Engineering Materials

Materials are essential for the development and growth of human civilization. The designation of successive historical epochs, such as the Stone, Copper, Bronze, and Iron Ages, reflects the importance of materials. Engineering materials are the foundation of technology. Material science is concerned with the behavior of materials and the variation of their properties.

Atomic Structure

The smallest unit of a material, the atom, consists of three particles: electrons, protons, and neutrons. Electrons are negatively charged particles with a magnitude of $1.6 \times 10^{-19}$ C (Coulombs), whereas protons are positively charged particles of equal magnitude. Neutrons are neutral particles with no charge. Protons and neutrons, collectively called nucleons, are bound by nuclear forces into atomic nuclei. A neutral atom has the same number of electrons and protons.

Neutrons and protons have very similar masses, roughly equal to 1 atomic mass unit (a.m.u.) each. The mass of an electron is negligible compared to that of a proton or neutron.

Basic Terminology

  1. Atomic Number: A chemical element is characterized by the number of protons in its nucleus. The atomic number ($Z$) is the number of protons found in the nucleus of an atom and is equal to the number of electrons for an electrically neutral atom. The atomic number ranges from 1 (Hydrogen) to 92 (Uranium), the highest of the naturally occurring elements.
  2. Atomic Weight: The atomic weight of an element is the average relative weight of its atom compared to the weight of one atom of carbon-12, which is taken as 12. All atoms with different atomic weights but belonging to the same element are called isotopes.
  3. Atomic Mass Unit (a.m.u.): One atomic mass unit is equal to $1.6605 \times 10^{-27}$ kg. One mole of an atom has a mass in grams equal to its atomic mass in a.m.u. For example, one mole of carbon has a mass of 12 g.
  4. Avogadro’s Number: Avogadro’s number is defined as the number of carbon-12 atoms in 12 grams of unbound carbon-12 in its rest-energy electronic state. It is the reciprocal of the atomic mass unit: \[ N_A = \frac{1}{1.6605 \times 10^{-27}} = 6.0223 \times 10^{26} \text{ /kmole} = 6.0223 \times 10^{23} \text{ /mole} \] The number of atoms in one mole is called Avogadro’s number ($N_A$).