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ATOMS & CRYSTALS


Elemental metals, IRON, COPPER, ALUMINIUM etc are composed of atoms, with a single atom can be sufficient to participate in a chemical reaction (with a second), to produce a COMPOUND. A MOLECULE is the smallest part of an ELEMENT (or compound), that can exist in a free state and still retain all the properties of that element (or compound). [ref 1] The above mentioned metals are examples of elements that can freely exist as a single atom (MONOATOMIC), as opposed to others (such as OXYGEN), that freely exist retaining all their properties only as two or more atoms combined (DIATOMIC/TRIATOMIC and so on).


It follows therefore that IRON (Fe) can exist freely as a single atom with an atomic weight of 193.10. Iron can also combine with itself and other atoms to produce materials with quite distinct properties, including CAST IRON and STEEL (each more than two atoms combined to exist as a MOLECULE). Both cast iron and steel are ALLOYS of iron.


Atoms combine to form new compounds by BONDING. [ref 2] Bonding typically arises either through one of the following events: (1) the attraction of positive and negative IONS (2) ELECTRONS orbiting more than one atom, or (3) the lowering of an electron’s energy in proximity to more than one nucleus (the last being a METALLIC BOND). The bonding of atoms leads to their organisation into a SPACE LATTICE (sometimes called an ARRAY), forming a single basic CRYSTAL. The lattice of a solid substance is an arrangement of atoms in one of fourteen possible combinations. [ref 3] The combinations most relevant to metallurgy are BODY CENTRED CUBIC (9 atoms), FACE CENTERED CUBIC (14 atoms) and HEXHAGONAL CLOSE PACKED (17 atoms). The first two are of particular importance as both COPPER and ALUMINIUM are face centred cubic, IRON is body centred cubic (at ambient temperatures).


The atoms of alloys also organise in a space lattice formation, however the nature of organisation of the atoms within an alloy lattice varies according to the CLASSIFICATION of a particular ALLOY SYSTEM. For example, alloys that are classed as a SOLID SOLUTION when solidified (these alloys cannot be reduced back into individual elements), will have atoms of the alloying element randomly distributed throughout the space lattice of the parent element (COPPERNICKEL, COPPERGOLD and COPPERZINC [≤37%Zn] combinations are typical of this). INTERMETALLIC COMPOUNDS on the other hand, organise the atoms of each element into a defined position within each space lattice (COPPER - ALUMINIUM, SILVER – TIN and COPPER – ZINC [40-50%Zn] for example). [ref 4].  The third main classification is the EUTECTIC class, these broadly behave like solid solutions when liquid (ie completely mix), but separate out into component elements when solidified.


To more clearly appreciate some of the above points, it is helpful to briefly look at what happens to metals (and then alloys) when they are heated and cooled.

INFO: The Brussels ATOMIUM (designed by Andre Waterkeyn for the Belgium International Exhibition 1958), is based on the space lattice arrangement of an IRON crystal (corner down). With nine atoms arranged in BODY CENTRED CUBIC form, the Atomium represents a crystal form approximately 165 billion times smaller than itself [ref 5].

 

atomic lattice

The two common space lattice arrangements:
Top: Face Centered Cubic.
Bottom: Body Centered Cubic.

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