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Engineering Fundamentals. Atomic Structure. Atomic Structure .
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Engineering Fundamentals Atomic Structure
Atomic Structure • A study of the properties of materials reveals vast differences not only across groups of materials within these groups. It is known that there is a direct relationship between the propertiesof a material and the atomic structure of that material.
The theory of the atom: • Bohr-Chadwick model of the atom developed in 1832. • Anyone know the generic representation of the atom?
Protons • Possess a positive charge and exist together with neutrons in the nucleus of the atom.
Neutrons • Possess no electrical charge and are heavier than protons. Generally, the larger the size of the atom the higher the neutron-proton ratio in the nucleus.
Electron • Have a negative charge equal to the positive charge of a proton. Electrons exist in shells or orbitals surrounding the nucleus. The electrons, particularly those in the outer most orbitals (valance electrons) determine many engineering properties of materials.
Chemical bonding • The strength and direction of the chemical bonding present in the material determines the arrangement of atoms. • Ions, radicals and molecules also experience bonding. • Chemical bonds are classified as either primary or secondary bonds. This depends on the degree of electron involvement. Primary bonds are stronger and more stable.
Chemical bonding continued… • There are three main types of primary bonds. • These are?
Ionic bonding • Are the attractive forces existing between a positive and a negative ion. These ions are formed when the atoms involved lose or gain electrons in order to stabilise their valence electron shells.
Covalent bonding • Are formed when pairs of electrons are shared by several atoms. Stable covalent bonds are formed between many no-metals. This is due to these elements usually possessing half-filled outer electron shells which resist the direct electron transfer typical of an ionic bond.
Metallic bonding • May be seen as a result of individual metallic atoms losing their outer valence electrons which then exist in the metal as a cloud or electron gas.
Secondary bonds • Are a comparatively weak intermolecular bond formed as a result of dipole attractions. The dipoles are formed as a result of the unbalanced distribution of electrons in asymetrical molecules. Dipoles may be permanent as in water molecules or they may be temporary, in which case they are known as Van Der Waals forces.
The crystalline state • A crystal can be defined as a homogenous solid possessing a regular three dimensional lattice arrangement of atomic particles. • Non-crystalline materials are termed amorphous. Examples of these are glass and some plastics.
The crystalline state continued… • The atoms in crystalline materials are arranged in a 3D repeating pattern, which is known as its lattice structure. The lattices are best described as a network of lines drawn in space with the intersection of these lines being lattice points where the atoms, ions and molecules making up the material are located.
The crystalline state continued… • Three main crystal structures: • Body centred cubic (BCC); • Hexagonal close packed (HCP); • Face-centred cubic (FCC).
