CHEMICAL BONDING
A chemical bond is defined as the force of attraction between two or more atoms or Ions or molecules that holds various constituent particles”. The combination of these particles (atoms, ions, or molecules) is called chemical bonding; the followings are the reasons for their combination.
Concept of decrease of energy of combining atoms: When the two atoms form a bond, their potential energy decreases, i.e. bonded atoms have lower energy than non-bonded atoms and are more stable due to chemical bonding.
Electronic theory of valency or octet rule :
In 1916 the scientist’s Lewis, Kossel, and Langmuir proposed that while acquiring noble gases configuration that is He(Z=2), Ne: 2, 8; Ar: 2,8, 8; Kr:2,8,18,8 and so on. The elements of noble gases have 8 electrons in their outermost orbit(except He) and do not react to form a chemical bond hence the other atoms also try to acquire 8 electrons In their outermost orbit and become most stable. This is called the ‘octet rule, But helium has only two electrons called ‘duplet’ and is also as stable as an octet. The octet rule is followed by an atom or Ion by forming a chemical bond between the atoms either by transfer of electrons in case of ionic bonds or by sharing of electrons between the atoms, forming covalent bonds or by donation of a lone pair of electrons between the atoms forming a coordinate bond to complete their octet.
“The tendency of the atoms to complete 8 electrons in the outermost shell is called “octet rule” or “electronic theory of valency“.
The main features of the electronic theory of valency are as follows.
(i) When an atom adopts 8 electrons in its outermost orbit ( 2 in the case of He) by transferring, gaining, or sharing it becomes chemically stable and does not take part in chemical combination.
(ii) The atoms having less than 8 electrons in their outermost shell are chemically active and have a tendency to combine with other atoms. If the atoms have less than 4 electrons in their outermost shell, has a tendency to lose them and if they have more than 4 electrons in the outermost shell, tend to gain electrons during the chemical combination to attain a stable configuration.
(iii) The two atoms chemically combine together either by the transfer of an electron from the outermost shell of one atom to another or by sharing of 1,2,3 or 4 electrons between the valance shell of both combining atoms. Such a chemical combination produces a stable configuration of the rule of the octet.
(iv) The tendency of an atom to transfer or share its electron is a measure of its chemical reactivity.
(ii) The atoms having less than 8 electrons in their outermost shell are chemically active and have a tendency to combine with other atoms. If the atoms have less than 4 electrons in their outermost shell, has a tendency to lose them and if they have more than 4 electrons in the outermost shell, tend to gain electrons during the chemical combination to attain a stable configuration.
(iii) The two atoms chemically combine together either by the transfer of an electron from the outermost shell of one atom to another or by sharing of 1,2,3 or 4 electrons between the valance shell of both combining atoms. Such a chemical combination produces a stable configuration of the rule of the octet.
(iv) The tendency of an atom to transfer or share its electron is a measure of its chemical reactivity.
lonic/or Electrovalent bond:
The ionic/or electrovalent bond is formed by the loss of one or more electrons from the valence shell of an atom that is highly electropositive to the valence shell of a highly electronegative atom. The atom which loses electrons forms a cation and the atom which accepts electrons forms an anion. There is an electrostatic force of attraction between the opposite Ions resulting in the formation of an ionic bond. During the formation of an Ionic bond, energy is released. The cations and anions generated by the transference of electrons acquire inert gas configuration (ns 2 or ns2np6). Examples of the formation of ionic bonds In some electrovalent compounds are given below.
$$ \left(a\right)\:Na\:\left(2,\:8,\:1\right)\frac{-e^-}{ }Na^+\left(2,\:8\right) $$
$$ Cl\:\left(2,\:8,\:7\right)\frac{-e^-}{ }Cl^-\left(2,\:8,\:8\right) $$
Na+ + Cl– → NaCl (Due to electrostatic force of attraction)
$$ \left(b\right)\:Mg\left(2,\:8,\:2\right)\:\frac{-2e^-}{ }\:Mg^{2+}\left(2,\:8\right) $$
$$ O\:\left(2,\:6\right)\:\frac{+2e^-}{ }O^{2-}\left(2,\:8\right) $$
Mg2+ + O2+ → MgO
$$ \left(c\right)\:Al\left(2,\:8,\:3\right)\frac{-3e^-}{ }Al^{3+}\left(2,\:8\right) $$
$$ 3\:Cl\left(2,\:8,\:7\right)\:\frac{+3e^-}{ }3Cl^-\left(2,\:8,\:8\right) $$
Al3+ + 3Cl– → AlCl3
Characteristics of Ionic compounds :
- They are formed by the combination of metal and non-metals.
- Ionic bonds are formed between cations and anions by the electrostatic force of attraction.
- These compounds are highly soluble in polar solvents like H2O, NH3, Liq. SO2 etc and insoluble in nonpolar solvents like CCl4, C6H6, and Liq. CS2 etc.
- They ionize in solution or in a fused state.
- They are good conductors of heat and electricity in solution or in a molten state. But in a crystalline state, they are bad conductors of electricity because ions are not movable.
- They show high values of melting and boiling points.
- They possess a non-directional polar linkage. Hence they are incapable of exhibiting any type of isomerism.
- They undergo fast reactions.
- They have high density.
- They are highly brittle:
- They have regular symmetry.
Factors Influencing the Formation of Ionic bond:
In the formation of an ionic bond between A and X, the following steps will occur.
(i) Atom ‘A’ releases an electron by absorbing energy equal to its ionization energy (I.E.) and converted into cation c+.
A(g) + energy absorbed (I.E) → c+(g) + e–
(ii) In this step atom Xaccepts the released electron from atom A and converted it into an anion,
X(g) + e– → a–(g) + Energy released (EA)
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(iii) Lastly cations c+ and anion a- combine together by an electrostatic force of attraction to give a stable ionic crystal c+a- In this step, energy equal to lattice energy of the Ionic compound c+a- Is released, which is represented as
$$ \Delta \:H_{Lattice}=-\frac{e^2}{r_c^++r^-_a} $$
where the negative sign indicates the energy is released; a represents ionic charges on c+ and a− ions; rc+ andra− are ionic radii of cation and anion respectively. In other words, the greater the value of lattice energy of the resulting ionic compound, the greater will be the ease of its formation
$$ c^+_{\left(g\right)}+a_{\left(g\right)}^-\rightarrow c^+a^-\left(s\right)\:+\:Energy\:released\:\left(\Delta H_{lattice}\right) $$
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Thus, the overall energy change (E-Ionic ) in the formation of ionic crystal c+a−(s) is given by :
$$ E_{Ionic}=\left(I.P.\right)_A-\left(EA\right)_X-\frac{e^2}{r^+_c+r^-_a} $$
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If c+a– is stable, the ionic energy (Elonic) is negative.
1,0.
1,0.
i.e $$ \frac{e^2}{r^+_c+r^-_a}\:>\:\left[\left(IP\right)_A-\left(EA\right)_X\right] $$
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Thus, the different steps discussed above indicate that the following factors facilitate the formation of an ionic crystal by a metal and a non-metal.
(i) Atom ‘A‘ must-have low ionization energy.
(ii) Atom ‘Â X ‘ should have a high value of electron affinity.
(iii) The value of lattice energy of c+a- ionic crystal must be high.
(iv) There should be a large electronegativity difference (>1.80)Â between the atoms ‘Â AÂ ‘ and ‘Â X ‘ 1. e. A should be highly electropositive and X should be highly electronegative. For example, the energy change involved in the formation of ionic bonds in NaCl is discussed in the Born-Haber cycle. (ref, fig, 2.08).
(ii) Atom ‘Â X ‘ should have a high value of electron affinity.
(iii) The value of lattice energy of c+a- ionic crystal must be high.
(iv) There should be a large electronegativity difference (>1.80)Â between the atoms ‘Â AÂ ‘ and ‘Â X ‘ 1. e. A should be highly electropositive and X should be highly electronegative. For example, the energy change involved in the formation of ionic bonds in NaCl is discussed in the Born-Haber cycle. (ref, fig, 2.08).
Variable electrovalency (oxidation states):
The elements which change their valency and show two or more valencies are said to possess variable valency. The metals belonging to p and d-blocks show variable electrovalency. The reason behind this behavior is either an inert pair effect in p-block or incomplete (n−1)d-orbitals in the case of transitional elements.
(a) p-block elements :
| Heavier elements of group IIIA | :Ga(+3, +1); | In(+3, +1); | Ti(+3, +1); |
| Heavier elements of group IVA | :Ge(+4, +2); | Sn(+4, +2); | Pb(+4, +2); |
| Heavier elements of group VA | :Sb(+5, +3); | Bi(+5, +3); |
These metals show greater stability in a lower oxidation state as compared to a higher one. This is due to the inert pair effect.
(b) Transitional elements :
The transitional elements having the electronic configuration (π−1)s2,p6, d1 to 10ns1 to 2 form cations with variable positive electrovalency. For example, Fo shows +2,+3, and titanium show +2,+3, and +4oxidation states due to loss of 3 d and 4 s electrons respectively. Because of the fact that the (n−1)d and ns orbitals in the atoms of these elements have almost the same energy, the removal of electrons from (n−1) d-orbitals takes place as easily as electrons of ns-orbitals.
When all the valence electrons (electrons of incomplete shell) are removed from an atom, the residual part is called the ‘core’. The core may have a stable configuration similar to inert gases (more common in p-block elements) or pseudo-inert gas configuration as observed in transitional elements and heavier p-block elements. These are the two reasons for the variable electrovalency of the elements.
(c) Pseudo-inert gas configuration :
Those elements have 18 electrons in their outermost orbits such as Cu, Ag, Au of IB, and Zn, Cd, and Hgof IIB groups.
| IB | Cu=3s23p63d10s1 | Cu+=3s23p63d10 | Cu2+=3s23p63d9 |
| Ag=4s24p64d105s1 | Ag+=4s24p64d10 | Ag2+=4s24p64d9 | |
| Au=5s25p65d106s1 | Au+=5s25p65d10 | Au3+=5s25p65d8 | |
| IIB | Zn=3s23p64d105s2 | Zn2+=3s23p64d10 | |
| Cd=4s24p64d105s2 | Cd2+=4s24p64d10 | ||
| Hg=5s25p85d106s2 | Hg2+=5s25p85d10 |
Monovalent cations of the IB group are not stable because of the presence of unstable (n−1) s2p6d10 configuration. Consequently, they are converted into Cu2+, Ag2+, and Au3++ which are relatively more stable and have 18 electrons in the Kernel, and are held more strongly with increased nuclear charge.
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