Metalwork - Materials - Chemical Bonds

Big Bang
Chemical Bonds are the joining or bonding forces that connect Atoms together to form what we see all around us in the world. In fact, only for Chemical Bonds the Universe would still look like it did after the Big Bang. You should probably have a look at the section on Atomic Structure before you try and figure out what's going on here.

Why Do Atoms Bond?

There are 112 different types of atoms know at the moment. Most of these atoms do not 'wish' to exist on their own and so they join up with other atoms that have the same 'problem'. Unfortunatly, the facts are not quite so simple. The number of Electrons an atom has determine wheather it has to bond or not.

Argon AtomWhy are these Electrons important? Here's the deal: all atoms prefer to have a certain number of electrons in their Outer Shell. However this 'magic' number of electrons is rarely the number an atom naturally has. The atom you can see on the right is called Argon. If you count the electrons around the central nucleus you should arrive at the number 18. ARGON is a 'happy' atom as it has a 'magic' number. Its outer shell is full, and you will find it does not want to, and is very hard to force to, join up with any other atoms!.

A 'family' of atoms called the Noble Gases, of which ARGON is a menber, are all 'snotty' like this. All of the other atoms, however are only too 'happy' to join up, so long as they meet eachothers needs. To do this atoms will take on or lose electrons to gain their 'magic' number. It is this taking on, giving up, or even sharing of electrons which determine the type of bonding. There are two basic types of bonds:

  1. Primary Bonds
  2. Secondary Bonds
Secondary Bonds are weak and Primary Bonds are strong. We will look at the Primary Bonds now. The Primary Bonds are called Metallic Bonds, Covalent Bonds and Ionic Bonds. When a bond is formed, or a bond is broken, bond energy is involved. A strong bond may require more energy to break than a weak bond. This can lead to strong molecules as in a solid, and to weak molecules as in a gas. By bonding together atoms can all be 'happy'. (If only life was so Simple!)

Metallic Bonding

You can probably guess what type of material is usually formed by a Metallic Bond....yes a metal! But why? Well most metal atoms only have a few 'lonely' electrons in their outer shell and so they are willing to lose these atoms without putting up too much of a fight. We will take an atom of Aluminium as a example. The Aluminium atom has only 3 electrons in its outer shell, but it would like to have 8. The atom now has a decision to make, lose 3 electrons or gain 5. It should be obvious that it is easier to lose 3 than to gain 5. So when metal atoms join up or bond they all get rid of their excess electrons. Now we're going to get a bit more technical.. When an atom loses an electron its charge changes to positive, and a positively charged atom is called a Cation. Where do all the electrons go, I hear you ask? Well they just flow around between the Cations, and are 'shared' by them all, (so in a way our friend, the Aluminium atom has both lost 3 electrons and gained 5 electrons). In the diagram below you can see the Cations in blue surrounded by green which is the electrons. All of the electrons together are generally called 'a sea of electrons' by the scientists. Therefore each metal atom is bonded to a 'sea' of electrons. Remember electrons are negatively charged.

Example of Metallic Bonding
Therefore a Metallic Bond consists of Cations in a 'sea' of electrons. The Cations can slip over eachother when a force, such as the blow of a hammer, is applied and this gives metals Ductility, which is the ability of a metal to be reshaped without breaking. Also the electrons can move very easily around the metal making metals very good conductors of heat and electricity.

Covalent Bonding

Well it doesn't look like the name of this type of bonding is going to help much! Actually the name is quite appropriate, but it is too much to go into at the moment. For now it is suffice to say Covalent Bonding essentially means 'the sharing of electrons' Again we're back to the fact that atoms are not 'happy' unless their outer shell is full. What better way to solve the problem can there be but to get together with other atoms that are in the same prediciment, and share the electrons. Well if that's enough for you then you better scroll back to the top... because here comes the realy interesting stuff...

Hydrogen and Oxygen AtomsWe will look at a Water Molecule to help to explain what is going on here. The diagram on the right shows 2 Hydrogen atoms and 1 Oxygen atom. The Hydrogen atoms are 'unhappy' because their outer shells are not full. They would like to have 2 electrons each. And the Oxygen atom is not 'happy' because its outer shell is not full. It would like to have 8 altogether in its outer shell, (ITS EASIER THAN LOSING 6). So the Oxygen atom says to the two Hydrogen atoms "I'm short 2 electrons, and together you have 2, you're both short one each and I have 6 in my outer shell, so why don't we get together and have a party!'.

Covalent Bonding of a water moleculeAnd here, on the right, you can see the result. Together they form a water molecule. (H20). And here things get realy sticky....As you can see there are more electrons, (the black dots), occupied at the end of the molecule where the Hydrogen atoms are. This causes the opposite side of the whole Molecule to have a positive charge. As a result the Hydrogen end of the whole molecule has a negative charge. This is an example of Electrostatic Induction. Can you figure out how water molecules stick together? Remember 'like forces repel and unlike forces attract'.

Therefore Covalent Bonding can be described as 2 or more atoms sharing a pair of electrons with strong Electrostatic Forces.

Ionic Bonding

The name of this type of bonding gives us a clue. Remember when we taked about Metallic Bonding we spoke of Cations. Cations are positively charged atoms. A charged atom is called an Ion. We know what a positive ion is called, but a negative ion is called an Anion. So Ionic Bonding has to do with the bonding of Ions. We are going to look at Sodium Chloride(NaCl) as an an example.....what? You know it better as Salt. Sodium has 1 electron in its outer shell, so guess what...its not 'happy'. Chlorine has 7 electrons in its outer shell...yeah its not 'happy' either. So when they get together Sodium gives its extra electron to Chlorine. Now both atoms are happy. A side effect of this is that Sodium now has less electrons than it has protons, and so its charge is positive, making it a Cation. And Chlorine now has more electrons than it has protons making it negative, an Anion. The atoms now come together because the have opposite charges and they are attracted to eachother. In the diagram below notice how the atoms alternate between positive and negative. The Ions are held together by Electrostatic Forces.

Example of Ionic Bonding
So in an Ionic Bond atoms give up and take on electrons resulting in a change in charge which causes the atoms to be attracted to eachother.