Covalent bonding

Question 1

What is covalent bonding?

Answer 1

• Occurs between non-metal elements
• Covalent bonds are formed by the electrostatic attraction between a shared pair of electrons and the positively charged nuclei
• It leads to the formation of giant covalent structures

Question 2

What is the difference in electronegativity for covalent bonds?

Answer 2

• Difference is fewer than 1.8 units

Question 3

How many electrons are involved in double and triple bonds?

Answer 3

• A single bond consists of 2 shared bonding electrons (H2)
• A double bond consists of 4 shared bonding electrons (2 pairs) (O2)
• A triple bond consists of 6 shared bonding electrons (3 pairs) (N2)

Question 4

What is the octet rule?

Answer 4

• For most covalent compounds, the atoms in a molecule share electrons to achieve a full out shell of electrons
• For most this means 8 in the valence shells

Question 5

What is bond length and bond strength?

Answer 5

• Length: the distance between the nuclei of the bonded atoms
• Strength: described in terms of bond enthalpy. Basically the amount of energy needed to break the bond between the atoms
• Bond strength can affect the reactivity of a compound (N2 unreactive and has triple bond) and (O2 more reactive and has double bond)

Question 6

Why are longer bonds weaker than shorter bonds and what does this have to do with the atomic radius?

Answer 6

• When the atomic radius increases down a group, the length of the bond between the atoms increases
• It is due to the increased distance between the nuclei and the shared pairs of bonding electrons, hence weaker electrostatic attraction between the atoms.

Question 7

Explain the length of the bonds when multiple bonds are involved and why they are stronger.

Answer 7

• In multiple bonds, there are more shared electrons between the atoms and the electrostatic attraction between the two nuclei is greater
• The greater the attraction, the closer the nuclei of the atoms are
• Multiple bonds are shorter and stronger than single bonds

Question 8

What is the general trend for bond length and strength?

Answer 8

The bond length deceases and strength increases as the number of bonds and shared electrons increases.

Question 9

What are coordinate covalent bonds?

Answer 9

• They are not a regular covalent bond as both the bonding electrons come from one atom (and not one electron each from both)
• Carbon monoxide (CO) contains one
• Represented by an arrow
• Once the coordinate bond is formed, it is identical to a ‘regular’ covalent bond

Question 10

What is a dimer and give an example?

Answer 10

• Basically two repeating units
• A larger molecule composed of two identical smaller molecules and can be linked by coordinate covalent bonds or hydrogen bonds.
• For example Aluminum chloride AlCl3 to Al2Cl6.
• AlCl3 is electron-deficient (lacking in electrons) and needs two electrons to complete the octet. This only occurs at high temperatures.
• In Al2Cl6 there is one coordinate bond coming from only one Cl, the rest are covalent bonds
• At lower temperatures, two molecules of AlCl3 combine into Al2Cl6
  Check structure in book

Question 11

What are bond orders?

Answer 11

• The number of bonds between a pair of atoms
• The higher the bond order, the greater the strength of the bond
• Some poly-atomic ions e.g. CO3 -2 has fractional bonds
• To calculate it, divide the sum of the individual bond orders by the number of bonding groups. (number of bonds, has one double bond) / (number of atoms it bonds to)

Question 12

Recap: What is electronegativity? What does the difference in electronegativity determine?

Answer 12

• The tendency of an atom to form a negative ion (gain electrons). Non-metals have a higher tendency.
• The difference determines the type of bonding that takes place between atoms

Question 13

Show the difference in electronegativity and the type of bonding:

Answer 13

More than 1.8 –> Ionic
0 < x < 1.8 (inclusive) polar covalent
0= non-polar covalent

Question 14

Explain polarity in bonding.

Answer 14

The greater the difference in electronegativity between bonded atoms, the greater the polarity of the bond and the greater its ionic character.
- Smaller difference is more non-polar covalent

Question 15

What are pure polar covalent bonds and non-polar covalent bonds?

Answer 15

• There is no difference in electronegativity e.g. O2, Cl2, N2
• The bonding electrons are equally shared between the two atoms
• When the difference in electronegativity increases, the bond becomes more polar.
• Bonds with a difference between 0.1 and 0.4 units are considered non-polar or weakly polar

Question 16

What are polar covalent bonds and what is a bond dipole? What do the partial charges mean?

Answer 16

• Occur between atoms that have a difference in electronegativity of between 0 and 1.8 units.
• The bonding electrons are not equally shared between the atoms. The unequal sharing of electrons in a bond creates a bond dipole
• Bond dipole will result in one atom in the bond having a partial positive charge and the other having a partial negative charge

δ+ and δ-
The positive is assigned to the less electronegative element (hydrogen)
The negative assigned to the more electronegative element (halide)
Arrow (dipole) goes from least electronegative to most electronegative

Question 17

Explain the difference in electronegativity in the halides.

Answer 17

• The H-F bond has the greatest polarity (greatest difference) but is still a covalent bond (non-metal and non-metal)
• The H-I has least polarity (non-polar) little difference
• Generally the polarity of the hydrogen halide bonds decrease down a group (difference becomes less)