NIS – CHEMISTRY

Lecture 37Covalent Bonds

Ozgur Unal

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The Covalent BondWhat type of bond exist between the ions?NaCl MgCl2Ca3(PO4)2

Are the following compounds ionic compounds?CO2H2ONH3

The bonds between CO2, H2O and NH3 are called covalent bonds.

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Covalent Bond FormationIons lose or gain electrons to gain chemical stability (octet rule) forming ionic compounds.Nonmetals share electrons with other nonmetals to gain chemical stability Formation of covalent bondsThe chemical bond that results from sharing valence electrons is a covalent bond.In a covalent bond, the shared electrons are considered to be part of the outer energy levels of both atoms involved.A molecule is formed when 2 or more atoms bond covalently.

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Covalent Bond FormationExample: H2, N2, O2, F2, Br2, I2 when two atoms of each element share electrons.These 2 atom molecules are chemically more stable than the individual atoms.Consider Fluorine: 1s2 2s2 2p5 7 valence electrons

Check out Figure 8.2Check out Figure 8.3Two F atoms share 2 electrons (a pair of electrons)

The pair of electrons shared are called bonding pair.The unshared valence electrons are called lone pairs.

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Single Covalent BondsWhen only one pair of electrons is shared, it is a single covalent bond.Example: F2, H2, Cl2Check out Figure 8.4 for H2.Lewis dot structure can be used to show the bonding pairs and lone pairs in a molecule.

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Single Covalent BondsGroup 17 and single bondsHalogens have 7 valence electrons 1 electron is needed to form an octet.Group 17 elements form single covalent,bonds with atoms of other nonmetals.They also form covalent bonds with identical atoms.

Group 16 and single bondsAtoms of group 16 can share 2 electrons Two covalent bonds (either single or double)Example: H2O (water)

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Single Covalent BondsGroup 15 and single bondsGroup 15 elements form 3 covalent bonds.Example: NH3, NF3, NCl3

Group 14 and single bondsGroup 14 elements form 4 covalent bonds.Example: CH4 (methane)http://bcs.whfreeman.com/thelifewire/content/chp02/02020.html

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The Covalent BondsExample: The pattern on the glass shown in Figure 8.6 was made chemically etching its surface with hydrogen fluoride (HF). Draw the Lewis dot structure for a molecule of HF.

Example: Draw a generic Lewis dot structure for a molecule formed between atoms of Group 1 and Group 16 elements.

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The Sigma BondSingle covalent bonds are also called sigma bonds, represented by the Greek letter sigma (σ).When 2 atoms share electrons, their valence atomic orbitals overlap end-to-end, concentrating the electrons in a bonding orbital between the 2 atoms.Sigma bonds can form when an s orbital overlaps with another s orbital or a p orbital, or two p orbitals overlap.Check out Figure 8.7 for examples

NIS – CHEMISTRY

Lecture 38Multiple Covalent Bonds

Ozgur Unal

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Multiple Covalent BondsSome nonmetals share more than one pair of electrons to have the electron configuration of a noble gas.Sharing multiple pairs of electrons forms multiple covalent bonds.A double covalent bond and a triple covalent bond are examples of multiple covalent bonds.Example: C, O, N and S can form multiple covalent bonds.

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Multiple Covalent BondsDouble bonds:A double covalent bond forms when two pairs of electrons are shared between two atoms.Example: O2

Triple bonds:A triple covalent bond forms when three pairs of electrons are shared between two atoms.Example: N2

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The pi BondA multiple covalent bond consists of one sigma bond and at least of one pi bond.A pi bond, represented by the Greek letter pi (π), forms when parallel orbitals overlap and share electrons.The shared electron pair of a pi bond occupies the space above and below the line that represents where the two atoms are joined together.Check out Figure 8.9

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The Strength of Covalent BondsIn a molecule, nuclei and electrons attract each other, electrons repel each other and nuclei also repel each other.When this balance of forces is upset, the covalent bond may be broken.Some covalent bonds are easy to break (weak), some are hard to break (strong).

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The Strength of Covalent BondsThe strength of a bond depends on the bond length – the distance between the two bonded nuclei at the position of maximum attraction.Bond length is determined by the sizes of the two bonding atoms and the number of electron pairs shared.The shorter the bond length, the stronger the bond strength.Check out Table 8.1

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The Strength of Covalent BondsForming bonds release energy, breaking bonds require energy.The amount of energy required to break a specific covalent bond is called bond-dissociation energy Always positive.The smaller the bond length, the greater the bond dissociation energy.

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Bond Dissociation EnergyA chemical reaction involves reactants and products.The total energy change in a chemical reaction is determined from the energy of the bonds broken and formed.An endothermic reaction occurs when a greater amount of energy is required to break the existing bonds in the reactants than is released when the new bonds form in the products.Example: Separating H2O into H2 and O2An exothermic reaction occurs when more energy is released during product bond formation than is required to break bonds in the reactants.Example: Burning C with O2 to have CO2