Chemical Bonding

Question 1

Why are noble gas atoms stable?

Answer 1

They all have 8 electrons in their outer shell (except for He)

Question 2

What us the octet rule?

Answer 2

Except for the noble gases, atoms of most other elements chemically combine with each other so that each atom has 8 electrons in its outer shell

Question 3

How is the octet rule achieved?

Answer 3

• Transferring electrons from one atom to another to form ionic bonds or
• Sharing pairs of electrons to form covalent bonds

Question 4

Define ionic bonding

Answer 4

The electrostatic attraction between oppositely charged ions

Question 5

Define covalent bonding

Answer 5

The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms

Question 6

Define metallic bonding

Answer 6

The electrostatic attraction of a lattice of positive metal ions and a ‘sea’ of delocalised electrons

Question 7

Define dative covalent bond

Answer 7

The strong electrostatic attraction between a shared pair of electrons and the nuclei of the bonded atoms, where only one of the atoms supplies both of the electrons shared

Question 8

Define average bond enthalpy

Answer 8

Average bond enthalpy can be used as a measurement of covalent bond strength

Question 9

What do the shapes of molecules depend on?

Answer 9

• Electron pairs repel each other to get as far apart as possible
• Lone pairs of electrons repel more strongly than bonding pairs of electrons
• Shape is determined by the number and type of electron pairs around the central atom

Question 10

What are the 6 different shapes of molecules? What is the bond angle of each shape

Answer 10

1 - Linear (180)
2 - Non-linear (1 lone par 117.5, 2 lone pairs 104.5)
3 - Tetrahedral (109.5)
4 - Pyramidal (107)
5 - Trigonal planar (120)
6 - Octahedral (90)

Question 11

Define the term electronegativity

Answer 11

Electronegativity is the ability of an atom to attract the bonding electrons towards itself in a covalent bond

Question 12

Describe non-polar bonds

Answer 12

Both atoms in the bond have the same electronegativity so the bonding electrons are evenly distributed between the bonded atoms e.g. in H-H or Cl-Cl

Question 13

Describe polar bonds

Answer 13

• One of the two atoms is more electronegative than the other atom
• The bonding electrons are attached more closely to the more electronegative atom.
• As the electron cloud is more dense around the more electronegative atom there is a permanent dipole across the bond

Question 14

What is a permanent dipole?

Answer 14

A small difference in charge across a bond that results from a difference in electronegativities of the bonded atoms

Question 15

How is polarity affected if a molecule is symmetrical or unsymmetrical?

Answer 15

• Symmetrical molecules are non-polar because dipoles cancel out
• Unsymmetrical molecules are polar because do not cancel out

Question 16

What are the 3 types of intermolecular forces?

Answer 16

1 - Induced dipole-dipole attractions (london forces/dispersion forces)
2 - Permanent dipole-dipole attractions
3 - Hydrogen bonds

Question 17

Describe induced dipole-dipole attractions and explain how they arise

Answer 17

Very weak intermolecular forces between molecules that exist between all molecules
- At any moment, there may be an uneven distribution of electrons in a molecule
- This causes a temporary dipole (instantaneous dipole) to be present
- The temporary dipole in one molecule causes an induced dipole in a neighbouring molecule
The slightly positive charge of a dipole in one molecule attracts the slightly negative charge of a dipole in a neighbouring molecule to produce a London force

Question 18

Describe permanent dipole-dipole attractions and explain how they arise

Answer 18

• There are weak forces between polar molecules
  E.g. HCl (hydrogen chloride)
• The H-Cl bond is permanently polar because Cl is more electronegative than H, and the molecule is unsymmetrical so the dipoles do not cancel
• The H of one HCl molecule attracts the Cl of a neighbouring molecule to produce a permanent dipole-dipole force of attraction between the molecules
• Permanent dipole-dipole attractions are stronger than induced dipole-dipole attractions

NOTE: Both induced and permanent dipole-dipole attractions can be referred to as Van Der Waals forces

Question 19

Describe hydrogen bonding and explain how they arise

Answer 19

• A hydrogen bond is a strong dipole-dipole attraction between molecules containing O-H, N-H or F-H bonds
• Hydrogen bonds exist between either a H atom in one molecule and a lone pair on a highly electronegative atom (O,N or F) on another molecule
• Hydrogen bonds are the strongest type of intermolecular forces of attraction (per single force)

NOTE: if asked to draw a hydrogen bond between two molecules ensure you include the lone pairs of electrons in the drawing