Chemical Bonding

Question 1

Structure of metals

Answer 1

○ Metals have giant metallic lattice structures consisting of metal cations that are arranged in a lattice and surrounded by mobile ‘sea’ of delocalised electrons
○ The valence electrons are delocalised and do not belong to any cation but rather to the crystal as a whole

Question 2

Metallic bonds

Answer 2

○ The electrostatic attraction between a lattice of positive ions and the ‘sea’ of delocalised electrons
○ They are non-directional as the delocalised electrons are attracted by many nuclei

Question 3

Factors affecting metallic bonds

Answer 3

○ Number of delocalised electrons (= number of valence electron available for bonding)
§ The more the delocalised electrons released by the metal –> the stronger their electrostatic attraction to the metal cation –> stronger the metallic bond
○ Charge density of a metal cation
§ Charge density of a cation is proportional to its charge/size ratio
§ Cations with higher charge will attract electrons more strongly
§ When the cationic radius is smaller, the delocalised electrons are closer to the nucleus, and are more strongly attracted (ionic radius decreases across the period and increases down the group)
§ A metal cation with a higher charge density –> exerts a stronger electrostatic attraction to the ‘sea’ of delocalised electrons –> stronger metallic bond strength

Question 4

Physical properties of metals

Answer 4

○ High melting and boiling points
§ A large amount of energy is required to overcome the strong electrostatic attraction between the lattice of metal cations and the ‘sea’ of delocalised electrons in a giant metallic structure
○ Good conductors of electricity in both solid and molten states
§ The mobile delocalised electrons act as charge carriers.
§ Metals with more valence electrons are better electrical conductors, due to the presence of more delocalised electrons to act as charge carriers
○ Ductile (drawn into wire) and malleable (hammered into sheets without breaking)
§ When a force is applied, a layer of cations can glide over another easily without breaking the metallic bond to maintain the electrostatic attraction between the metal cations and the ‘sea’ of delocalised electrons

Question 5

Structure of ionic compounds

Answer 5

○ Ionic compounds have giant ionic lattice structure consisting of oppositely charged ions that are arranged in a regular pattern or lattice
○ They are arranged such that the attraction between the oppositely charged ions is a maximum and repulsion between similarly charged ions is minimum

Question 6

Ionic bonds

Answer 6

○ It is the electrostatic attraction between the oppositely charged ions
○ The formation of ions involves the complete transfer of electrons from a highly electropositive atom (a metal) to a highly electronegative atom (non-metal). Cations and anions are formed as a result.
○ Non-directional as ions attract each other in all directions.

Question 7

Factors affecting the strength of ionic bonds

Answer 7

○ The strength of ionic bond is related to the lattice energy of the ionic compound and hence depends on 2 factors
§ Charges of the ions
§ Ionic radius of the ions
○ (Lattice energy:)
§ Energy released when 1 mole of the solid ionic compound is formed from its constituent gaseous ions
§ It is the result of electrostatic forces of attraction between the oppositely charged ions in an ionic compound
Magnitude of lattice energy is affected by the charge and size of the ions

Question 8

Physical properties of ionic compounds

Answer 8

○ High melting and boiling points
§ A large amount of energy is required to overcome the strong electrostatic attraction between the oppositely charged ions in a giant ionic lattice
○ Conducts electricity in molten and aqueous states but not in solid state
§ In molten or aqueous states, the oppositely charged ions are free to move hence act as mobile charge carriers to conduct electricity
§ In the solid state, the ions are held in fixed positions hence cannot act as mobile charge carriers to conduct electricity
○ Hard but brittle
§ In the giant ionic lattice –> ions are held by strong ionic bonds –> ionic compounds are hard since a lot of energy is required to overcome the electrostatic attractions
§ Very brittle and may be split cleanly (cleaved) when the crystal is tapped sharply along a particular plane –> this is due to the fact that the ions are arranged in an orderly lattice of layered ions –> thus it is possible to displace one layer of ions relative to the other layer, causing ions of similar charge to come together –> repulsion between the like-charges causes the two portions of the crystal to fall apart
○ Soluble in water and polar solvents (usually) but insoluble in non-polar solvents
§ When an ionic compound dissolves –> each ion on the crystal’s surface attracts the oppositely charged parts of the polar water molecules
§ Hence, the ionic compounds can form ion-dipole interactions with water molecules and the ions become hydrated –> this hydration process releases energy
§ The formation of ion-dipole interactions is exothermic enough (i.e. Release sufficient energy due to hydration) to overcome the strong electrostatic attraction between oppositely charged ions and causes the detachment of ions from the ionic lattice
§ Hence ionic compounds dissolve in polar solvents such as water
§ Not all ionic compounds are soluble in water as the lattice energy is so exothermic that hydration energy (from ion-dipole interaction) is insufficient to overcome the strong ionic bonds holding the lattice (e.g. MgO)
§ Non-polar solvents (e.g. Hexane, C6H12) are unable to form significant interactions with the ions in the ionic lattice and hence, unable to supply or compensate for the large amount of energy required to dissociate the ionic lattice

Question 9

Covalent substances

Answer 9

giant molecular:
- atoms held together by strong electrostatic attraction between the 2 positive nuclei and shared pair of electrons (covalent bonds)
- e.g. diamond, graphite, silicon, silicon (IV) oxide, silicon carbide and boron nitride

simple molecular:
- atoms held together by strong electrostatic attraction between the 2 positive nuclei and shared pair of electrons (covalent bonds)
- weak intermolecular attractions
(electrostatic attraction between the molecules)
1. id-id
2. pd-pd
3. hydrogen bonds
- e.g. iodine, carbon dioxide, water, aluminum chloride, hydrogen chloride

Question 10

Giant molecular structure

Answer 10

○ Covalent substances with giant molecular structure consisting of atoms bonded together by covalent bonds in a three-dimensional network. Different giant molecules display different types of structures
- e.g. on diamond and graphite

Question 11

Simple molecular structure

Answer 11

○ Covalent substances with simple molecular structures are held together by weak intermolecular forces of attraction between the molecules. As a result, most of them, particularly liquids and gases, do not have fixed shapes or arrangements.
○ Solids with simple molecular structure consist of discrete molecules or atoms held together by weak intermolecular interactions arranged in a lattice. E.g. Of simple molecular solid is iodine.

Question 12

Covalent bonds

Answer 12

• Usually formed between 2 non-metals
  
  • Defined as the electrostatic attraction between the shared pair of electrons and the positively charged nuclei

Question 13

Sigma bonds

Answer 13

Formed when the orbitals from two atoms overlap head-on.

Question 14

About sigma bonds

Answer 14

• the electron density of a sigma bond is concentrated between the nuclei of the 2 bonding atoms
• head-on overlap can occur between:
  2 s orbitals
  2 p orbitals
  1 s orbital and 1 p orbital

Question 15

pi bond

Answer 15

p orbitals of the 2 atoms overlap sideways.

Question 16

about pi bond

Answer 16

• occurs after a sigma bond has formed between 2 atoms
• the pi bonding orbital has an electron cloud below and above the nuclear axis but with zero electron density along the axis

Question 17

sigma vs pi bond

Answer 17

• sigma bond stronger
• cuz, overlapping of orbitals is more effective than that of a pi bond

Question 18

Covalent bond strength

Answer 18

§ The strength of a covalent bond is measured by its bond energy which is defines as the energy required to break 1 mole of that covalent bond between two atoms in the gaseous state under standard conditions
§ A molecule with strong covalent bonds generally has less tendency to undergo chemical change than does one with weak bonds

Question 19

Factors affecting strength of covalent bonds

Answer 19

○ Number of bonds between atoms
§ For the same bonding atoms –> an increase in the number of bonds increases the number of shared electrons between the two atoms i.e. There is increased electrostatic attraction between the bond-pairs and the two nuclei, hence bond strength is increased
§ Hence the strength of triple bond > double bond > single bond
○ Effectiveness of overlap of the orbitals
§ The covalent bond strength is the distance between the nuclei of two bonded atoms
§ In general, larger orbitals are more diffuse so overlap is less effective resulting in weaker electrostatic attraction shared pair of electrons and the two positive nuclei, resulting in a weaker covalent bond
§ Therefore larger atoms tend to form weaker covalent bonds

Question 20

Valence Shell Electron Pair Repulsion Theory (VSEPR)

Answer 20

• VSEPR theory can be used to predict
  ○ The shape or the molecular geometry of a molecule (or ion)
  i.e. The arrangement of atoms in 3-dimensional space with respect to the central atom
  
  • The theory makes use of the
    ○ Repulsion between the electron pairs surrounding the central atom of a molecule to predict the shape
  • Hence, the
    ○ arrangement of electron pairs around the central atom of a molecule (or polyatomic ion) or,
    ○ the electron pair arrangement or,
    ○ electric geometry
    must be determined first

Question 21

Main postulates of VSEPR

Answer 21

• Electron pairs in the valence shell around a central atom repel each other and orientate themselves in space as far apart as possible so as to minimise the repulsion between them (tells us about the basic shape / electric geometry of the molecule or ion)
  ○ Basic shape / electric geometry of a molecule is determined by the number of electron pairs (bond pairs as well as lone pair electrons) around a central atom.
  ○ The electrons in a multiple bond (double or triple bonds) are regarded as one region of electron cloud, as the double or triple bond is also localised between two atoms
  ○ Steps to determine the shape of the given species:

and review steps

Question 22

LP-LP vs LP-BP

Answer 22

• The repulsion between lone-pair - lone-pair of electrons is greater than those between lone-pair - bond-pair of electrons which is in turn greater than bond-pair - bond-pair of electrons (seeks to predict and explain slight distortions of the bond angles from those given in the table on the first picture above)
  ○ As a lone-pair of electrons is only attracted by the nucleus of the central atom, it is more spread out than a bond-pair of electrons which is attracted by both nuclei.
  ○ As a result, lone-pairs of electrons tend to exert a greater repulsive force on adjacent electron pairs and tend to compress the bond angles.
  ○ Hence, molecules that have lone pair(s) of electrons on the central atom are expected to have a smaller bond angle / angle between bond-pair - bond-pair of electrons

Question 23

Repulsion due to electronegativity

Answer 23

• Repulsion between bond pairs of electrons increases with an increase in the electronegativity of the central atom (seeks to predict and explain slight distortions of the bond angles)
  ○ Repulsion between bond pairs is also increased by an increase in the electronegativity of the central atom, as illustrated by comparing the bond angles of H2O and H2S.
  ○ The more electronegative central atom tends to draw electron density of the bond-pair towards itself.
  ○ The bond-pair electrons are thus nearer to the nucleus and exert more repulsion.

Question 24

Intermediate bond types

Answer 24

1. an ionic bond with some covalent character
2. a covalent bond with some ionic character, i.e. polar molecules

Question 25

Covalent character in ionic bond

Answer 25

The attraction of the cation for the valence shell electrons of the anion caused polarisation (distortion) of the electron cloud in the anion, pulling it into the region between the 2 nuclei. This results in some form of electron sharing i.e. covalent character.
The more polarised the anion, the higher the degree of covalent character

Question 26

Degree of covalent character depends on …

Answer 26

• Polarising Power of the Cation
  (i.e. ability to polarise the anion), which depends on its charge density (∝
  charge
  size ). Hence the higher
  the charge and the smaller the cation, the stronger is its polarising power.
• Polarisability of the Anion
  (i.e. ease of being polarised), which depends on its charge and size. Electron clouds of large
  anions are easier to distort than those of small anions.

An ionic compound with significant covalent character would exhibit some physical and chemical
properties of a covalent compound.

Question 27

Ionic character in covalent bond

Answer 27

When two atoms of similar electronegativities (e.g. same elements) form a covalent bond, the
electron–pair is equally shared. The bond is a non-polar covalent bond.

When two atoms of different electronegativities (e.g. different elements) form a covalent bond, the
electron–pair is not equally shared. The bond becomes polar and is said to possess ionic
character. It is called a polar covalent bond.

Question 28

Bond Polarity and Electronegativity

Answer 28

Electronegativity is the ability of an atom to attract electrons to itself within a covalent bond.
The bond polarity is determined by the difference in electronegativity of the bonding atoms. The
greater the difference in electronegativity, the more polar the covalent bond.

Question 29

Dipole moment

Answer 29

The degree of polarity of a bond is measured by its dipole moment which is a product of the charge
and the distance between the charges.
The larger the difference in the electronegativity between the two atoms, the greater the
magnitude of the dipole moment the more polar the bond.
A dipole moment is shown by the symbol “⎯→” where the arrow points towards the more
electronegative atom,

Question 30

Criteria for a molecule to be polar

Answer 30

(1) it has polar bonds, and
(2) it has a net dipole moment.

Question 31

What does the polarity of the molecule depend on?

Answer 31

A molecule is polar if it has polar bonds in a non-symmetrical arrangement such that dipole
moments do not cancel out (The net dipole moment is non–zero).
A molecule is non–polar if it has non-polar bonds only or the net dipole moment is zero.
The polar bonds in a symmetrical arrangement will result in a total cancellation of the dipole
moments associated with these bonds (i.e. the linear, trigonal planar, tetrahedral, trigonal bipyramidal,
octahedral and square planar arrangement).