Covalent bonding and coordinate (dative covalent) bonding

Xenon forms a number of covalently bonded compounds with fluorine. What do you understand by the term covalent bond?

When two non-metal atoms combine, they share one, or more, pairs of electrons. A shared pair of electrons is called a single covalent bond, or a bond pair.

Aluminium chloride molecules join together to form a compound with the formula Al₂Cl₆. 

Draw a displayed formula (showing all atoms and bonds) to show the bonding in one Al₂Cl₆ molecule. Show the dative covalent bonds by arrows.

Propanone has a double bond. One of the bonds is a σ bond (sigma bond). The other is a π bond (pi bond). Explain the difference between a σ bond 

and a π bond in terms of how they are formed.

When hybridised orbitals overlap linearly (end-on) we call the bond a σ bond (sigma bond). The electron density of each σ bond is symmetrical about a line joining the nuclei of the atoms forming the bond.

Bonds formed by the sideways overlap of p orbitals are called π bonds (pi bonds). A π bond is not symmetrical about the axes joining the nuclei of the atoms forming the bond.

Alkenes contain a carbon to carbon double bond that consists of a σ bond and a π bond.

Complete the diagram to show the areas of electron density for each bond. Label the σ bond and the π bond.

Bond length and bond energies can be used to compare the reactivity of covalent molecules. Compare the reactivity of hydrogen halides using the information in Table below

The order of reactivity of hydrogen halides is HI > HBr > HCl because HI has the largest bond length and lowest bond energy. That means that less energy is required to break this bond, leading to less attraction between the nucleus and electron

A student suggests that the PCl₅ molecule has sp³ hybridisation. Explain why the student is not correct.

PCl5 does not have sp³ hybridisation as sp³ orbitals are generated from one s orbital with 3 p orbitals. From PCl₅ structure, there are 5 covalent bonds in this molecule which are formed by the end-on overlap of atomic orbitals forming 5 σ bonds. That means that each σ bond will be a hybrid of 5 atomic orbitals.

What is sp, sp² and sp³ hybridisation?

The process of mixing atomic orbitals (for example one s orbital and three p orbitals) in this way is called hybridisation. 

The hybrids are called sp³ hybrids. In sp³ hybrids, each orbital has `1/4` s character and `3/4` p character. 

When one s orbital and two p orbitals are hybridised, the hybrids are called sp² hybrids. 

When one s orbital and one p orbital are hybridised, the hybrids are called sp orbitals.

Which of the following contains a dative covalent bond?

A. `N_2`

B. `NH_3`

C. `CN^-`

D. `NH""_4^+`

The answer is D.

`NH""_4^+` ion is formed by a reaction between `NH_3` and `H^+` ion and the ammonia molecule donates its lone pair of electrons to a `H^+` ion.

The dative covalent bonds is formed when one atom provides both the electrons needed for a covalent bond. For dative covalent bonding we need: 

• one atom having a lone pair of electrons 
• a second atom having an unfilled orbital to accept the lone pair; in other words, an electron-deficient compound.

And this bond is shown by a bond with an arrow.

A is incorrect because this is a nitrogen molecule having a triple covalent bond

B is incorrect because this molecule has three single covalent bonds between N-H and one lone pair of electrons on the nitrogen atoms

C is incorrect because this ion has a triple covalent bond between C and N

Which type of covalent bond is formed by the end-on overlap of atomic orbitals?

A. Sigma bond

B. Pi bond

C. Ionic bond

D. Metallic bond

The answer is A.

When hybridised orbitals overlap linearly (end-on) we call the bond a σ bond (sigma bond). The electron density of each σ bond is symmetrical about a line joining the nuclei of the atoms forming the bond.

B is incorrect because pi bonds are formed by the sideways overlap of adjacent p orbitals, not end-on overlap

C and D are incorrect because sigma bonds are found in covalent structures

Which of the following does not contain a triple bond?

A. `SO""_4^(2-)`

B. `CO`

C. `N_2`

D. `CN^-`

The answer is A.

`SO""_4^(2-)` ion contains two single bonds and two double bonds

B is incorrect because `CO` contains a triple bond between C and O atoms and there is also a lone pair electrons on both atoms.

C is incorrect because `N_2` contains a triple bond between two atoms and a lone pair electrons on each atom.

D is incorrect because `CN^-` contains a triple bond between C and N atoms and there is also a lone pair electrons on both atoms.