What is meant by atomic arrangements?

Atoms are the smallest units of matter. Atoms can be subdivided into electrons, protons, and several other subatomic particles. Groups or combinations of atoms form molecules that primarily decide the property and behavior of materials. These molecules have the property to attract other molecules based on the valence electrons in the atoms' outermost orbit. An orderly arrangement of molecules is known as crystal structure, which forms the fundamental topic of concern in materials science and engineering. The arrangement of molecules is different for different materials and hence, each material exhibits different lattice arrangements. The lattice arrangements are functions of external loads, deformations, and temperature. On experiencing high temperatures or low temperatures, the behavior of the lattice arrangement of a material change.

Response of crystal structure with respect to changes in external temperature occurs due to an imbalance in the intermolecular bonding, medium-strength intermolecular interactions, and weak Vander Waal's forces. The thermal equilibrium and thermal properties are altered due to temperature changes. Moreover, the concept is largely used in several heat treatment processes to harden materials, induce, and alter the various properties of materials such as physical, mechanical, and chemical properties. At extreme temperatures, the structure of atomic arrangement changes along with grain boundary refinement. This is an important area of study in the materials science and engineering domain.

Representation of helium atoms
CC BY-SA 3.0 | Image credits: https://commons.wikimedia.org | User:Yzmo

States of matter and temperature effects

From the elementary level, it is known that matter consists of mainly three states, solid, liquid, and gas. At low temperatures, the internal energy is decreased, which reduces the molecular activities. Also, at low temperatures, the intermolecular force of attraction is more. As a consequence, the molecules have less tendency to move apart. As the temperature is reduced further, a point is reached when the molecular activity ceases, and the molecules are closely packed together. At this condition, the substance behaves like a solid. Molecular activity increases with the temperature rise, the molecules have enough energy that allows them to exert forces on one another. This phenomenon or behavior forms the liquid phase. In the gaseous state, the molecules absorb enough heat energy and utilize that energy to overcome the energy of interatomic interactions, which makes them independent and free to move.

Temperature effects and phase change

A phase is a state of a system or matter having distinct properties. A phase of a system can be changed by temperature changes. A substance changes its phase from solid to liquid or liquid to gas and vice versa by absorbing or releasing latent heat. A substance in a solid state, when supplied by heat energy, the molecular vibrations increase which raises the sensible energy of the substance. Continuous supply of heat energy leads to a stage where the molecules of the substance absorb the latent heat energy to overcome the atomic interactions to behave like a liquid. The phase change always occurs at a constant temperature. A similar explanation can be provided for the transition from the liquid to the gas phase.

Water vapor is present in the atmosphere in the gaseous form of water. Water vapors either form by evaporation or by boiling water in the liquid phase. During boiling and evaporation, the molecules of water absorb the latent heat to detach themselves from corresponding molecules to become free and become vapor. If the molecules of the vapor are cooled by lowering the temperature, the energy possessed by the molecules will also reduce, at a constant temperature stage, the molecules will release the latent energy to change its phase into liquid. This phenomenon is also known as condensation. In a similar context, this principle can be used to explain the formation of solid or ice. The energy released in the formation of ice is preferably known as latent heat of fusion.

Temperature effect on lattice parameters

As discussed in the quick overview section, an orderly molecular arrangement is known as a crystal lattice or crystal structure. From these crystal structures, if a small unit is considered such that it signifies the entire crystal structure geometry, the small unit is known as a unit cell, which is a parallelepiped. The combination of several of these units gives rise to the entire crystal structure. A unit cell is characterized by lattice parameters or lattice constants. These are constants for each unit cell and are not variables. A lattice parameter is characterized by six constants, three constants that define the lengths of the parallelepiped meeting at a vertex, and three constants that define the angles of those vertexes. These parameters are found to be a direct function of temperature and pressure. These parameters are found to vary linearly with the rise in temperatures with increased thermal expansion. The altered magnitudes of the lattice parameters as a phenomenon of temperature effect are analyzed by x-ray diffraction devices.

Temperature effect and heat treatment

Many mechanical processes such as welding, hot working, machining, and so on, make use of heat. Due to heat, the metallurgical structure changes due to atomic rearrangements, and the property of the material is either altered or enhanced. This characteristic is mainly used in the heat treatment process of metals in industries. Heat treatment is the science of heating metals lower than their melting point and sometimes above or below their recrystallization temperature. New grain growths are characterized by the recrystallization process. The process of heat treatment ends with cooling the material in a controlled way, cooling results in a drop in temperature that reorients the atomic arrangements giving rise to specific properties. For instance, refer to the figure below, the figure shows the atomic arrangements of two forms of iron. The left figure shows the atomic arrangement of iron at low temperatures known as alpha iron, whereas the right figure shows the altered atomic arrangements of iron at high temperatures, known as gamma iron. This signifies that temperature forms the basis of atomic structure alteration.

Atomic arrangements of alpha iron (left) at low temperature and gamma iron (right) at high temperatures
CC BY-SA 3.0 | Image credits: https://en.wikipedia.org | Cdang

During the heating phase, the atoms shift from their positions and occupy new positions during the cooling phase. The crystal structure orients into a new arrangement that governs the property of the material altered by heat treatment. There are various methods used in the heat treatment process they are:

  • Annealing
  • Tempering
  • Case hardening
  • Carburizing
  • Normalizing
  • Quenching, and
  • Precipitation hardening

Context and Applications

The topic is extensively taught in many undergraduate and postgraduate degree courses of:

  • Bachelors in Technology (Mechanical Engineering)
  • Bachelors in Science (Physics)
  • Bachelors in Science (Chemistry)
  • Masters in Technology (Metallurgical Engineering)
  • Masters in Technology (Materials Science and Engineering)

Practice Problems

1. Which of the following are the sub-atomic particles?

  1. Electrons
  2. Protons
  3. Molecules
  4. Both electrons and protons

Answer: Option d

Explanation: Both electrons and protons are the smallest particles found inside an atom. They are known as sub-atomic particles.

2. Which of the following devices is used to measure the lattice parameter?

  1. Microscope
  2. X-ray diffraction device
  3. Electron microscope
  4. All of these

Answer: Option b

Explanation: The lattice parameters can be measured by using an X-ray diffraction device.

3. What is the effect on temperature during a phase change process?

  1. Varies
  2. Remains constant
  3. Only increases
  4. Only decreases

Answer: Option b

Explanation: During a phase change process, the temperature of a substance remains constant.

4. What is the impact on the lattice parameters if the temperature is increased?

  1. Increases linearly
  2. Increases parabolically
  3. Increases exponentially
  4. No effect

Answer: Option a

Explanation: The lattice parameters are a linear function of temperature. With the increase in temperature, the lattice parameters increase linearly.

5. Which of the following is a heat treatment process?

  1. Normalizing
  2. Spectroscopy
  3. Carburizing
  4. Both a and c

Answer: Option d

Explanation: Both normalizing and carburizing are heat treatment processes.

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