Download Ionic Bonding www.AssignmentPoint.com Ionic bonding is a type of

Ionic Bonding
www.AssignmentPoint.com
www.AssignmentPoint.com
Ionic bonding is a type of chemical bond that involves the electrostatic
attraction between oppositely charged ions. These ions represent atoms that
have lost one or more electrons (known as cations) and atoms that have gained
one or more electrons (known as anions). This transfer of electrons is known as
electrovalence in contrast to covalence. In the simplest case, the cation is a
metal atom and the anion is a nonmetal atom, but these ions can be of a more
complex nature, e.g. molecular ions like NH4+ or SO42−. In simpler words, an
ionic bond is the transfer of electrons from a metal to a non-metal in order for
both atoms to obtain a full valence shell.
It is important to recognize that clean ionic bonding – in which one atom
"takes" an electron from another – cannot exist: All ionic compounds have some
degree of covalent bonding, or electron sharing. Thus, the term "ionic bonding"
is given when the ionic character is greater than the covalent character—that is,
a bond in which a large electronegativity difference exists between the two
atoms, causing the bonding to be more polar (ionic) than in covalent bonding
where electrons are shared more equally. Bonds with partially ionic and
partially covalent character are called polar covalent bonds.
Ionic compounds conduct electricity when molten or in solution, but typically
not as a solid. There are exceptions to this rule, such as rubidium silver iodide,
where the silver ion can be quite mobile. Ionic compounds generally have a
high melting point, depending on the charge of the ions they consist of. The
higher the charges the stronger the cohesive forces and the higher the melting
point. They also tend to be soluble in water. Here, the opposite trend roughly
holds: The weaker the cohesive forces, the greater the solubility.
www.AssignmentPoint.com
Overview
Atoms that have an almost full or almost empty valence shells tend to be very
reactive. Atoms that are strongly electronegative (as is the case with halogens)
often only have one or two empty orbitals in their valence shell, and frequently
bond with other molecules or gain electrons to form anions. Atoms that are
weakly electronegative (such as alkali metals) have relatively few valence
electrons that can easily be shared with atoms that are strongly electronegative.
As a result, weakly electronegative atoms tend to distort their electrons cloud
and form cations.
Formation
Ionic bonding can result from a redox reaction when atoms of an element
(usually metal), whose ionization energy is low, give some of their electrons to
achieve a stable electron configuration. In doing so, cations are formed. The
atom of another element (usually nonmetal), whose electron affinity is positive,
then accepts the electron(s), again to attain a stable electron configuration, and
after accepting electron(s) the atom becomes an anion. Typically, the stable
electron configuration is one of the noble gases for elements in the s-block and
the p-block, and particular stable electron configurations for d-block and f-block
elements. The electrostatic attraction between the anions and cations leads to the
formation of a solid with a crystallographic lattice in which the ions are stacked
in an alternating fashion. In such a lattice, it is usually not possible to
distinguish discrete molecular units, so that the compounds formed are not
molecular in nature. However, the ions themselves can be complex and form
molecular ions like the acetate anion or the ammonium cation.
www.AssignmentPoint.com
For example, common table salt is sodium chloride. When sodium (Na) and
chlorine (Cl) are combined, the sodium atoms each lose an electron, forming
cations (Na+), and the chlorine atoms each gain an electron to form anions
(Cl−). These ions are then attracted to each other in a 1:1 ratio to form sodium
chloride (NaCl).
Na + Cl → Na+ + Cl− → NaCl
However, to maintain charge neutrality, strict ratios between anions and cations
are observed so that ionic compounds, in general, obey the rules of
stoichiometry despite not being molecular compounds. For compounds that are
transitional to the alloys and possess mixed ionic and metallic bonding, this may
not be the case anymore. Many sulfides, e.g., do form non-stoichiometric
compounds.
Many ionic compounds are referred to as salts as they can also be formed by the
neutralization reaction of an Arrhenius base like NaOH with an Arrhenius acid
like HCl
NaOH + HCl → NaCl + H2O
The salt NaCl is then said to consist of the acid rest Cl− and the base rest Na+.
The removal of electrons from the cation is endothermic, raising the system's
overall energy. There may also be energy changes associated with breaking of
existing bonds or the addition of more than one electron to form anions.
However, the action of the anion's accepting the cation's valence electrons and
www.AssignmentPoint.com
the subsequent attraction of the ions to each other releases (lattice) energy and,
thus, lowers the overall energy of the system.
Ionic bonding will occur only if the overall energy change for the reaction is
favorable. In general, the reaction is exothermic, but, e.g., the formation of
mercuric oxide (HgO) is endothermic. The charge of the resulting ions is a
major factor in the strength of ionic bonding, e.g. a salt C+A− is held together
by electrostatic forces roughly four times weaker than C2+A2− according to
Coulombs law, where C and A represent a generic cation and anion
respectively. Of course the sizes of the ions and the particular packing of the
lattice are ignored in this simple argument.
Structures
Ionic compounds in the solid state form lattice structures. The two principal
factors in determining the form of the lattice are the relative charges of the ions
and their relative sizes. Some structures are adopted by a number of
compounds; for example, the structure of the rock salt sodium chloride is also
adopted by many alkali halides, and binary oxides such as MgO. Pauling's rules
provide guidelines for predicting and rationalizing the crystal structures of ionic
crystals
Bond strength
For a solid crystalline ionic compound the enthalpy change in forming the solid
from gaseous ions is termed the lattice energy. The experimental value for the
lattice energy can be determined using the Born-Haber cycle. It can also be
www.AssignmentPoint.com
calculated (predicted) using the Born-Landé equation as the sum of the
electrostatic potential energy, calculated by summing interactions between
cations and anions, and a short-range repulsive potential energy term. The
electrostatic potential can be expressed in terms of the inter-ionic separation and
a constant (Madelung constant) that takes account of the geometry of the
crystal. The further away from the nucleus the weaker the shield. The BornLandé equation gives a reasonable fit to the lattice energy of, e.g., sodium
chloride, where the calculated (predicted) value is −756 kJ/mol, which
compares to −787 kJ/mol using the Born-Haber cycle.
Polarization effects
Ions in crystal lattices of purely ionic compounds are spherical; however, if the
positive ion is small and/or highly charged, it will distort the electron cloud of
the negative ion, an effect summarised in Fajans' rules. This polarization of the
negative ion leads to a build-up of extra charge density between the two nuclei,
i.e., to partial covalency. Larger negative ions are more easily polarized, but the
effect is usually important only when positive ions with charges of 3+ (e.g.,
Al3+) are involved. However, 2+ ions (Be2+) or even 1+ (Li+) show some
polarizing power because their sizes are so small (e.g., LiI is ionic but has some
covalent bonding present). Note that this is not the ionic polarization effect that
refers to displacement of ions in the lattice due to the application of an electric
field.
Comparison with covalent bonding
In ionic bonding, the atoms are bound by attraction of opposite ions, whereas, in
covalent bonding, atoms are bound by sharing electrons to attain stable electron
www.AssignmentPoint.com
configurations. In covalent bonding, the molecular geometry around each atom
is determined by valence shell electron pair repulsion VSEPR rules, whereas, in
ionic materials, the geometry follows maximum packing rules. One could say
that covalent bonding is more directional in the sense that the energy penalty for
not adhering to the optimum bond angles is large, whereas ionic bonding has no
such penalty. There are no shared electron pairs to repel each other, the ions
should simply be packed as efficiently as possible. This often leads to much
higher coordination numbers. In NaCl, each ion has 6 neighbors and all bond
angles are 90 degrees. In CsCl the coordination number is 8. By comparison
carbon typically has a maximum of four neighbors.
Purely ionic bonding cannot exist, as the proximity of the entities involved in
the bonding allows some degree of sharing electron density between them.
Therefore, all ionic bonding has some covalent character. Thus, bonding is
considered ionic where the ionic character is greater than the covalent character.
The larger the difference in electronegativity between the two types of atoms
involved in the bonding, the more ionic (polar) it is. Bonds with partially ionic
and partially covalent character are called polar covalent bonds. For example,
Na–Cl and Mg–O interactions have a few percent covalency, while Si–O bonds
are usually ~50% ionic and ~50% covalent. Pauling estimated that an
electronegativity difference of 1.7 (on the Pauling scale) corresponds to 50%
ionic character, so that a difference greater than 50% corresponds to a bond
which is predominantly ionic. Ionic character in covalent bonds can be directly
measured for atoms having quadrupolar nuclei (2H, 14N, 81,79Br, 35,37Cl or
127I). These nuclei are generally objects of NQR nuclear quadrupole resonance
and NMR nuclear magnetic resonance studies. Interactions between the nuclear
quadrupole moments Q and the electric field gradients (EFG) are characterized
via the nuclear quadrupole coupling constants QCC = e2qzzQ/h where the eqZZ
www.AssignmentPoint.com
term corresponds to the principal component of the EFG tensor and e is the
elementary charge. In turn, the electric field gradient opens the way to
description of bonding modes in molecules when the QCC values are accurately
determined by NMR or NQR methods.
In general, when ionic bonding occurs in the solid (or liquid) state, it is not
possible to talk about a single "ionic bond" between two individual atoms,
because the cohesive forces that keep the lattice together are of a more
collective nature. This is quite different in the case of covalent bonding, where
we can often speak of a distinct bond localized between two particular atoms.
However, even if ionic bonding is combined with some covalency, the result is
not necessarily discrete bonds of a localized character. In such cases, the
resulting bonding often requires description in terms of a band structure
consisting of gigantic molecular orbitals spanning the entire crystal. Thus, the
bonding in the solid often retains its collective rather than localized nature.
When the difference in electronegativity is decreased, the bonding may then
lead to a semiconductor, a semimetal or eventually a metallic conductor with
metallic bonding.
www.AssignmentPoint.com