Bonding

Lessons Topics

1-2 Bonding•understand ionic bonding, covalent bonding, co-ordinate bonding and metallic bonding in terms of electrons and forces.

3Electronegativity•understand electronegativity and that the electron distribution in a covalent bond may not be symmetrical• know that covalent bonds between different elements will be polar to different extents

3-5 • understand qualitatively how molecules may interact by dipole forces and hydrogen bonding• understand the importance of hydrogen bonding in determining the boiling points of compounds and the structures of some solids (e.g. ice) and to understand changes of state

5-7 Bonding and structure•recognise the four types of crystal and know the structures of the following crystals: sodium chloride, magnesium, diamond, graphite, iodine and ice•be able, in terms of electron pair repulsion, to predict the shapes of, and bond angles in, simple molecules and ions.

Metallic bondingDescribe how bonding occurs in metals

Properties of metalsThe delocalisation of electrons can be used to explain some properties of metals

Strong forces between lattice ions and electrons lead to high m.p. and malleability

Ionic bonding

Name the three types of strong bonds between atoms

Covalent - (Love)

Ionic - (Altruism)

Metallic -(Indifference)

Why do elements in the same group have similar patterns of bonding?

Because of the octet rule. Atoms try to gain noble gas configurations

Sodium has one outer electron. It gains stability by giving an electron to chlorine. Chlorine also become stable by gaining one electron.

Sodium has lost a negative charge and so become a positive ion.Chlorine gains a negative charge and becomes a negative ion

Ionic bonding in MgCl2

In each case, the ions are held together by attractive electrostatic forces. They form giant structures

NaCl

Task

Draw dot cross diagrams to show how CaCl2 is formed

The Octet Rule

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Ionic bonding and orbitals

Properties of ionically bonded compounds

List the properties of ionic compounds

Solid at room temperatureGiant structureHigh melting pointsCrystallineBrittle - shatter easilyConductors in liquid or solvated statePolar, Hydrophilic

Why are they brittle?

Ions of the same charge are now adjacent

Covalent bonding

11

If ionic bonds form between metals and non-metals, between what class of element do covalent bonds form?

Non-metals and non-metals

Why?

Electron gainElectron loss

Nirvana

Why do covalent bonds form?Covalent bonds often form between atoms with too many electrons in their valence shells to give away, but not enough to easily fill.

Thus they share electrons with their neighbours, in such a way that including the shared electrons the shells are full

Delocalizing electrons over two atoms instead of one lowers the energy of the system

Properties of covalent bondsColvalent bonding forms discrete molecules. State 4 facts about covalent bonds

Atoms share pairs of electrons

Each atom has a stable, noble gas configuration

They have molecular formulae

The molecules are neutral

Lewis structures, are similar to dot-cross diagrams

Use the information above to draw a dot-cross diagram for methane

What holds covalent bonds together?

What force exists between two atoms?

The Electrostatic force

Draw a diagram to show these forces

The atomic separation of particles in a nucleus is determined by the balance of these forces

Extension

More about covalent bonds

Unlike Group 1 and group 7 elements, most elements need to gain, lose or share more than one electron

Draw a dot cross diagram for oxygen

Question

Draw dot cross diagrams for the following:

HydrogenOxygenCarbon dioxideCarbon tetrafluoride

PropertiesHow strong is the inter molecular attraction between covalently bonded molecules?

Not very!

List 3 properties of covalently bonded molecules

Low melting and boiling points

Often amorphous

Often poor conductors of electricity

Remain molecular if they dissolve in water

Can be involved in other types of bonding…….

Lone pairs

Lone pairs

What is a lone pair? Lone pairs occur in elements from group 5, 6 and 7

Lone pair

How many lone pairs does Oxygen have?

Lone pairs affect the shape of the molecule

ElectronegativityHow desperate are you for this lesson to end? (tick one) Very

Slightly

Not at all

N/A

What group do you belong to? Group I & VII

Group II & VI

Group III & V

Group 4

If you answered “very”, then you are like elements in Groups I and VII. They are very desperate to lose or gain an electron!

This demonstrates the concept of electronegativity: "The power of an atom in a molecule to attract electrons to itself."

The Pauling scale

Why are the halogens missing?

Describe the trends of electronegativity across the PT

Increases

Dec

reas

es

What does electronegativity depend upon?

1. Nuclear charge2. Distance between the nucleus

and the outer electrons3. The shielding by inner electrons

Atomic radii – what’s the link?

Rules1. The smaller the atomic radius, the closer the nucleus is to the shared

electrons, the larger the electronegativity2. The larger the nuclear charge (for a given shielding effect), the greater the

electronegativity

Inequality

What happens if the partner of oxygen doesn’t want to give away it’s electron??

The molecule becomes polar

Polar molecules

What molecule is this? HCl

polarIs it polar, or non polar?

threeHow many lone pairs are there?

What other bonding possibilities are there

Electron probability (density) map

Question:Which of the following molecules will be polar?

HCl, H2O, CCl4, CH2 OAll of them, except CCl4

How do we know if a molecule is going to be polar?

Step 1: Draw a reasonable Lewis structure for the substance

Step 1: If the difference in electronegativity for the atoms in a bond is greater than 0.4, we consider the bond polar.

Step 3: If there are no lone pairs on the central atom, and if all the bonds to the central atom are the same, the molecule is nonpolar.

If the central atom has at least one polar bond and if the groups bonded to the central atom are not all identical, the molecule is probably polar.

Step 4: Describe the polar bonds with arrows pointing toward the more electronegative element. Use the length of the arrow to show the relative polarities of the different bonds. Check for symmetry.

If we put arrows into the geometric sketch for CO2, we see that they exactly balance each other, in both direction and magnitude. This shows the symmetry of the bonds.

Example:

Step 1: The Lewis structure for CO2 is

Is CO2 polar or non polar ?

Step 2: The electronegativities of carbon and oxygen are 2.55 and 3.44.

The 0.99 difference in electronegativity indicates that the C-O bonds are polar, BUT all of the bonds to the central atom are the same, which indicates that the molecule nonpolar.

The lewis structure is:

Example 2Why is CCl4 non-polar?

Even though the C-Cl bonds are polar, their symmetrical arrangement makes the molecule nonpolar.

The molecular geometry of CCl4 is tetrahedral

Summary of bonding

Other bonding possibilities

There are three types of intermolecular force van der Waals dipole-dipole forces hydrogen bonding

Dipoles:

Individual bonds can be polar, but molecules with polar bonds can also have a dipole moment caused by all of the polar bonds in the molecule.

What is a turning moment?

Turning moments are found where asymmetric forces operate

Dipoles

What causes the intermolecular force?

An unequal distribution of electron density due to the high e-negativity of Chlorine. It is an electrostatic force

Uncharged molecule can still have an electric dipole moment.

Electric Dipoles arise from opposite but equal charges separated by a distance.

Molecules that possess a dipole moment are called Polar molecules

Dipole –dipole forcesDipole-Dipole forces exist between neutral polar molecules

Dipoles affect the boiling points of a substance – the reason why water is a liquid at room temperature

Dipoles and symmetry

A polar molecule is one with a permanent dipole moment. A polar molecule must have a slightly positive end opposite a slightly negative one.

What about symmetrical molecules?

If a molecule is 'spherical' enough, then each end of the molecule will have the same properties and in must be non-polar.

(Ext) Induced dipoles are the reasons for induced charge in electrostatics.

At the molecular level, the proximity of a charged particle can distort the electron cloud of another neutral atom and the two will stick together. This is called an induced dipole

Van der Waals forces

What happens in non-polar species?

In the noble gases, there is no “molecular stickiness” so how can they be liquefied?

Other forces exist – called dispersive forces. They are very important in non-polar molecules and atoms, but exist in all atoms and molecules.

Consider helium. Where would you be likely to find the electrons at a moment in time?

Transient dipoles

The movement of the electrons, even in the He atom, cause an instantaneous dipole to be formed.

The time-averaged dipole moment of the atom is still zero.

This dipole, however short lived, can induce a dipole in a neighbouring atom, causing a force.

This force is always attractive but even shorter ranged (and weaker) than permanent dipole-induced dipole forces.

Van der Waals

They are in addition to other types of force

They act only at certain time and in certain places

The size of vdW forces decreases with increasing Z

The increase in b.p with Z in Group 0 is due to vdW

vdW forces do not act between non-polar molecules

The bigger the molecule, the larger the vdW

True False

vdW forces rely upon spherical symmetry

Hydrogen bondingIf van der Waals forces act between all molecules and atoms (ie they are ubiquitous), what range do hydrogen bonds have and are they as strong?

• H-bonds are a special case of permanent dipole-dipole interactions.

• They are stronger than van der Waals forces and around 10% as strong as covalent bonds

• Molecules with hydrogen bonds have higher boiling points than molecules that don’t.

What are the two prerequisites for H-bonding?

A lone pair of electrons on the electronegative atom.

A hydrogen atom covalently bonded to an electronegative atom … N, O or F.

If only one of these conditions is met, you don’t get hydrogen bonding.

TaskDecide what these molecules are and whether they will take part in H-bonding

Ammonia

Has hydrogen bonds.Nitrogen is very electronegative, and it has one lone pair of electrons in ammonia

Methane

No hydrogen bonds. Carbon is not very electronegative, and it has no lone pairs of electrons in methane.

Water

Has hydrogen bonds.Oxygen is very electronegative, and it has two lone pairs of electrons in water

Hydrogen bonding in water

Boiling points of period 2 and 3 hydrides

Why do the hydrides of N, O and F buck the trend?

Complete work sheet 3.6

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Summary:

Summary of intermolecular bondingEnergy/kJ

mol-1

Van der Waals

Summary

1. Produce a table summarising the properties of Covalent, ionic, metallic and inter- molecular bonds.

2. How are these properties reflected in physical characteristics, such as m.p. and b.p, conductivity etc