Covalent Bonds

From lecture 4 of Structure Bonding and Equilibria

Briefly describe covalent bonding

Covalent Bonding. Sharing of electrons between atoms. In general, covalent bonds are found in elements with high electronegativity.

What is the relative strength of a covalent bond

In general they are stronger then hydrogen bonds (≈30 kJ mol^-1) but are weaker than ionic bonds (500-5,500 kJ mol^-1)

In general how does number of electrons shared effect strength and length of a bond

-In general Multiple bonds are shorter and stronger than single bonds. The greater number of electrons that are shared the shorter and stronger the bonds. 

Single share 2,double share 4,triple share 6. 

Describe the Lewis model

Simplest display of structure showing only the bonds outer electrons and the elements

What 4 parts make up the Octet rules

The octet rule 

1. Elements in the row Li-Ne never have more than a share of 8 electrons in the outer shell. 

2. Elements Li-Ne may form compounds with fewer than eight electrons 

3. Main group compounds from groups further down the periodic table may "expand their octet" to form "hypervalent" species 

4. It is not applicable to transition metals, lanthanides and actinides

Define Bond Dissociation Energies (BDEs)

These are values of when a specific bond in a compound dissociates, the energy required to break a specific chemical bond in a molecule

How does the size of an atom affect its bond strength

As atoms get larger they become more diffuse( spread out) and as such find it more difficult to directly overlap orbitals forming weaker bonds.

Big atom = Weaker bonds, as less direct overlap

Why do more electronegative atoms form stronger bonds

They have comparatively smaller orbitals, and as such, electrons are more strongly attracted to the nucleus.

Bonds are shorter and stronger as orbital overlap is better.

Describe an example which go against greater electronegativity always creating stronger bonds. (Using H3C-NH2 and C2H6)

-The presence of the more electronegative N is expected to increase bond strength but this isn’t the case.

-Due to nitrogen’s lone pair of electrons it will have greater repulsion forces. As such, this destabilises the bond making it weaker.

-Therefore due to the extra repulsion the bond is more easily broken.