Covalent Bonding

Covalent Bonding

Define a covalent bond.

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

How is a covalent bond formed?

By the overlap of two atomic orbitals, each containing a single electron.

What are bonding electrons?

The two electrons in the area of overlap between two atomic orbitals.

What is another term for bonding electrons?

A shared pair of electrons.

What particles are attracted to the shared pair of electrons in a covalent bond?

The positively charged nuclei of the bonded atoms.

What is orbital overlap?

The overlapping of atomic orbitals from two atoms, allowing their electrons to form a shared pair.

Which atomic orbitals are mainly involved in covalent bonding at this level?

s and p orbitals.

How is a sigma (σ) bond formed?

By end-on overlap of atomic orbitals.

Where is the electron density in a sigma (σ) bond?

Directly between the nuclei of the two bonded atoms.

What types of orbital overlap can form a sigma (σ) bond?

s–s, p–p or s–p end-on overlap.

How is the σ bond in H₂ formed?

By end-on overlap of two 1s orbitals, each containing one electron.

Where is the highest electron density in H₂?

Between the two hydrogen nuclei.

How is the σ bond in Cl₂ formed?

By end-on overlap of two p orbitals, each containing one unpaired electron.

What happens when two chlorine atoms form Cl₂?

Their singly occupied p orbitals overlap end-on to form a σ bond.

How is a pi (π) bond formed?

By sideways overlap of two parallel p orbitals.

Where is the electron density in a π bond?

Above and below the internuclear axis.

Can a π bond form without a σ bond?

No. A σ bond must form first.

Where are π bonds found?

In double and triple covalent bonds.

What bonds make up a double bond?

One σ bond and one π bond.

What bonds make up a triple bond?

One σ bond and two π bonds.

What bonds are present in the C=C bond in ethene?

One σ bond and one π bond.

What bonds are present in the N≡N bond in nitrogen?

One σ bond and two π bonds.

Which is stronger: a σ bond or a π bond?

A σ bond.

Why is a σ bond stronger than a π bond?

End-on orbital overlap is greater than sideways orbital overlap, producing a stronger electrostatic attraction between the nuclei and the bonding electrons.

Why is a π bond weaker than a σ bond?

Sideways overlap of p orbitals is less effective than end-on overlap.

Why are alkenes generally more reactive than alkanes?

Alkenes contain a weaker π bond that can be broken relatively easily during addition reactions.

Define bond length.

The distance between the nuclei of two atoms that are covalently bonded together.

How is covalent bond strength measured?

By the energy required to break one mole of the bonds in the gaseous state.

What is the general relationship between bond length and bond strength?

The shorter the bond, the stronger the bond.

Why are shorter covalent bonds generally stronger?

The nuclei are closer to the shared electrons, producing stronger electrostatic attraction.

What happens to bond strength as bond length increases?

Bond strength generally decreases.

For bonds between the same two elements, how does bond length change from single to double to triple?

It decreases: single > double > triple.

For bonds between the same two elements, how does bond strength change from single to double to triple?

It increases: single < double < triple.

Put C–C, C=C and C≡C in order of increasing bond strength.

C–C < C=C < C≡C.

Put C–C, C=C and C≡C in order of decreasing bond length.

C–C > C=C > C≡C.

Put N–N, N=N and N≡N in order of increasing bond strength.

N–N < N=N < N≡N.

Put N–N, N=N and N≡N in order of decreasing bond length.

N–N > N=N > N≡N.

Why must bond length and bond strength comparisons usually be made between similar bonds?

Other factors, such as lone-pair repulsion, can affect bond strength.

Why can a bond be shorter but still weaker than another bond?

Factors such as lone-pair repulsion can weaken the bond despite its shorter length.

Why is the F–F bond unusually weak?

Each fluorine atom has three lone pairs, causing strong electron–electron repulsion between the atoms and weakening the bond.

What is the relationship between atomic size and bond length down a group?

Atomic size increases down a group, so bond length generally increases.

Explain the trend C–C > Si–Si > Ge–Ge in bond strength.

Atomic size increases down the group, making the bonds longer and reducing the electrostatic attraction between the nuclei and bonding electrons, so bond strength decreases.