Bonding and structure
Ionic bonding
Ionic bonding is strong electrostatic attraction between oppositely
charged ions.
Electrostatic interactions between oppositely charged ions are greater
than electrostatic repulsions between ions with same charge.
Size and charge of ions determine strength of ionic bonding.
Smaller ionic radius and higher charges on ions make electrostatic forces
of attraction stronger.
Dot-and-cross diagram of ionic bonding:
Draw cation with no outer electrons, rather than with 8 electrons.
Down group, ionic radius increases because ions have more electron
shells, so ions get larger.
Isoelectronic ions are ions with same number of electrons and so same
electronic configuration, but different number of protons.
For set isoelectronic ions, ions with more protons have smaller ionic
radius, as greater nuclear charge pulls in electron shells more.
Physical properties of ionic compounds:
◊ High melting temperatures, as ionic solids consist of giant lattice
network of oppositely charged ions, and combined electrostatic
forces of attraction among ions are large.
◊ Brittle, as when stress is applied, layers may slide over each other
slightly meaning ions of same charge are now side by side and
repel each other. This breaks crystal.
◊ Electrical conductivity for ionic compounds when molten or in
aqueous solution, as ions can move and will migrate to electrodes
of opposite sign when p.d. is applied. Solid Li3N is exception, and
can conduct electricity.
◊ Many are soluble in water, as hydration of ions with polar water
molecules releases energy which can break apart and dissolve ionic
lattice structure.
, Ability of ionic compound to conduct electricity and undergo electrolysis
(when molten or in aqueous solution) provides evidence for existence of
ions. For example, when direct electric current is passed through molten
NaCl, Na is formed at cathode and Cl is formed at anode.
Covalent bonding
Covalent bond is strong electrostatic attraction between nuclei of two
atoms and shared pair of electrons between them.
To draw dot-and-cross diagrams, write out electronic configuration (box
notation) of each of atoms in molecule, then see where electrons can be
promoted.
Dative covalent bond is formed when empty orbital of one atom overlaps
with orbital containing lone pair of electrons of another atom.
Dative bonding is also known as co-ordinate bonding.
Al2Cl6 molecule forms by dative bonding:
NH4+ and NO3- ions form by dative bonding:
Bond length is distance between nuclei of two atoms that are covalently
bonded together.
The shorter the bond, the stronger the bond. This is because electrostatic
attraction is greater between the two nuclei and electrons in overlapping
orbitals.
Shapes of simple molecules or ions
Valence shell electron pair repulsion (VSEPR) theory states that shape of
molecule/ion is caused by repulsion between pairs of electrons that
surround central atom. Electron pairs arrange themselves around central
atom at maximum to ensure minimum repulsion.
Ionic bonding
Ionic bonding is strong electrostatic attraction between oppositely
charged ions.
Electrostatic interactions between oppositely charged ions are greater
than electrostatic repulsions between ions with same charge.
Size and charge of ions determine strength of ionic bonding.
Smaller ionic radius and higher charges on ions make electrostatic forces
of attraction stronger.
Dot-and-cross diagram of ionic bonding:
Draw cation with no outer electrons, rather than with 8 electrons.
Down group, ionic radius increases because ions have more electron
shells, so ions get larger.
Isoelectronic ions are ions with same number of electrons and so same
electronic configuration, but different number of protons.
For set isoelectronic ions, ions with more protons have smaller ionic
radius, as greater nuclear charge pulls in electron shells more.
Physical properties of ionic compounds:
◊ High melting temperatures, as ionic solids consist of giant lattice
network of oppositely charged ions, and combined electrostatic
forces of attraction among ions are large.
◊ Brittle, as when stress is applied, layers may slide over each other
slightly meaning ions of same charge are now side by side and
repel each other. This breaks crystal.
◊ Electrical conductivity for ionic compounds when molten or in
aqueous solution, as ions can move and will migrate to electrodes
of opposite sign when p.d. is applied. Solid Li3N is exception, and
can conduct electricity.
◊ Many are soluble in water, as hydration of ions with polar water
molecules releases energy which can break apart and dissolve ionic
lattice structure.
, Ability of ionic compound to conduct electricity and undergo electrolysis
(when molten or in aqueous solution) provides evidence for existence of
ions. For example, when direct electric current is passed through molten
NaCl, Na is formed at cathode and Cl is formed at anode.
Covalent bonding
Covalent bond is strong electrostatic attraction between nuclei of two
atoms and shared pair of electrons between them.
To draw dot-and-cross diagrams, write out electronic configuration (box
notation) of each of atoms in molecule, then see where electrons can be
promoted.
Dative covalent bond is formed when empty orbital of one atom overlaps
with orbital containing lone pair of electrons of another atom.
Dative bonding is also known as co-ordinate bonding.
Al2Cl6 molecule forms by dative bonding:
NH4+ and NO3- ions form by dative bonding:
Bond length is distance between nuclei of two atoms that are covalently
bonded together.
The shorter the bond, the stronger the bond. This is because electrostatic
attraction is greater between the two nuclei and electrons in overlapping
orbitals.
Shapes of simple molecules or ions
Valence shell electron pair repulsion (VSEPR) theory states that shape of
molecule/ion is caused by repulsion between pairs of electrons that
surround central atom. Electron pairs arrange themselves around central
atom at maximum to ensure minimum repulsion.
