is determined by the number pf electron pairs in the outer shell. these electron pairs repel each other and arrange themselves as far apart as possible
what structure does 2 outer shell electron pairs give?
linear. 180 degree bond angle.
eg. BeCl2
what structure does 3 outer shell electron pairs give?
trigonal planar. 120 degree bond angle.
eg BF3
what structure does 4 outer shell electron pairs give?
tetrahedral. 109.5 degree bond angle.
eg. CH4
what shape does 5 outer shell electron pairs give?
trigonal bi-pyramid. 120 degree horizontal bond angle and 90 degree vertical bond angle.
eg. PCl5
what structure does 6 outer shell electron pairs give?
octahedral. 90 degree bond angle
eg. SF6
lone pairs:
don’t take part in bonding, but do participate in electron repulsion. each lone pair reduces bond angles between bonded atoms by 2.5 degree.
order of electron repulsion:
LP-LP > LP-BP > BP-BP
structure of 3 outer shell electron pairs + 1 lone pair =
pyramidal. each angle 117.5 degrees
eg. PH3
structure of 4 outer shell electron pairs + 2 lone pairs =
square planar. 90 degree bond angle
eg. XeF4
what is electronegavity?
the ability of an atom to attract the bonding electrons in a covalent bond
what is a dipole?
a separation of charge across a distance (a difference in charge)
what is a polar covalent molecule?
a molecule that has an overall dipole when you take into account any dipoles across the bonds
what is a non polar covalent molecule?
a molecule that has no overall dipole when you take into account any dipoles across the bonds
what is a polar covalent bond?
a covalent bond in which the bonding atoms have a difference in electronegavity and so has a permanent dipole.
what is a non polar covalent bond?
a covalent bond in which the bonding atoms have the same electronegativity and so there is no permanent dipole
the bonding atom with the higher electronegativity takes which delta?
the negative one as it acquires a partial negative charge
the bonding atom with the lower electronegativity takes which delta?
the positive one as it acquires a partial positive charge
what is a permanent dipole?
a small charge difference across a bond that results from a difference in the electronegativites of the bonded atoms.
name the 3 types of intermolecular forces
permanent dipole-dipole forces
London forces
hydrogen bonds
how do permanent dipole-dipole forces arise?
if the atoms involved in a bond have different electronegativities, the bond will be polar and the molecule will have a permanent dipole. these are then attracted to neighbouring molecules, positive - negative
how do London forces arise?
- at any moment, oscillations in an atom produce an instantaneous dipole - with more electrons in one area of the outer shell than another
- these dipoles induce further dipoles in neighbouring molecules
- induced dipoles attract one another
how do hydrogen bonds arise?
an electron deficient Hydrogen atom attracts a lone pair of electrons on a highly electronegative atom (oxygen, nitrogen or fluorine) bonded within a different molecule
what is a hydrogen bond?
a strond dipole - dipole attraction between an electron deficient H atom in one molecule and a lone pair of electrons on a different, highly electronegative atom such as oxygen, fluorine or nitrogen.
order of strength of bond forces:
ionic + covalent > hydrogen bonds > dipole-dipole forces > London forces
special properties of water due to hydrogen bonds:
ice is less dense than water
water has higher than expected melting and boiling points
why is ice less dense than water?
in ice, water molecules are in an orderly pattern in an open lattice held apart by rigid hydrogen bonds. when ice melts some of these hydrogen bonds collapse and so molecules can move closer together, becoming more dense.
why does water have higher boiling + melting points than expected?
hydrogen bonds are stronger than other molecular forces, the extra strength of which has to be overcome when melting/boiling, therefore requires more energy and has higher melting/boiling points
