المواد الهندسية الجزء الاول

Some of the important properties of solid materials depend on geometrical atomic arrangements, and also the interactions that exist among constituent atoms or molecules.
FUNDAMENTAL CONCEPTS:
Each atom consists of a very small nucleus composed of protons and neutrons, which is encircled by moving electrons. Both electrons and protons are electrically charged, the charge magnitude being 1.60 * 10-19 C which is negative in sign for electrons and positive for protons; neutrons are electrically neutral. Masses for these subatomic particles are infinitesimally small; protons and neutrons have approximately the same mass 1.67*10-27 Kg, which is significantly larger than that of an electron 9.11*10-31 Kg.
Each chemical element is characterized by the number of protons in the nucleus, or the atomic number (Z). For an electrically neutral or complete atom, the atomic number also equals the number of electrons.This atomic number ranges in integral units from 1 for hydrogen to 92 for uranium, the highest of the naturally occurring elements.
The atomic mass (A) of a specific atom may be expressed as the sum of the masses of protons and neutrons within the nucleus.Although the number of protons is the same for all atoms of a given element, the number of neutrons (N) may be variable.Thus atoms of some elements have two or more different atomic masses, which are called isotopes. The atomic weight of an element corresponds to the weighted average of the atomic masses of the atom’s naturally occurring isotopes. The atomic mass unit (amu) may be used for computations of atomic weight.