Exploring Atomic Structure and Physical Properties of Materials

This report delves into the relationship between atomic structure and the physical properties of materials, focusing on crystal structures and polymer types. It begins by describing Face-Centered Cubic (FCC), Body-Centered Cubic (BCC), and Hexagonal Close-Packed (HCP) crystal structures, detailing their atomic arrangements and coordination numbers. The report further differentiates between crystalline and amorphous polymers, highlighting their contrasting properties such as strength, melting point, transparency, and chemical resistance, along with examples of their applications. The analysis provides a foundational understanding of how atomic-level arrangements dictate the macroscopic behavior of materials in mechanical engineering.

Mechanical Engineering.
Structures and Materials Properties
LO1 Explain the relationship between atomic structure and physical properties of materials.
Part 1:
a. Crystal structure Types
Face-Centered Cubic Crystal structure (FCC).
As shown in the figure 1 below, the Face-Centered Cubic is a crystal structure which has atoms
located at both the centers of the cubic faces of a cubic unit cell all the corners of the cube. This
follows that, there is mutual sharing of every corner atom by the unit cells (eight in number).
Consequently, the six cubic face center atoms are also shared with an atom seen to be adjacent.
This brings the total number of shared atoms to 12 hence the coordination number for the FCC
structure said to be 12 (Dattun, 2018).
In this structure, the atoms pack relatively closer.
Figure 1. Face-Centered Cubic Crystal structure (FCC)
Body-Centered Cubic Crystal structure (BCC).
The Body-Centered Cubic Crystal structure for a cubic unit cell is defined by location of atoms
(eight) at each of the cube corners and one atom located at the center of the entire cube, as shown
in the figure 2 below. Consequently, there still mutual sharing of every corner atom by the unit
cells (eight in number) from the arrangement of these atoms. Since 8 atoms are shared, the BCC
crystal structure therefore has a coordination number of 8. A BCC unit cell can be illustrated by a
total number of 2 atoms, one located at the center and 8 eighths as the corner atoms. According
to (Bergman, Berglund and Isaksson, 2014 et al), this structure does not allow for close packing
of the atoms as seen in the FCC structure. The BCC is mostly the high temperature form of
metals close-packed at low temperatures.
Examples of materials; chromium, potassium, tungsten, vanadium, etc.

Figure 2. Body-Centered Cubic Crystal structure (BCC) (Rangamathan, 2015).
Hexagonal Close-Packed Crystal structure (HCP).
The Hexagonal Close-Packed Crystal structure has got three layers of atoms. There are six atoms
arranged in the shape of a hexagon in each of the layers. In the middle of each hexagon, there is
also one atom located. As shown in the figure below, there are 6 atoms in the HCP unit cell and
12 corner atoms.
Twelve of these atoms are shared bringing the coordination number for this structure to 12.
This type of structure is most evident in elemental metals such as magnesium, zinc and
cadmium.
Fig 3. Hexagonal Close-Packed Crystal structure (HCP) (Wahab and Wahab, 2018)
b. Difference between Crystalline Polymers and Amorphous Polymers.
Crystalline Polymers
Are often highly ordered in terms of their crystal structure hence giving rise to their strength and
rigidity. These polymers have got a reasonably specific chain conformation and regular chain
structure (Marthur, 2017).
Properties
Relatively high strength- due to strong intermolecular bonding.
They have uniformly packed molecules.
Bear a sharp melting point.
From appearance, they are opaque and translucent.
Has got high chemical and thermal resistance.
Are mostly rigid.
Relatively high resistance to creep shrinkage.
Examples of applications of crystalline polymers are; Polyethylene, polypropylene,
copolymers, nylons and polyesters (Dongying Ju and Tan, 2015).

Amorphous Polymers.
Generally, amorphous solids are characterized by unsymmetrical shape and crystal orientation.
These polymers do not have a specific chain conformation or regular chain structure.
Consequently, the physical their properties in different directions are similar (Polymers Editorial
Office, 2015).
Properties
Relatively low strength- due to weak intermolecular bonding between the particles.
They have irregularly packed molecules.
They do not exhibit a sharp melting point.
From appearance, they are mostly transparent.
Have got poor/low chemical resistance.
High toughness.
High flexibility (Deguchi and Uehara, 2017).
Examples of applications of these polymers are; Polyetherimide, polystyrene, ABS,
polysulfone.
References.
Dattun, J.A. (2018). Face Centered Cubic Structure (FCC) | MATSE 81: Materials In Today’s
World. [online] Psu.edu. Available at: https://www.e-education.psu.edu/matse81/node/2133.
Deguchi, T. and Uehara, E. (2017). Statistical and Dynamical Properties of Topological
Polymers with Graphs and Ring Polymers with Knots. Polymers, 9(12), p.252.
Dongying Ju and Tan, Y. (2015). Advanced material science and technology : selected, peer
reviewed papers from the 7th International Forum on Advanced Material Science and
Technology, June 26-28, 2011 [i.e. 2010], Dalian, China, IFAMST-7. Pt. 2. Stafa-Zuerich Ttp,
Trans Tech Publ.
Marthur, N. (2017). Size-Exclusion Chromatography and Its Optimization for Material
Science. Journal of Material Science & Engineering, 06(04).
Polymers Editorial Office (2015). Acknowledgement to Reviewers of Polymers in
2014. Polymers, 7(1), pp.115–119.

Rangamathan, M. (2015). Body-Centered Cubic Crystal System - an overview | ScienceDirect
Topics. [online] www.sciencedirect.com. Available at:
https://www.sciencedirect.com/topics/chemistry/body-centered-cubic-crystal-system [Accessed
28 Mar. 2020].
Wahab, M.A. and Wahab, K.M. (2018). Genesis of Hexagonal Close Packed (HCP) Structures
from Packing of Identical Spheres (Atoms). Materials Focus, 7(2), pp.223–231.