Chemical Bonding
Chemical Bond (Intramolecular Force) and Nature of Chemical Bond
Most atoms are not stable and are not capable of independent existence. That is why substances composed of single atoms are very rare and their examples include only few monoatomic elements like He, Ar, Kr, Ne, Xe. The majority of the substances around us is generally made up of compounds and mixtures (which themselves are built of elements). Obviously, there must be some force which holds atoms together in a molecule or crystals otherwise the atoms would simply fly apart and no compound could exist.
“The linkage or force which holds atoms together in a molecule or a crystal is called Chemical Bond or Intramolecular Force.” OR “The attractive force which binds atoms together in the form stable molecule or a crystal (comprising of formula units) of a compound is called Chemical Bond.”
The chemical bonds are electrical in nature involving mutual attraction between unliked charged species. A stable molecule is only formed when the total energy of the combined atoms in the molecule is less than the total energy of the isolated (separate) atoms, so energy is evolved during bond formation. Lower the energy, stable will be the molecule. Thus bond making is always exothermic.
Valence Shell and Valence Electrons
The part of the atom involved in the formation of chemical bond is the outermost shell or valence shell containing valence electrons. The electrons in the outermost shell (i.e. the highest energy orbit) of an atom are called the Valence Electrons. The valence electrons are farthest from the nucleus and are thus considered to be the most loosely held electrons. Only the valence electrons are involved in the formation of chemical bonds (and the inner or core electrons do not take part in bond formation). Every element exhibits a large increase in ionization energy when core-electrons are removed from its noble gas core, supporting the idea that only the outermost electrons, those beyond the noble gas core, are involved in the sharing and transfer of electrons that give rise to chemical bonding and reactions. Thus the valence shell electronic configuration of an atom controls the atoms’s chemistry.
All the elements in a particular group of the periodic table possess the same number of valence electrons which are shown by their group numbers. No atom has more than 7 valence electrons except noble gases Ne, Ar, Kr, Xe, Rn which have 8 valence electrons (Noble gas He has only 2 valence electrons). e.g.
(1). Alkali metals of group IA (Li, Na, K, Rb, Cs) have 1 valence electron.
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(2). Alkaline earth metals of group IIA (Be, Mg, Ca, Sr, Ba) have 2 valence electrons
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(3). Elements of group IIIA (B,
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(4). Elements of group IVA (C, Si, Ge, Sn, Pb) have 5 valence electrons
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(5). Elements of group VA (N, P, As, Sb, Bi) have 5 valence electrons
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(6). Chalcogens of group VIA (O, S, Se, Te) have 6 valence electrons.
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(7). Halogens of Group VIIA (F, Cl, Br, I) have 7 valence electrons.
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Need and Reason for Bond Formation
1. Emission of Energy To get lower Energy for Acquiring Stability
The molecules become stable when there is a net increase in the attractive force. This attractive force results in the decrease in energy of the molecule. Lower the energy, stable will be the molecule.
2. Octet Rule or Duet Rule (Acquiring Inert Gas or 8-Electrons Configuration/
Noble gases formerly called Inert Gases like Ne, Ar, Kr, Xe etc have 8 electrons in their outermost (valence) shell. They are non-reactive and regarded as more stable. They have a stable outer shell consisting of 8 electrons (octet). The lightest noble gas helium has the stable outer shell of two electrons (duplet). According to Octet Rule:
“The outermost shell can have at maximum eight electrons and atoms tend to acquire an outermost shell of eight. Some atoms (like hydrogen, lithium, and beryllium) tend to attain an outermost shell of only two electrons like the lightest noble gas helium. This is in accordance with the general rule that all processes tend to move towards the state of maximum stability.”
It is a natural tendency of all atoms to complete their valence shell so as to attain the stable structure of their nearest noble gas i.e. an octet or a duplet of electron in their outermost orbit. The atoms of elements attain stability when they combine to form molecule in which each atom attains an inert gas like stable electronic configuration in its valence shell. This tendency of acquiring noble gas configuration is the basis of chemical bond formation.
Ways of Uniting Atoms
In the formation of chemical bonds between atoms of elements, the valence electrons are either transferred from the valence shell of one atom to the valence shell of another atom or shared between them usually attaining a stable outer shell consisting of 8 electrons (octet) except H2, Li, Be which have stable outer shell of two electrons (duplet). In other words the stable noble gas configuration can be achieved by following two ways:
(a) By Complete Transference of One or More Electrons
The one way of attaining stable noble gas configuration is the complete transfer of one or more valence electrons from one atom to another. This usually happens when atoms of metallic elements combine with atoms of non-metallic elements. Metals being electropositive lose their valence electrons to form cations and non-metals being electronegative gain those electrons to form anions. Both acquiring stable outer shell of 8 (octet) are held together by strong electrostatic force between them forming ionic bond.
(b) By Mutual Sharing of Electrons
The second way of attaining stable outer shell is the mutual sharing of one or more electrons between atoms of different elements and atoms of similar elements to form covalent bond.
The different methods of acquiring an octet or duplet of electrons in the outermost shell to achieve stable configuration of noble gas, gives rise to different types of chemical bondings is the real basis of bond formation
Exceptions to Octet Rule (Failure or Limitations of Octet Rule)
Dr Inam Ul Haq Jazbi
Although, according to octet rule, the central atom (as well as surrounding atoms) in most of the covalent compounds attain 8 electrons, yet there are several covalent molecules in which the central atom does not obey octet rule. The octet rule applies mainly to the second-period elements of the periodic table which have only 2s and 2p subshells that can hold a total of 8 electrons thereby attaining the electronic configuration analogous to the noble gas neon. The second-period elements, unlike third-period (or higher periods) elements, do not have 2d levels, so they can never expand their valence shells and atoms of second-period elements can never be surrounded by more than 8 electrons in any of their compounds.
Exceptions to the octet rule fall into three categories:
1. Odd-Electron Molecules
2. An Incomplete Octet or Lessened Octets (Octet-Deficient Molecules)
3. The Expended Octet or Valence Shell Expansion (Hypervalent Molecules)
?Dr Inam Ul Haq Jazbi
Types of Chemical Bonds
There are three types of chemical bond i.e.
1.Ionic/Electrovalent Bond
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[by the transfer of one or more electrons from one atom to another giving ions]
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2.Covalent bond
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[by the mutual sharing of one or more electrons between bonded atoms]
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3.Co-ordinate Covalent Bond
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[by the one sided sharing of an electron pair between bonded atoms].
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4.Metallic Bonds
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[by the electrostatic forces between metal cations and delocalized electrons]
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Ionic Bond or Electrovalent Bond
?Dr Inam Ul Haq Jazbi
Definition of Ionic Bond
In 1916 a German scientist W. Kossel proposed the concept of ionic bond.
“The electrostatic force of attraction arise due to complete transfer of one or more valence electrons from one atom to another that binds oppositely charged ions (i.e. positive and negative ions) together is termed as Ionic or Electrovalent Bond.”According to Lewis theory, ionic bond is formed by the complete transfer of electron (s) from an atom with low I.P. to another with high E.A.
During the formation of ionic bond, a metallic atom loses one or more valence electrons acquiring positive charge and change into cation while a non-metallic atom gains these electrons acquiring negative charge and changing into anions. The cation and anion attaining the stable electronic configuration of nearest noble gas, being oppositely charged are attracted towards each other and held together by electrostatic forces acting between them. The cations and anions are mutually surrounded by each other in an orderly arrangement and are associated to form giant crystal lattice [releasing lattice energy which reduces the energy of system and making it stable.]
Ionic bonds are non-directional i.e. it extends equally in all directions. Due to non-directional nature of ionic bonds, ionic crystals neither have any geometry nor show stereo (space) isomerism.
Conditions for forming Ionic Bond
Ionic bonds are usually formed between two dissimilar atoms of elements i.e. metals and non-metals having an electronegativity difference of more than 1.7. Strong electropositive elements of group IA and IIA and highly electronegative elements of group VIA and VIIA combine to form ionic bonds e.g. NaF, NaCl, NaBr, NaI, KF, KCl, KBr, KI, NaH, KH, LiF, MgO, CaO, Na2O, K2O, KO2, Na2O2, NaOH, KOH, etc.
1.The atoms taking part in bond formation should be dissimilar in nature i.e. oppositely charged particles.
2. The lowering of energy of system should take place after bond formation.
Factors Affecting for Ionic Bond
1. High Electronegativity Difference more than 1.7
2. Low Ionization Potential and High Electron Affinity
3. Emission of High Lattice Energy
1. High Electronegativity Difference more than 1.7
Ionic bonds are formed by the elements of low electronegativity (e.g. metals) and the elements of high electronegativity (e.g. non-metals). A difference of electronegativity value of about 1.7 between two elements leads to the formation of ionic bond.
2. Low Ionization Potential and High Electron Affinity
The ionization potential (I.P) of metal atom should be low so that electrons may easily be removed from it to form cation. The metals of group I, II and III (Al) A due to their low I.P. and low E.N., lose one, two or three electrons to form M+, M2+ and M3+ ions respectively. While electron affinity (E.A.) of non-metals should be high. The non-metals in group VIA and VIIA due to their high E.A. and high E.N., tend to gain two and one electron to form A2– and A1– ions respectively.
3. Emission of High Lattice Energy
The energy released when gaseous ions arrange to form crystal lattice is called Lattice Energy. Higher the value of lattice energy of crystal, greater is the ease of formation of ionic bond and greater will be stability of ionic compound.
?Dr Inam Ul Haq Jazbi
?Dr Inam Ul Haq Jazbi
Formation of Ionic Bond in Sodium Chloride
In order to form ionic bond, ∆E.N. between bonded atoms must be greater than 1.7. The between Na and Cl is 2.1 (3-0.9). Thus sodium metal and chlorine gas combine electrovalently or ionically to form formula unit of an electrovalent compound, sodium chloride (NaCl). The overall reaction can be shown as:
The formation of ionic bond in sodium chloride involves the loss of one valence electron by sodium atom to form monopositive sodium cation (Na+) with stable octet and this electron lost by Na is picked up by a chlorine atom completing its octet to form uninegative chloride anion (Cl–). Na+ and Cl– ions acquire neon and argon structure respectively. These oppositely charged ions are held together by electrostatic forces of attraction and are associated to form a crystal lattice.
The formation of ionic bond in sodium chloride (NaCl) involves the following steps:
Energy Changes in Formation of NaCl
The energy changes are as follows:
This loss of energy (147 kJ/mole) is more than compensated when oppositely charged ions i.e. Na+ and Cl– attract each other and are associated to form a crystal lattice (consisting of a closely packed array in which each ion is surrounded by six opposite ions), releasing lattice energy which can be defined as “the energy released when one gram mole of gaseous ions arrange to form crystal lattice”. The lattice energy compensates the loss of energy which occurs in the formation of ions.
Release of high lattice energy and great attractive forces in the crystal lattice greatly reduce the energy of the system thereby making it stable. [The total decrease in energy of the system is (+495 – 348 – 788 = 651 kJ/mole)].
Conclusion
Thus, it is essential for the formation of ions and ionic bond that the sum of energies released in 2nd and 3rd steps must be larger than what is required in the first step.
Covalent Bond or Electron Pair Bond
Definition
The idea of the electron pair bond or covalent bond was first introduced by G.N. Lewis in 1916. When two or more atoms of the same or different elements having same or nearly same electronegativities react, transfer of electrons does not occur instead the atoms attain noble gas structure by completing their valence shell by means of sharing of unpaired valence electrons obeying octet rule and a covalent bond is formed. In covalent bond each atom has to contribute equal no. of unpaired valence electrons.
“The bond which is formed by mutual sharing of one or more pairs of electrons called shared pair of electrons that links the atoms in a molecule is known as Covalent Bond.”
OR
“A covalent bond is a joint internuclear attractive force which is originated due to the mutual sharing of unpaired valence electrons between the bonded atoms.”
Conditions for Forming Covalent Bond
The covalent bond is usually formed between two same or different non-metallic elements (or between weak electropositive and non-metallic elements) having same or nearly same electronegativities. ). I[n such cases, atoms have no tendency to gain or lose electrons. Under these conditions, transfer of electrons does not involve. Instead electrons are shared equally by the two atoms, obeying Octet Rule]. The covalent bonding is usually found in two non-metallic elements or atoms. According to Pauling’s Electronegativity Scale, if the difference in the electronegativities of bonded atoms is up to 1.7, then the bond is covalent bond. If the difference in electronegativities is zero or less than 0.5, then the bond is pure covalent or non-polar covalent bond. e.g. H–H, O=O, NºN, NCl3, CH4, CH3–CH3, H2C=CH2, F–F, Cl–Cl, I–I, Br–Br, etc. If the difference in electronegativities is more than 0.5 but less than 1.7, then the bond is polar covalent or partially ionic in character e.g. H –F , H – Cl , H – O – H etc.
Covalent Bond Formation in Chlorine Molecule
Types of Covalent Bond According to Number of Shared
Pairs
Covalent bond is of three types:
(i) Single Covalent Bond.
(ii) Double Covalent Bond.
(iii) Triple Covalent Bond.
1. Single Covalent Bond
Such a covalent bond formed by the sharing of only one pair of electrons (i.e. 2 electrons) between bonded atoms each contributing one electron is called Single Covalent Bond and is denoted by single short line (–) between bonded atoms. e.g. H–H, F–F, Cl–Cl, Br–Br, I–I, H–Cl, H–Br, H–I, H–F, H2O, H2S, NH3, PH3, PCl3, NCl3, CH4, CCl4, CH3Cl, CH3–CH3, AlCl3, BCl3, BF3, FeCl3 etc. The electronic formulae or Lewis Formulae or Cross-dot formulae of some compounds containing single bond are given below:-
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