5.4 chemical bonding 🫂 Flashcards
explain why all chemical bonds are electrostatic in nature
- ionic bonds are strong electrostatic attraction between oppositely charged ions
- covalent bonds are electrostatic attraction between the shared pair of electrons and the positively charged nuclei (protons inside nucleus)
- metallic bonds are strong electrostatic attraction between a lattice of cations and sea of delocalised electrons
what does the strength of metallic bond depend on?
no. of valence electrons aavailable for delocalisation per atom / size of metal cation
what does the strength of ionic bond depend on?
lattice energy
define lattice energy
lattice energy is the energy released when one mole of the solid ionic compound is formed from its constituent gaseous ions
what does the magnitude of lattice energy depend on?
q+q- / (r+)+ r- (cation and anion radius)
define sigma and pi bond
sigma bonds are a head-on overlap of orbitals with 1 region of overlap
pi bonds are a side-on overlap of orbitals with 2 regions of overlap (formed only when a sigma bond already exists)
define dative / coordinate bonds
electrostatic forces of attraction that exist between the donor and acceptor atoms nuclei and the shared pair of electrons
define electronegativity
electronegativity is the ability of an atom to attract a shared pair of electrons towards itself in a covalent bond
how does a polar bond arise?
the pair of electrons are not equally shared between the two atoms, giving rise to a permanent dipole in the bond, making the bond polar
define bond (dissociation) energy
the amount of energy needed to break one mole of covalent bonds in gaseous state under standard conditions
factors affecting strength of covalent bond
1) number of bonds between atoms
2) effectiveness of orbital overlap (atomic radius) and bond length
- larger atomic radius, larger orbitals that are more diffused, less effective orbital overlap, bond length longer, weaker bond
3) polar VS non-polar
- increased electrostatic force of attraction between oppositely charged dipoles
describe instantaneoous dipole-induced dipole (id-id) interactions and how it arises
id-id refers to the electrostatic attraction that exists between oppositely-charged poles of temporary charges in molecules.
it arises from momentary polarisation of the electron cloud at any instant when the molecules are in continual, random motion
factors affecting strength of id-id
1) total number of electrons in an atom
- greater number of electrons, larger size of electron cloud, greater momentary polarisation, stronger id-id
2) shape of molecules (organic molecules only)
- linear molecule has greater surface area than a branched molecule with same number of atoms to facilitate a greater degree of id-id
describe permanent dipole-permanent dipole (pd-pd) interactions and how it arises
pd-pd refers to the electrostatic forces of attraction between oppositely-charged permanent dipoles of polar molecules.
it arises due to differences in electronegativity of the atoms and the dipole moments do not cancel out, resulting in a net dipole moment.
describe hydrogen bonds and how it arises
hydrogen bonds refers to the electrostatic forces of attraction involving the hydrogen atom from a H-F/O/N covalent bond and a lone pair from F/O/N of another molecule.
It arises because F/O/N is small and highly electronegative, leading to a very polar F/O/N - H bond . electrostatic forces of attraction form between the poorly shielded hydrogen atom on one molecule and a lone pair of electrons from the F/O/N atom in another molecule.
effect of hydrogen bonds on physical properties
1) higher mp/bp
2) anomalous Mr due to dimerisation
3) difference in isomers due to intra and inter-molecular hydrogen bonding
why are most ionic compounds soluble in water?
in water, strong ion-dipole interactions between ions and water (solvent) molecules and this process releases sufficient energy for the detachment of ions from the ionic lattice for solvation
why are certain ionic compounds not soluble in water?
lattice energy of the ionic compound is too exothermic, hence energy released in the formation of ion-dipole interactions between ions and water molecules is insufficient to overcome the strong ionic bonds
what unique property of water is a result of hydrogen bonding?
ice (water in solid state) is less dense than water in liquid state because of its open lattice structure
why can atoms period 3 and above expand the octet configuration?
availability of vacant low-lying d orbitals to accomodate electrons for chemical bonding
steps to draw dot-cross
- identify central atom (first in formula, least electronegative, not H and O)
- for polyatomic ions, negative charge extra electron to most electronegative atom; positive charge one less electron of least electronegative atom
- make the surrounding atoms have stable octet configuration
- octet rule can be flouted only for period 3 and above elements (can have less than 8 also)
- dative bond for atoms with more than 8 electrons
explain the VSEPR (valence shell electron repulsion) theory
the VSEPR theory states that electron pairs are arranged around the central atom in a molecule as far apart as possible so as to minimise repulsion between them.
bond pair-bond pair > bond pair-lone pair > lone pair-lone pair
explain why the shape of the molecule is [trigonal pyramidal]
the 4 electron pairs around nitrogen are directed in a tetrahedral arrangement to minimise repulsion. Since there are 3 bond pairs and 1 lone pair of electrons, ammonia is trigonal pyramidal in shape.
how do you determine whether a MOLECULE is polar?
1) there are polar bonds present in the molecule
2) dipole moments do not cancel off because bonds are asymmetrical (and surrounding atoms are not all the same), hence there is a net dipole moment
what does extent of ionic character in covalent bond depend on?
increases with increased electronegativity difference between the covalently bonded atoms
how does covalent character in an ionic bond arise?
it arises from the polarisation of electron cloud, involving distortion of the anion electron cloud by a neighbouring cation. this induces covalent character in the ionic bond.
what are other factors affecting bond angles (aside from shape?)
1) electronegativity of central atom: a more electronegative central atom attracts the shared pair of electrons more closely to itself, increasing bp-bp repulsion hence bond angle increases
2) electronegativity of surrounding atoms: electronegative surrounding atoms attract the shared pair of electrons away from central atom, decreasing bond angle
3) size of atoms bonded to central atom: larger atoms could experience mutual repulsion of electron clouds that can result in a larger bond angle
factors affecting extent of covalent character in the ionic bond
1) polarising power of the cation = charge density = charge / cationic radius
2) polarisability of anion = size of anion
