Engineering - The Structure of Materials
There is a close relationship between the structure of the atoms within a material and the properties that the material possesses.
The Structure of An Atom
Atoms are very small and an Electromagnetic Microscope is used to view them. An atom is made up of a Nucleus and Electrons flying around it. The Nucleus is made up of Protons, which are positively charged, and other particles. In relative terms, the Electrons, which are negatively charged, fly around the Nucleus at a very large distance. The number of Electrons is usually equal to the number of Protons for each type of atom. Atoms which do not have the same number of Electrons and Protons are called Ions. Because the charges of the Protons and the Electrons are opposite they are attracted to eachother, and this is very important.
Have you read about Atomic Sturcture in the Junior Cert. Metalwork section ?
Electrostatic Forces work in much the same way as magnetic forces. Like forces repel and unlike forces attract.
Electrostatic Induction is what occurs in cases such as when you rub a balloon and then it can stick to your jumper. What happens is that the negative charge on the balloon, caused by the rubbing, repels the negative charges in your jumper causing the positive charges to be attracted to the balloon. Thus they stick.
Have you read about Electrostatic Forces in the Junior Cert. Metalwork section ?
The Periodic Table of the Elements
The Periodic Table of the Elements has been developed over the centuries, and is a 'list' defining all of the elements known to man. As a result of the work of many scientists, a lot of information can be recovered from the Periodic Table, such as the number of Protons in a certain atom, the mass of the atom, and much, much more.
Goto the Interactive Periodic Table if you wish to learn more about who developed it, or about particular atoms.
How Atoms Bond Together
Atoms generally bond together in 3 different ways. They are :
Whichever way the atoms bond depends on both the number of Electrons the atom has in it's outer shell and the atom it is bonding with. Goto Chemical Bonding to learn more about this topic in total.
All materials have properties, and as was stated earlier these properties are dependant on the atomic make-up of the material, both the atoms themselves and the manner in which they are bonded. Goto the Properties page for a more detailed description of the common properties of materials.
Here are some basic properties of Metals and Non-Metals :
States of Matter
There are 3 States of Matter. They are :
The State of Matter depends on the temperature and the pressure. Goto States of Matter to learn more about this topic.
Solidifying Metal And Dendretic Growth
The cooling of a metal from a liquid to a solid is extremely important. As a metal reaches its cooling point small particles cool first. Solidification takes place in a pattern. This pattern is called Dendretic Growth and looks like the branches of a tree. Each small particle grows to form a crystal or grain. Crystals grow together to form a solid.
The solidification starts at one point and spreads out like the branches of a tree. Eventually the Dendrite branches meet and form grains as in the final diagram.
Crystal Patterns and Unit Cells
In metals the atoms bond together in patterns that repeat.
A single pattern is called a Unit Cell. A group of patterns is called a Lattice.
The Unit Cells join together in three dimensions to form the Lattice, through Dendritic Growth, finally resulting in a Crystal or Grain. The formation of the grain follows the sequence shown below.
The images above are only representations. They are the basic unit structures that form the grain.
Slip In BCC & FCC Metals
Atoms in a BCC structure are not closely packed and so a large force is required to cause them to slip. As a result shearing is less likely to occur. Therefore brittle metals have a BCC atomic structure.
Atoms in a FCC structure are more closely packed together. Therefore slip occurs more easily. Therefore shear is more likely to occur.
Here are some examples of what metals have which atomic structure.
Atomic Imperfections in Metals
Crystal Defects are any imperfections within the crystal structure.
Another type of defect commonly found in metal crystalyne structures is called a Point Defect. There are four basic types :