BYNAJMIYATUL FADILAH MOHAMAD

Topic 5: Chemical Bonding

LEARNING OUTCOMES

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

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

2. Summarise how ions are formed

3. Demonstrate how ionic bonds are formed using the Lewis diagramand chemical formulae

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

5. Compare the structure and properties of ionic compounds andcovalent compounds

6. Describe the strength and properties of metallic bonding

7. Summarise the intermolecular forces between molecules.

THE STABILITY OF NOBLE GAS STRUCTURE

Group 18 in the Periodic Table noble gases.Characterised by completely filled “s” and “p”

sub shells.All t gases except helium (it has 1s2

configuration) have stable s2p6 electron configuration /eight electrons in their outer most shell unreactive elements = inert gases.

Please refer module page 111, Table 5.1: Electron Configuration of Noble Gases

THE OCTET RULE

Atoms tend to gain, lose or share electrons until they are surrounded by eight valence electrons.

Electron configuration Sodium atom 1s22s22p63s1

Sodium atom sheds one of its electrons in the 3s sub shell.

Na+ is formedStable electron configuration of 1s22s22p6 like

the noble gas, neon.

THE OCTET RULE

Electron configurationChlorine (Cl) atom 1s22s22p63s23p5

In order for it to obtain eight valence electrons, chlorine atom needs one electron in its outermost third shell.

The electron that is shed by sodium atom is taken up by chlorine atom.

When chlorine atom receives an electron, it becomes the chloride ion, Cl, with a stable electron configuration of 1s22s22p63s23p6.

This configuration is the same as the electron configuration of the noble gas, Argon.

THE FORMATION OF IONS

When an atom gains or loses its electrons to attain the stable octet electron arrangement in chemical reactions, it becomes a charged particle or ion.

An 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 IONS

An example of a cation.Sodium ion, Na+ 1s22s22p63s1

11 electrons in the outer shells of the atom.Atomic number is 11 = 11 protons in the nucleus.Sodium atom is a neutral atom as the positive charges

of the protons balance the negative charges of the electrons.

If sodium atom loses an electron, it will have more protons than electrons.

Please refer module page 114, Figure 5.2.

THE FORMATION OF IONS

An anion example.Chloride ion, Cl1s22s22p63s23p5

Atomic number 17 = 17 electrons in its outer shells = 17 protons in the nucleus of the atom.

This makes chlorine atom a neutral atom since the positive charges of its protons balance the negative charges of its electrons.

Chlorine atom gains an electron, there are now more electrons than protons, resulting in a charge of -1.

This produces the chloride ion, Cl.

Please refer module page 115, Figure 5.3.

THE FORMATION OF IONS

Monoatomic ion is an ion that consists of only one atom.

Na+ Cl-

A polyatomic ion is an ion that contains more than one atom.

NH4+ SO42-

IONIC BONDS

An ionic bond is formed between ions of opposite charges.In general, ionic bonds are formed between metals of

groups 1, 2 and 13 and non metals of Groups 15, 16 and 17 of the Periodic Table.

In forming an ionic bond:The metal atoms lose valence electrons to form cations.The 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.

Formation of Ionic Bonds in MagnesiumChloride

Interaction of metals (Mg) and non-metals (Cl2) gas. Magnesium atom has a proton number of 12

1s22s22p63s2

To obtain stability, a magnesium atom loses two valence electrons to form a magnesium ion, Mg2+, with an octet electron arrangement of 1s22s22p6.

A chlorine atom has a proton number of 17 1s22s22p63s23p5

Needs one electron to form the chlorine ion, Cl , with an octet electron configuration, 1s22s22p63s23p6.

Two valence electrons from the magnesium atom are transferred to two chlorine atoms (each receiving an electron).

Electrostatic forces of attraction exist between the cation, Mg2+, and the two anions, Cl.

As a result, an ionic bond is formed in magnesium chloride.

Dot-and-cross Diagram (Lewis Diagram)

1. Write the chemical symbol for the element.

2. Put a dot or a cross for each valence electron in the element.

Lewis electron-dot diagrams can be used to represent chemical reactions involving valence electrons.

Example:

Chemical Formulae of Ionic Compounds

Ionic compounds consist of oppositely charged ions joined by ionic bonds.

Please refer to module, page 120, Table 5.3.

Structure and Properties of Ionic Compounds

1. They are solids with high melting points (typically more than 400C).

Sodium chloride, NaCl, has a high melting point of 800°C.

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.

Structure and Properties of Ionic Compounds

2. Many are soluble in polar solvents such as water.

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

4. Ionic liquids conduct electricity well because they contain mobile charged particles or ions.

5. They conduct electricity in aqueous solutions.6. They are often formed between a metal and a

non-metal.

COVALENT BONDS

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

Formed within or between non-metal atoms from Groups 15, 16 and 17 of the Periodic Table.

Each atom contributes an equal number of electrons for sharing.

The types of covalent bond formed depend on the number of pairs of electrons shared between two atoms.

Molecules of Elements

Covalent bonding occurs in molecules of elements.

Hydrogen MoleculeHydrogen atom has an electron configuration

1s1 with one valence electron.H2, from H atoms, each hydrogen atom

contributes one electron to share a pair of electron between them

Molecules of Compounds

Covalent bonding also occurs in compounds of non-metals. Water Molecule Has one atom of oxygen and two atoms of hydrogen. Each hydrogen atom has a proton number of one and an electron

configuration of 1s1 with one valence electron. Each hydrogen atom needs one more electron to attain a stable duplet

electron arrangement similar to helium. Oxygen atom has a proton number of eight and an electron configuration of

1s22s22p4 with six valence electrons.

Each oxygen atom needs two more electrons to attain a stable octet electron arrangement.

Each hydrogen atom contributes one electron for sharing while oxygen atom contributes two electrons.

Thus, one oxygen atom shares two pairs of electrons with two hydrogen atoms in order to attain the stable noble gas electron arrangement for all three atoms.

Non-polar covalent bond

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

Hydrogen moleculeEach hydrogen atom equally shares the two

electrons between each other.

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.

In a water molecule, the electron density is more towards the oxygen atom than the hydrogen atom, resulting in unequal sharing of the electrons.

As the electron density is pulled towards the oxygen atom, it leaves a partial positive charge on each hydrogen atom and a partial negative charge on the oxygen atom.

Notice that the “+” and “-” symbolise the partial positive and negative charges respectively.

These two charges of opposite sign and separated by a distance is called a dipole.

Structure and Properties of CovalentCompounds

1. They are low melting solids, liquids or even gases (typically less than 300C).

2. Many are insoluble in polar solvents such as water.3. Most are soluble in non-polar solvents such as benzene, C6H6.

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

5. Aqueous solutions are usually poor conductors of electricity because most do not contain mobile charged particles or ions.

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

Giant Molecular Compounds

Consist of atoms that are held together in large networks or chains by covalent bonds.

DiamondThe element in diamond is carbon, which has an

electron configuration of 1s22s22p2 with four valence electrons.

Each carbon atom can form single covalent bonds by sharing each of its electrons with four other carbon atoms.

The structure is tetrahedral with a carbon atom at the centre of a regular tetrahedron and linked to four other carbon atoms at its corners.

Four physical properties which can be used to describe diamond.1. It has a very high melting and boiling points,

almost 4,000°C. Strongly held together by covalent bonds.

2. It is very hard. Carbon atoms held together by strong covalent bonds in a uniform tetrahedral structure.

3. It does not conduct electricity even when molten. Does not contain mobile charged particles or ions.

4. It neither dissolves in water nor organic solvents

Giant Molecular Compounds

GraphiteAlso carbon but

structure lie in parallel layers, network of regular hexagons.

Held together only by weak forces of the Van der Waals type.

Giant Molecular Compounds

Physical properties of graphite

It has a high melting point and boiling point. Carbon atoms in each layer of graphite are strongly held together by covalent

bonds.

It is a soft substance. The forces between layers of graphite are very weak to the extent that they

can slide readily on each other.

It has a low density The arrangement of atoms in graphite is in layers with spaces between the

layers.

It neither dissolves in water nor organic solvents.

It is a good conductor of electricity.

Giant Molecular Compounds

Silicon DioxideUsually found in the form of sand and quartzElectron arrangement :

1s22s22p63s23p2 = four valence electrons available for bonding.

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 Compounds

Physical properties of graphiteIt has a high melting point around 1,700C.

The silicon and oxygen atoms are held strongly by covalent bonds.

It is hard due to arrangement of atoms held together by strong

covalent bonds.It is not a conductor of electricity.

It neither dissolves in water nor organic solvent

METALLIC BONDSStrength of Metallic

BondsWhat are delocalised

electrons?The metal cations and

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

Conductors and Insulators Conductors

Metals conduct heat and electricity

There are delocalised electrons surrounding the metal cations that are mobile and they are free to move

Conduction of heat and electricity in metals is due to the mobility of the electrons.

Insulators

Do not conductor electricity

non-metals are not surrounded by valence electrons that are mobile.

INTERMOLECULAR FORCES

The forces between moleculesForces of attraction are much weaker than

covalent bonds

Van der Waals Forces

Weak attractive forces between moleculesThree types of Van der Waals forces:1. Dipole-dipole Forces

2. London Dispersion Forces

3. Hydrogen Bonding