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Slide 1
CHM 101 INTRODUCTORY CHEMISTRY II Lecturers: Dr. Abayomi, Dr.
Oluseyi & Ms. Adetunde PHYSICAL CHEMISTRY - ONE
Slide 2
Electrovalent / Ionic bonding Lecture One CHEMICAL BONDING
Electrovalent / Ionic bonding
Slide 3
Chemical Bonding The attractive chemical force which keeps the
atoms in any molecule together is commonly described as a chemical
bond. A chemical bond is the physical phenomenon of chemical
substances being held together by attraction of atoms to each other
through sharing as well as exchanging of electrons or electrostatic
forces.
Slide 4
CHEMICAL BOND Two concepts - Valence Electrons - Octet
Rule
Slide 5
VALENCE ELECTRONS Not all electrons in a given atom participate
in bonding Only valence electrons are available for bonding
(electrons in the outer most shell) For representative and
noble-gas elements these electrons are always found in the s or p
sub shells
Slide 6
VALENCE ELECTRONS Using electron configuration to determine the
number of valence electrons C: 1s 2 2s 2 2p 2 O: 1s 2 2s 2 2p 4 Na:
1s 2 2s 2 2p 6 3s 1 Using electron-dot structure (Lewis symbol) to
designate the number of valence electrons. (place first 4 dots
separately on four sides and pair up as needed) CC :O:O
Na.....
Slide 7
Slide 8
Some atoms have too many electrons (one or two extra). These
atoms like to give up their electrons.
Slide 9
Some atoms are really close to having a full shell. Those atoms
go around looking for other atoms who want to give up an
electron
Slide 10
Important facts about valence electrons Representative elements
in the same group of the periodic table have the same number of
valence electrons The number of valence electrons for
representative elements is the same as the group number in the
periodic table The maximum number of valence electrons for any
given element is eight
Slide 11
OCTET RULE Electrons arranged with 8 valence electrons are more
stable than all others The valence electron configuration of the
noble gases are considered the most stable (all have 8 valence
electrons; helium has 2) All noble gases have the outermost s and p
subshell completely filled
Slide 12
OCTET RULE The noble gases are the most unreactive of all
elements Atoms of many elements tend to acquire the 8 valence
electron configuration through chemical reactions Atoms of elements
tend to gain, lose, or share electrons to produce a noble-gas
electron configuration This results in the formation of compounds
This tendency is known as the OCTET RULE
Slide 13
When elements form compounds, they either lose, gain or share
electrons so as to achieve stable electron configurations similar
to the next higher or lower noble gas in the periodic table. This
idea forms the basis of the electronic theory of bonding.
Slide 14
Loss of electron(s) by atoms Atoms of the first three elements
in a period (Groups I, II & III) lose electrons from their
outermost shell to form positively charged ions (Electropositive).
Metals donate electrons to form positive ions These ions have an
electron structure like Neon the previous noble gas.
Slide 15
Gain of electron(s) by atoms Elements at the end of the period,
Groups VI & VII, gain electrons to form negatively charged
ions. Nonmetals accept electrons to form negative ions These ions
have the same electron structure like the next noble gas, Argon.
Element Electron structureno. of e- inion formed electron structure
of atomoutermost shell of ion S2,8,66S 2- 2,8 Cl2,8,77Cl - 2,8
Slide 16
Sharing of electrons by atoms Other elements across the period,
Groups IV & V achieve electron structures similar to noble
gases by sharing electrons with other atoms and not by electron
transfer. (this will be discussed in the next lecture)
Slide 17
Electrovalent / Ionic bonding An ionic (or electrovalent) bond
is formed when metals in Group I or II react with non metals in
Groups VI & VII. When the reaction occurs, electrons are
transferred from the metal to the non metal until the outer
electron shells of the resulting ions are identical to those of a
noble gas.
Slide 18
Na + Cl:[Na] + [:Cl:] - NaCl CaCl 2.. Ca +.. Cl:.. [Ca] 2+
[:Cl:] -.. Formed by attraction between two oppositely charged ions
as a result of the transfer of electron(s) from one atom to another
atom(s) Formed between metal and nonmetal ions through
electrostatic attraction
Slide 19
Energetics Removing an electron from Na(g) to form Na + (g)
Na(g) Na + (g) + e - E = +496 kJ/mol Adding an electron to Cl(g) to
form Cl - (g) Cl(g) + e - Cl - (g) E = -349 kJ/mol Attraction
between the unlike charges draws ions together causing energy to be
released Heat of formation of ionic substances is quite exothermic
Na(s) + 1/2Cl 2 (g) NaCl(s) H f o = -410.9 kJ IONIC BOND
Slide 20
Ionic compounds do not contain discrete molecules but ordered
arrays of positive and negative ions (result of energy released)
Formula unit that indicates combining ratio E.g. NaCl
Slide 21
The number of electrons lost by metal atoms equals the number
gained by nonmetals so the compounds charge is NEUTRAL
Slide 22
Other examples of Ionic Compounds Formation of magnesium
chloride,(MgCl 2 ) -The electronic configuration of magnesium
(Atomic. No. 12) is 2,8,2. It has two electrons in its valence
shell. -The electronic configuration of chlorine (Atomic. no. 17)
is 2,8,7. It has seven valence electrons. In terms of the Lewis
(electron dot) structures, one can write :
Slide 23
Formation of aluminium fluoride (AlF 3 ).
Slide 24
Properties of Ionic (or electrovalent) Bond An ionic bond is
formed due to the coulombic attraction between the positively and
negatively charged ions. An ionic bond is non-directional, i.e.,
the strength of interaction between two ions depend upon distance,
but not on the direction. An ionic bond gets broken when the
substance is dissolved in a polar solvent such as water, or when
the substance is melted.
Slide 25
Factors Influencing the Formation of an Ionic Bond Low
ionisation enthalpy of the metallic element which forms the cation.
Large electron gain enthalpy (electron affinity) of the
non-metallic element which forms the anion. Large lattice energy,
i.e., the smaller size and higher charge of the ions.
Slide 26
Properties of ionic compounds High melting and boiling points
Good conductors of electricity when molten but non - conductors
when solid Readily soluble in water and in polar solvents but
insoluble in non polar solvents Crystals of ionic solids are hard
and brittle