Unit 1 - Structure and Bonding

Question 1

Covalent Bonding
Pure Covalent Bonds

Answer 1

In a covalent bond, atoms share pairs of electrons. The covalent bond is a result of two positive nuclei being held together by their common attraction for the shared pair of electrons.

In covalent elements, both atoms have the same attraction for the shared electrons (electronegativity). When the electrons are shared equally, a pure covalent bond (non-polar covalent bond) is formed.

A pure covalent bond has no ionic character at all.

Question 2

Covalent Bonding
Polar Covalent Bonds

Answer 2

Polar covalent bonds are formed when the attraction of the atoms for the pair of bonding electrons is different.

Simply, a polar covalent bond is formed when when a shared pair of elctrons are not shared equally. This is due to one of the elements having a higher electronegativity than the other.

Question 3

Polar Covalent Bond Example

Answer 3

Hδ+ –> Fδ-

The symbols delta postive and delta negative indicate partial charges on atoms.

Fluorine has a greater electronegativity than Hydrogen. This means it will pull the bonding electrons towards itself. This makes Fluroine slightly negative and Hydrogen slightly positive. This is known as a dipole.

Question 4

Ionic Bonding

Answer 4

In ionic bonding, the element with the greater atttraction (higher electronegativity [non-metal]) gains electrons to form a negative ion. The element with the smaller attraction (lower electronegativity) loses electrons to form a positive ion.

Ionic bonds are the electrostatic attraction between positive and negative ions.

Ionic compounds form lattice structures of oppositely charged ions.

Question 5

Bonding Continuum

Answer 5

Pure covalent bonding and ionic bonding can be considered as being opposite ends of a bonding continuum with polar covalent bonding lying between these two extremes.
The bonding continuum shows the ‘continuum’ of states from covalent to ionic bonds.

Question 5

Ionic Formulae

Answer 5

A correct ionic formula containes ions with the same total number of positive and negative charges.

E.g Na+Cl- , Mg2+O2- , Na+2O2- , Cu2+(No-3)2

Question 6

Compounds formed…

Physical Properties and Bonding

Answer 6

Compounds formed between metals and non-metals are often, but not always, ionic.

Question 7

Intermolecular Forces

Answer 7

The noble gases do not form covalent or ionic bonds between their atoms and yet all of the noble gases will condense to liquids and eventually form solids, provided the temperature is sufficiently low.

**The fact that it is possible to liquefy and solidify the noble gases implies that attractive forces exist between atoms to hold them together in the liquid and solid state. **

Attractive forces also exist between molecular elements and compounds. The intermolecular forces acting between molecules are known as van der Waals forces.

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Question 8

London Dispersion Forces

Answer 8

London dispersion forces are forces of attraction that can operate between all atoms and molecules and are the weakest of the van der Waals forces.

London dispersion forces are formed as a result of electrostatic attraction between temporary dipoles and induced dipoles caused by movement of electrons in atoms and molecules.

London dispersion forces are cause by the uneven distribution of the constantly moving electrons around the nuclei of the atoms. This causes temporary dipoles on the atoms. The atoms then attract each other. The dipoles are constantly changing but there are always some in existence.

Question 9

Strength of LDFs

Answer 9

The strenght of London dispersion forces is related to the number of electrons within an atom or molecule.

As the number of electrons within the atoms increases, the strength of the London dispersion forces increases. As the strength of the London dispersion forces increases, the boiling point of the noble gases, halogens and alkanes increases.

Question 10

Polar Molecules

Answer 10

A molecule is polar if it has a permanent dipole. Some molecules containg polar covalent bonds end up with an overall polarity (permanent dipole) because the spatial arrangement of polar covalent bonds (bonds not arranged symmetrically) can result in a molecule being polar.

Question 11

Polar Molecule Requirements

Answer 11

For a molecule to be polar, it must have polar covalent bonds and when a line is drawn through the middle, all the atoms on one side are slightly positive and slightly negative on the other side.

other molecules have a symmetrical arrangement of polar bonds and the polarity cancels out. These molecules are described as non-polar.

Polar liquids are deflected by an electric field.

Question 12

Permanent Dipole - Permanent Dipole Interactions

Answer 12

The intermolecular forces between non-polar molecules are the weak London dispersion forces. PD-PD interactions are additional electrostatic forces of attraction between the permanent dipoles of polar molecules.

Permanent dipole-permanent dipole interactions are stronger than London dispersion forces for molecules with similar numbers of electrons (equivalent size).

Question 13

Hydrogen Bonding

Answer 13

Hydrogen atoms have a very low electronegativity (2.2). As a result, bonds consisting of a hydrogen atom covalently bonded to an atom of a strongly electronegative element such as flurone, oxygen of nitrogen are highly polar. Molecules containing O-H or N-H covalent bonds are always polar.

The pull of electrons away from the hydrogen results in an unusually strong permanent dipole capable of interacting with similar molecules.

A hydrogen bond is slightly stronger than other types of intermolecular forces but much weaker than covalent bonds.

Question 14

Forces of Attraction - Order of Strength

Answer 14

Covalent bond - 100-500 kJ mol-1
Hydrigen Bond - 40 kJ mol-1
Permanent dipole-permanent dipole - 30 kJ mol-1
London dispersion forces - 1-20 kJ mol-1

Question 15

Physical Properties:
Melting and Boiling Points

Answer 15

The melting and boiling points of a substance depends on the strength of the intermolecular forces between its molecules - Stronger forces = higher mp/bp.

When predicting melting and boiling points, the factors to consider are the
* type of van der Waals bonding present (LDF, PD-PD, H-bonding)
* total number of electrons in the molecule

The bp increases down a group due to the increases strenght of the LDF/increased no. of electrons.

Question 16

Melting and Boiling Points & Hydrogen Bonding

Answer 16

Substances with hydrogen bonding show higher than expected boiling points. The higher boiling points of ammonia, water and hydrogen fluoride are due to hydrogen bonding.

Question 17

Viscosity

Answer 17

Viscosity is a measure of how thick a liquid is. Thick liquids have high viscosities, runny liquids have low viscosities.

Viscosities can be compared by timing how long it takes a ball bearing to fall through the liquid (the longer the time, the more viscouse the liquid).

There is a greater degree of hydrogen bonding in propane-1,2,3-triol compared to propan-1-ol. This results in a greater force of attraction between the molecules and so the viscosity increases.

Question 18

Solubility

Answer 18

Ionic compounds and polar molecular compounds tend to be soluble in polar substances, such as water, and insoluble in non-polar solvents (alkanes e.g heptane). The polar nature of the water molecules means they are attracted to both positive and negative ions.

Non-polar molecular substances tend to be soluble in non-polar solvents and insoluble in polar substances. However, some small molecules are soluble because they can form hydrogen bonds with water molecules e.g ethanol

Question 19

Solubility - Predicting Solubility

Answer 19

When predicting solubility, the factors to consider are the
- presence in molecules of O-H or N-H bonds, which implies hydrogen bonding
- spatial arrangement of polar covalent bonds, which could result in a molecule possessing a permanent dipole