Lecture 6
ELECTRONEGATIVITY AND BOND TYPE
Electronegativity is the power of an atom to
attract electrons to itself in a chemical bond.
Electronegativity increases from left to right
along a period, and generally decreases down
groups in the periodic table.
Estimation of electronegativity
Pauling electronegativity is based on bond
energies, using the empirical observation that
bonds between atoms with a large
electronegativity difference tend to be
stronger than those where the difference is
small. This scale was historically the first to be
devised and although it lacks a firm theoretical
justification is still widely used.
Mulliken electronegativity is the average of the
first ionization energy and the electron affinity
of an atom, reflecting the importance of two
possibilities in bond formation, losing an
electron or gaining one. The scale has the
advantage that electronegativity values can be
estimated not only for the ground states of
atoms, but for other electron configurations
and even for polyatomic fragments.
Allred-Rochow electronegativity is proportional
to Zeff/r2, where Zeff is the effective nuclear
charge of valence orbitals , and r the covalent
radius of the atom. The value is proportional
to the effective electrostatic attraction on
valence electrons by the nucleus, screened by
inner shell electrons.
Electronegativity increases towards the right and
decreases towards the bottom in the periodic
table. It thus follows the same trend as atomic
ionization energies).
Elements in early groups have low values and are
called electropositive.
Elements of group 18 in early periods do not form
any stable compounds, and so the most
electronegative element is fluorine.
Pauling electronegativity values for the elements H–K.
Elements in the shaded region are metallic
Periodic trends of electronegativity using the Pauling scale
The bond type
How the electronegtivities of two elements A
and B (which may be the same) determine the
type of bond formed between them?
1- When A and B are both electropositive they form
a metallic solid, characterized by high
electrical conductivity and a structure where each
atom is surrounded by many others
Metallic bonding involves the delocalization of
electrons throughout the solid.
The electrons are shared between atoms as in
covalent bonding but in a less specific way and
without the directional character of covalent
bonds.
2- When A and B are both electronegative they
form covalent compounds. These may consist
of individual molecules (O2, H2O, etc.
3- When one atom is very electropositive and
the other very electronegative, a solid
compound is
formed that is often regarded as ionic. In this
picture there is a complete transfer of one or
more electrons, giving cations of the
electropositive element and anions of the
electronegative one.
.
Bond polarity
1- A covalent bond between two atoms of the same element is described as
homopolar.
2- one between different elements as heteropolar.
3- A polar bond is a covalent bond in which there is a separation of charge between
one end and the other - in other words in which one end is slightly positive and
the other slightly negative. Examples include most covalent bonds. The hydrogenchlorine bond
in HCl or the hydrogen-oxygen bonds in water are typical. In general, the greater the
difference in electronegativity between two atoms, the more polar the bond that will
be formed between them, with the atom having the higher electronegativity being at
the negative end of the dipole
bond polarity describes the unequal sharing of
electrons between two atoms, and is a feature
of heteropolar bonds when the two elements
concerned have a different electronegativity.
The more electronegative atom draws
electrons and thus acquires a partial negative
charge, with the other atom becoming
correspondingly positive.
Quiz
Why does electronegativity increase across a
periode?
Electronegativity increases across a period
because the number of charges on the
nucleus increases. That attracts the bonding pair
of electrons more strongly.
Why does electronegativity fall as you go down
a group?
because the bonding pair of electrons is
increasingly distant from the attraction of the
nucleus.
electric dipole moment,
the measure of the electrical polarity of asystem
of charges.
dipole moment – μ – (units of debyes D
(3.336x10-30 C·m)):
μ = Q·R
where Q is the charge and R is the bond length