CHAPTER 4 CHEMICAL BONDING AND MOLECULAR STRUCTURE OCTET

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Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. DOUBLE COVALENT BOND A covalent bond formed by the mutual sharing of. two pair of electrons is called a double covalent bond or simply a double bond A. double covalent bond is represented by two small horizontal lines between the. two atoms E g O O O C O etc, TRIPLE COVALENT BOND A covalent bond formed by the mutual sharing of. three pair of electrons is called a triple covalent bond or simply a triple bond A. triple covalent bond is represented by three small horizontal lines between the. two atoms E g N N H C C H etc, FORMATION OF A COVALENT BOND Formation of a covalent bond is. favoured by, i High ionisation enthalpy of the combining elements. ii Nearly equal electron gain enthalpy and equal electro negativities of. combining elements, iii High nuclear charge and small atomic size of the combining elements.
POLAR COVALENT BOND The bond between two unlike atoms which differ. in their affinities for electrons is said to be polar covalent bond E g H Cl. COORDINATE BOND The bond formed when one sided sharing of electrons. take place is called a coordinate bond Such a bond is also known as dative bond It. is represented by an arrow pointing towards the acceptor atom E g H3N BF3. Bond Length Bond length is defined as the equilibrium distance between the. nuclei of two bonded atoms in a molecule, Bond Angle It is defined as the angle between the orbitals containing bonding. electron pairs around the central atom in a molecule complex ion. Bond Enthalpy It is defined as the amount of energy required to break one mole. of bonds of a particular type between two atoms in a gaseous state. Bond Order In the Lewis description of covalent bond the Bond Order is given. by the number of bonds between the two atoms in a molecule. Resonance whenever a single Lewis structure cannot describe a molecule. accurately a number of structures with similar energy positions of nuclei bonding. and non bonding pairs of electrons are taken as the canonical structures of the. hybrid which describes the molecule accurately, Dipole moment The product of the magnitude of the charge and the distance. between the centres of positive and negative charge It is a vector quantity and is. represented by an arrow with its tail at the positive centre and head pointing towards. a negative centre Dipole moment charge Q distance of separation r. Please Visit www ncerthelp com For Video lectures of all subjects Class 9 to 12. Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. SIGMA BOND A covalent bond formed due to the overlapping of orbitals of the. two atoms along the line joining the two nuclei orbital axis is called sigma. bond For example the bond formed due to s s and s p p p overlapping along the. orbital axis are sigma bonds, Pi BOND A covalent bond formed by the side wise overlapping of p or d. orbitals of two atoms is called as pi bond For example the bond formed due to. the sideways overlapping of the two p orbitals is a pi bond. HYDROGEN BOND The bond between the hydrogen atom of one molecule and. a more electro negative element of same or another molecule is called as hydrogen. HYBRIDIZATION The process of mixing of the atomic orbitals to form new. hybrid orbitals is called hybridization All hybrid orbitals of a particular kind have. equal energy identical shapes and are symmetrically oriented in shape. The hybrid orbitals are designed according to the type and the atomic orbitals. merging together e g,Shape arrangeme Geometry Examples.
AX2E0 Linear BeCl2 HgCl2 CO2,AX2E1 Bent,NO2 SO2 O3 CCl2. AX2E2 Bent,AX2E3 Linear,XeF2 I3 XeCl2, Please Visit www ncerthelp com For Video lectures of all subjects Class 9 to 12. Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. BF3 CO32 NO3 SO3,AX3E2 T shaped,CH4 PO43 SO42 ClO4. AX4E1 Seesaw,AX5E0 bipyramid,ClF5 BrF5 XeOF4, Please Visit www ncerthelp com For Video lectures of all subjects Class 9 to 12.
Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. AX6E1 al XeOF5 IOF52 7,ONE MARK QUESTIONS, Q 1 What is the total number of sigma and pi bonds in the following molecules. a C2H2 b C2H4, Ans there are three sigma and two pi bonds in C2H2. there are five sigma bonds and one pi bond in C2H4. Q 2 Write the significance of a plus and a minus sign shown in representing the. Ans Molecular orbitals are represented by wave functions A plus sign in an orbital. indicates a positive wave function while a minus sign in an orbital represents a. negative wave function, Q 3 How do you express the bond strength in terms of bond order. Ans Bond strength represents the extent of bonding between two atoms forming a. molecule The larger the bond energy the stronger is the bond and the greater is the. bond order,Q 5 Define the bond length, Ans Bond length is defined as the equilibrium distance between the nuclei of two.
bonded atoms in a molecule, Please Visit www ncerthelp com For Video lectures of all subjects Class 9 to 12. Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. Q 6 Arrange the bonds in order of increasing ionic character in the molecules. LiF K2O N2 SO2 and ClF3,Ans N2 SO2 ClF3 K2O LiF, Q 7 The skeletal structure of CH3COOH as shown below is correct but some of the. bonds are shown incorrectly Write the correct Lewis structure for acetic acid. Ans The correct Lewis structure for acetic acid is as follows. Q 8 Define octet rule, Ans The elements tend to adjust the arrangement of their electrons in such a way. that they except H and He achieve eight electrons in their outermost shell This is. called octet rule,Q 9 Define lattice enthalpy, Ans The energy required when one mole of an ionic compound in crystalline form.
is split into the constituent ions is called lattice enthalpy. Q 10 Which type of bond is formed when the atoms have zero difference in. electronegativity,Ans Covalent bond,TWO MARKS QUESTIONS. Q 1 Define hydrogen bond Is it weaker or stronger than the van der Waals forces. Ans A hydrogen bond is defined as an attractive force acting between the hydrogen. attached to an electronegative atom of one molecule and an electronegative atom of. a different molecule may be of the same kind,There are two types of H bonds. Please Visit www ncerthelp com For Video lectures of all subjects Class 9 to 12. Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. i Intermolecular H bond e g HF H2O etc,ii Intramolecular H bond e g o nitrophenol. Hydrogen bonds are stronger than Van der Walls forces since hydrogen bonds are. regarded as an extreme form of dipole dipole interaction. Q 2 Write the favourable factors for the formation of ionic bond. Ans i Low ionization enthalpy of metal atom, ii High electron gain enthalpy eg H of a non metal atom.
iii High lattice energy of the compound formed, Q 3 Although geometries of NH3 and H2O molecules are distorted tetrahedral bond. angle in water is less than that of ammonia Discuss. Ans The molecular geometry of NH3 and H2O can be shown as. The central atom N in NH3 has one lone pair and there are three bond pairs In. H2O there are two lone pairs and two bond pairs, The two lone pairs present in the oxygen atom of H2O molecule repels the two bond. pairs This repulsion is stronger than the repulsion between the lone pair and the. three bond pairs on the nitrogen atom, Since the repulsions on the bond pairs in H2O molecule are greater than that in NH3. the bond angle in water is less than that of ammonia. Q 4 Explain the important aspects of resonance with reference to the ion. Please Visit www ncerthelp com For Video lectures of all subjects Class 9 to 12. Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. Ans According to experimental findings all carbon to oxygen bonds in are. equivalent Hence it is inadequate to represent ion by a single Lewis structure. having two single bonds and one double bond, Therefore carbonate ion is described as a resonance hybrid of the following.
structures, Q 5 H3PO3 can be represented by structures 1 and 2 shown below Can these two. structures be taken as the canonical forms of the resonance hybrid representing. H3PO3 If not give reasons for the same, Ans The given structures cannot be taken as the canonical forms of the resonance. hybrid of H3PO3 because the positions of the atoms have changed. Q 6 Use Lewis symbols to show electron transfer between the following atoms. to form cations and anions a Ca and O c Al and N,Ans a Ca and O. The electronic configurations of Ca and O are as follows. Ca 2 8 8 2 O 2 6, Oxygen requires two electrons more to complete its octet whereas calcium has two. electrons more than the nearest noble gas i e Argon Hence the electron transfer. takes place as,b Al and N, Please Visit www ncerthelp com For Video lectures of all subjects Class 9 to 12.
Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. The electronic configurations of Al and N are as follows. Al 2 8 3 N 2 5, Nitrogen is three electrons short of the nearest noble gas Neon whereas. aluminium has three electrons more than Neon Hence the electron transference can. be shown as, Q 7 Although both CO2 and H2O are triatomic molecules the shape of H2O. molecule is bent while that of CO2 is linear Explain this on the basis of dipole. Ans According to experimental results the dipole moment of carbon dioxide is. zero This is possible only if the molecule is linear so that the dipole moments of C. O bonds are equal and opposite to nullify each other. Resultant 0 D, H2O on the other hand has a dipole moment value of 1 84 D though it is a. triatomic molecule as CO2 The value of the dipole moment suggests that the. structure of H2O molecule is bent where the dipole moment of O H bonds are. Q 8 Write the significance applications of dipole moment. Ans Dipole moment is the measure of the polarity of a bond It is used to. differentiate between polar and non polar bonds since all non polar molecules e g. H2 O2 have zero dipole moments It is also helpful in calculating the percentage. ionic character of a molecule, Please Visit www ncerthelp com For Video lectures of all subjects Class 9 to 12.
Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. Q 9 Use molecular orbital theory to explain why the Be 2 molecule does not exist. Ans The electronic configuration of Beryllium is, The molecular orbital electronic configuration for Be 2 molecule can be written as. Hence the bond order for Be2 is,Nb Number of electrons in bonding orbitals. Na Number of electrons in anti bonding orbitals,Bond order of Be2 0. A negative or zero bond order means that the molecule is unstable. Hence Be2 molecule does not exist,Q 10 Distinguish between a sigma and a pi bond.
Ans The following are the differences between sigma and pi bonds. Sigma Bond Pi Bond, a It is formed by the end to end overlap It is formed by the lateral overlap of. of orbitals orbitals, b The orbitals involved in the These bonds are formed by the overlap. overlapping are s s s p or p p of p porbitals only. c It is a strong bond It is weak bond, d The electron cloud is symmetrical The electron cloud is not symmetrical. about the line joining the two nuclei, e It consists of one electron cloud There are two electron clouds lying. which is symmetrical about the above and below the plane of the atomic. internuclear axis nuclei, f Free rotation about bonds is Rotation is restricted in case of pi.
possible bonds, Q 11 Explain with the help of suitable example polar covalent bond. Please Visit www ncerthelp com For Video lectures of all subjects Class 9 to 12. Visit www ncerthelp com For All NCERT solutions, CBSE sample papers Question papers Notes for Class 6 to 12. Ans When two dissimilar atoms having different electronegativities combine to. form a covalent bond the bond pair of electrons is not shared equally The bond. pair shifts towards the nucleus of the atom having greater electronegativity As a. result electron distribution gets distorted and the electron cloud is displaced. towards the electronegative atom, As a result the electronegative atom becomes slightly negatively charged while the. other atom becomes slightly positively charged Thus opposite poles are developed. in the molecule and this type of a bond is called a polar covalent bond. HCl for example contains a polar covalent bond Chlorine atom is more. electronegative than hydrogen atom Hence the bond pair lies towards chlorine and. therefore it acquires a partial negative charge,THREE MARKS QUESTIONS. Q 1 Write Lewis dot symbols for atoms of the following elements Mg Na B O. Q 3 Draw diagrams showing the formation of a double bond and a triple bond. between carbon atoms in C2H4 and C2H2 molecules, Ans C2H4 The electronic configuration of C atom in the excited state is.
In the formation of an ethane molecule C2H4 one sp2 hybrid orbital of carbon. CHAPTER 4 CHEMICAL BONDING AND MOLECULAR STRUCTURE OCTET RULE During a chemical reaction the atoms tend to adjust their electronic arrangement in such a way that they achieve 8 e in their outermost electron This is called octet rule CHEMICAL BOND the chemical force which keeps the atoms in any molecule together is called a chemical bond IONIC BOND The columbic force of attraction which

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