Covalent bonding.

Introduction

What holds a covalent bond together

The Energy Well Diagram

Representing Covalent Bonds

Dot and Cross

Ball and Stick

Space fill

Ball and Stick with Electron Boundaries

Quick Recap

How can we represent covalent bonding?

What about double bonds or even multiple covalent bonds?

What about a triple bond?

Exercises on covalent bonds.

 

Introduction

  • Nearly all covalent bonding occurs between non-metals.
  • Simplest is H2; the 1s orbitals of the two hydrogen atoms overlap.
  • Each hydrogen atom now has two electrons in its outer energy level.
  • Noble gas configuration achieved!

Hydrogen Atoms

Hydrogen Molecule

The atomic orbitals have merged together to form a molecular orbital. Electrons can only par up if they already have opposite spin.

What holds a covalent bond together?

  • Nuclei repel, but are attracted by the pair of negative electrons.

  • This is an ordinary covalent bond.

  • Sometimes one atom provides both electrons for a covalent bond. This is called a dative bond. A surprise for later.

  • A covalent bond involves a balance between attraction and repulsion.

  • The atoms are continuously vibrating as though they are on springs.

  • If enough energy is provided, the bonds will break.

The ‘energy well’ diagram

Representing Covalent Bonds

The usual way of representing a bond is to draw a line between two elements H – H, but there are other ways.

 

Dot and Cross Diagram

Ball and Stick

Space Fill

Ball and Stick with Electron Boundaries

Quick Re-cap.

  • Ionic bonding generally occurs between elements with a large difference between their electronegativities, eg a metal and a non-metal.
  • Covalent bonding usually occurs between non-metals
  • Metal-Metal bonding is rare, and is called metallic bonding.

How can we represent covalent bonding?

Covalent bonding is about two atoms sharing electrons to gain a full octet (or greater).

Looking at hydrogen chloride – using dot and cross diagrams.

Both the hydrogen and the chlorine now have a full outer shell (octet), even though they are sharing electrons.

A quicker method of doing this is to leave out the circles that represent the energy levels. This demands tidiness!

Can we use this for more than 2 atoms?

Yes, look at methane, formula CH4, the simplest hydrocarbon.

Notice the electrons are as far apart as possible.

Remember this is a representation of a 3-dimensional structure, and we need to develop special rules for drawing these in 3D.

  • Each bonding pair spreads out as far as possible

 In 2D, this is how we could draw the structure.

 

  However in 3 dimensions, it looks a little like this:

 

[methane]

 The hydrogen atoms forms the corners a tetrahedron.

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What about double bonds…… or even multiple covalent bonds?

In organic chemistry there are double and triple bonds as well as single.

Bond Type

Number of electrons shared

Single

2

Double

4

Triple

6

Eg for Oxygen

So a representation of oxygen:

O=O

What about a triple bond?

This occurs in Nitrogen, 5 electrons in the outer sub-shell.

We can represent this as:

Now try these:

H2S, HI, CH3OH, PCl3, SiO2, NH3