Topic 5Chemical Bonding

Learning OutcomesBy the end of this topic, you should be able to:

Demonstrate how to stabilise the noble gas structure using the octet rule.

Summarise how ions are formed.

Demonstrate how ionic bonds are formed using the Lewis diagram and chemical formulae.

Learning OutcomesBy the end of this topic, you should be able to:

Demonstrate how covalent bonds are formed using the dot-cross formula or dash formula.

Compare the structure and properties of ionic compounds and covalent compounds.

Describe the strength and properties of metallic bonding.

Summarise the intermolecular forces between molecules.

Electronic Configuration

The Periodic Table

The Periodic Table is constructed using the Aufbau principle, where the atomic orbitals are filled in the order of increasing energy.

The order in which atomic subshells are filled in a many-electron atom: 1s < 2s < 2p < 3s < 3p < 4s < 3d < The electronic configuration

The electronic configuration

Number of electrons that can be filled2610141s2s2p3s3p3d4s4p4d4f5s5p5d5f6s6p6d6f7s7p7d

5.1 THE STABILITY OF NOBLE GAS STRUCTURE

The Stability of Noble Gas Structure

The noble gases are characterised by completely filled s and p sub shells.

All these gases except helium (it has 1s2 configuration) have stable s2p6 electron configuration or eight electrons in their outer most shell.The Stability of Noble Gas Structure

5.2 THE OCTET RULE

The Octet RuleOctet rule defines that atoms tend to gain, lose or share electrons until they are surrounded by eight valence electrons.

The Octet RuleNa atom: 1s22s22p63s1In order to obtain 8 valence electrons for stable electron configuration, the sodium atom sheds one of its electrons in the 3s sub shell. Sodium ion, Na+, is formed and it has a stable electron configuration of 1s22s22p6 like the noble gas, neon.

The Octet RuleCl atom: 1s22s22p63s23p5

Cl- ion: 1s22s22p63s23p6

5.3 THE FORMATION OF IONS

The Formation of IonAn anion is an ion with a negative charge and it is formed when an atom gains electrons.

A cation is an ion with a positive charge and it is formed when an atom loses electrons.

The Formation of IonMonoatomic ion is an ion that consists of only one atom.Examples: Na+, and chloride ion, Cl-

A polyatomic ion is an ion that contains more than one atom.Examples: NH4+, OH-, SO42-

The Formation of IonAn ion has different chemical and physical properties from those of the atom from which the ion is derived. Sodium atom, Na, is different from the sodium ion, Na+.

5.4IONIC BONDS

Ionic BondsAn ionic bond is formed between ions of opposite charges.

In forming an ionic bond,Metal atoms lose valence electrons to form cations.Non-metal atoms gain electrons to form anions. The electrons are then transferred from metal atoms to non-metal atoms. The cations and anions are attracted to each other by electrostatic forces forming an ionic bond.

Ionic BondsFormation of Ionic Bonds in Magnesium Chloride

Dot-and-cross Diagram (Lewis Diagram):

Ionic BondsDot-and-cross Diagram (Lewis Diagram)

Valence electrons are electons in the outermost shell of an atom.

These electrons are important because they are involved in chemical bonding.

Chemical Formulae of Ionic Compounds

Structure and Properties of Ionic CompoundsThey are solids with high melting points.Sodium chloride, NaCl, has a high melting point of 800C.

It exists in a regular, extended network of oppositely charged ions, Na+ and Cl-, in which each Cl- ion is attracted to many surrounding sodium ions, and each Na+ ion is attracted to many surrounding chloride ions.

For sodium chloride to melt, every ionic attraction in the network must be overcome and this requires a lot of energy.

Structure and Properties of Ionic CompoundsMany are soluble in polar solvents such as water.

Most are insoluble in non-polar solvents such as benzene, C6H6.

Ionic liquids conduct electricity well. They contain mobile charged particles or ions. Ionic compounds dissociate into ions when melted and are free to move in an electric field, thus conducting electricity.

Structure and Properties of Ionic CompoundsThey conduct electricity in aqueous solutions. Ionic compounds dissociate into ions in solution and are free to move in an electric field, thus conducting electricity.

They are often formed between a metal and a non-metal.

5.5COVALENT BONDS

Covalent BondsA covalent bond is a bond that is formed when atoms share electrons to attain the stable electron configuration with a duplet or an octet.

Usually, the covalent bonds are formed within or between non-metal atoms from Groups 15, 16 and 17 of the Periodic Table.

Covalent BondsIn a covalent bond, each atom contributes an equal number of electrons for sharing.

Types of covalent bond:Single bond - a pair of electrons is sharedDouble bond - two pairs of electrons are sharedTriple bond - three pairs of electrons are shared between two atoms.

Covalent BondsMolecules of ElementsHydrogen MoleculeChlorine MoleculeOxygen Molecule

Covalent BondsMolecules of CompoundsWater MoleculeAmmonia MoleculeMethane MoleculeCarbon Dioxide Molecule

Covalent BondsMolecules of CompoundsWater MoleculeAmmonia MoleculeMethane MoleculeCarbon Dioxide Molecule

Covalent BondsMolecules of CompoundsWater MoleculeAmmonia MoleculeMethane MoleculeCarbon Dioxide Molecule

Covalent BondsMolecules of CompoundsWater MoleculeAmmonia MoleculeMethane MoleculeCarbon Dioxide Molecule

Covalent BondsMolecules of CompoundsWater MoleculeAmmonia MoleculeMethane MoleculeCarbon Dioxide Molecule

Covalent BondsPolar and Non-polar Covalent Bonds

In covalent bonding, electrons are shared between two atoms.

The electrons in a covalent bond are equally shared between the two atoms.

The bond that forms in those electrons is a non-polar covalent bond.

Examples: Hydrogen, chlorine, oxygen, fluorine, and nitrogen molecules

Covalent BondsPolar and Non-polar Covalent Bonds

Covalent bonds in molecules of compounds such as the ones in water, ammonia and carbon dioxide in which electron pairs are shared unequally are called polar covalent bonds.

Structure and Properties of Covalent CompoundsThey are low melting solids, liquids or even gases.The covalent bonds between the atoms in a molecule such as water molecule are relatively strong.However, the forces or attraction between the water molecules are relatively weak. They require very little energy to break the forces between the molecules to cause melting and boiling.

Structure and Properties of Covalent CompoundsMany are insoluble in polar solvents such as water.

Most are soluble in non-polar solvents such as benzene, C6H6.

Covalent liquids do not conduct electricity because they do not contain mobile charged particles or ions.

Structure and Properties of Covalent CompoundsAqueous solutions are usually poor conductors of electricity because most do not contain mobile charged particles or ions.

They are often formed between two elements with similar electronegativities, usually non-metals.

Giant Molecular CompoundsExamples of giant molecular compoundsDiamondGraphiteSilicon dioxide (SiO2)Silicon carbide (SiC)

Giant Molecular CompoundDiamondElectron configuration of carbon: 1s22s22p2 Four valence electrons. Each carbon atom can form single covalent bonds with four other carbon atoms. The structure is tetrahedral with a carbon atom at the centre and linked to four other carbon atoms at its corners.

Giant Molecular CompoundDiamondPhysical properties

High melting and boiling points (almost 4,000C). The carbon atoms in diamond are strongly held together by covalent bonds.

HardThis results from the arrangement of the carbon atoms held together by strong covalent bonds in a uniform tetrahedral structure.

Do not conduct electricity even when moltenIt does not contain mobile charged particles or ions.

Do not dissolve in water or organic solvents

Giant Molecular CompoundGraphiteThe element in graphite is also carbon.

In graphite, the carbon atoms lie in parallel layers, in which the carbon atoms are covalently combined in a network of regular hexagons.

These layers are held together by weak forces of the Van der Waals.

Giant Molecular CompoundGraphite

Giant Molecular CompoundGraphitePhysical properties:

High melting point and boiling pointThe carbon atoms in each layer of graphite are strongly held together by covalent bonds.

Soft substanceThe forces between layers of graphite are very weak to the extent that they can slide readily on each other.

Low densityThe arrangement of atoms in graphite is in layers with spaces between the layers.

Giant Molecular CompoundGraphitePhysical properties:

Do not dissolve in water or organic solvents

Good conductor of electricity

Giant Molecular CompoundSilicon dioxideSilicon dioxide is usually found in the form of sand and quartz.

Electron arrangement of silicon: 1s22s22p63s23p2

Four valence electrons available for bonding

In silicon dioxide, each silicon atom forms single covalent bonds with four oxygen atoms.

This arrangement of atoms is built up to form a giant three-dimensional structure.

Giant Molecular CompoundSilicon dioxide

Giant Molecular CompoundSilicon dioxidePhysical properties:

High melting point (around 1,700C)The silicon and oxygen atoms are held strongly by covalent bonds.

HardThis is due to arrangement of atoms held together by strong covalent bonds.

Do not conduct electricityDo not dissolve in water or organic solvents

5.6METALLIC BONDS

Strength of Metallic BondsX-ray diffraction studies show that each metal atoms is packed tightly in a crystal lattice.

In a metal, the bonding electrons are delocalised over the entire crystal.

Each circled positive charge represents the nucleus and inner electrons of a metal atom.The grey area surrounding the positive metal ions indicates the mobile sea of electrons.

Strength of Metallic Bonds

Strength of Metallic BondsDelocalised electrons are free to move around.

The metal cations and the electrons are attracted to each other by strong electrostatic forces forming metallic bonds that hold the metal atoms together.

Strength of Metallic BondsThe strength of the metallic bonds depends on the number of valence electrons available for bonding in the metals, resulting in a corresponding increase in melting points.

Sodium - one valence electron per atom m.p. 97.5CChromium - six valence electrons m.p. 1,890C

Conductors and InsulatorsMetals Good conductors There are delocalised electrons surrounding the metal cations that are mobile and they are free to move.The conduction of electricity in metals is due to the mobility of the electrons.

Crystalline non-metals (such as diamond) - InsulatorsNon-metals are not surrounded by valence electrons that are mobile.

5.7INTERMOLECULAR FORCES

Intermolecular ForcesIntramolecular forces The forces within a molecule. Intramolecular forces hold atoms together in a molecule.

Intermolecular forcesThe forces between molecules

Van der Waals ForcesVan der Waals forces are weak attractive forces between molecules.

Three types of Van der Waals forces: Dipole-dipole forcesLondon dispersion forcesHydrogen bonding forces

Van der Waals ForcesDipole-dipole forcesPolar molecules have dipoles.

When we put polar molecules very close together, dipole-dipole forces exist between the molecules The positive end of the dipole of one molecule will attract the negative end of the dipole of another molecule.

Van der Waals ForcesLondon Dispersion ForcesGenerally, the electrons in an atom are spread evenly around the nucleus.

However, at any given instant, the electron distribution in the atom may change, giving the atom a temporary dipole moment.

After this occurs, the electrons in a neighbouring atom may be influenced to also produce a dipole. The newly formed dipole is called an induced dipole.

Van der Waals ForcesLondon Dispersion ForcesThe forces of attraction between the temporary dipole and induced dipole are called the London dispersion forces.

London dispersion forces are found in all molecules.

Van der Waals ForcesLondon Dispersion Forces

Van der Waals ForcesLondon Dispersion Forces

Van der Waals ForcesLondon Dispersion ForcesThe attraction between non-polar molecules can be explained similarly.

Using molecular bromine as an example, each molecule has zero polarity. However, due to the motion of electrons at any given instant a temporary dipole would arise that would then induce a dipole in an adjacent molecule.

Van der Waals ForcesLondon Dispersion ForcesDispersion forces usually increase with molar mass because molecules with larger molar mass tend to have more electrons, and dispersion forces increase in strength with the number of electrons.

Furthermore, larger molar mass often means a bigger atom whose electron distribution is more easily distributed because the outer electrons are less tightly held by the nuclei.

Van der Waals ForcesLondon Dispersion ForcesThe melting points of similar substances that consist of non-polar molecules:

As expected, the melting point increases as the number of electrons in the molecule increases.

Because these are all non-polar molecules, the only attractive intermolecular forces present are the dispersion forces.

Van der Waals ForcesHydrogen BondingThe hydrogen bond is a special type of dipole-dipole interaction between the hydrogen atoms in a polar bond (such as NH, OH, or FH) and an electronegative O, N, or F atom.

Van der Waals ForcesHydrogen BondingHydrogen bonding in water and ammonia:

The Effects of Hydrogen BondingThe density of water and iceLeft: Ice cubes float on water.Right: Solid benzene sinks to the bottom of liquid benzene

The three dimensional structure of ice

Each O atom is bonded to four H atoms, two by covalent bonds and the other two by hydrogen bonds.

The empty space in the structure accounts for the low density of ice.

The three dimensional structure of ice

The highly ordered three dimensional structure of ice prevents the molecules from getting too close to one another.

When heat is provided and ice melts, a number of water molecules are trapped in the cavities of the three-dimensional structure.

Van der Waals Forces