Measuring Units on the Atomic Scale

ATOMIC NATURE OF MATTER DOE-HDBK-1019/1-93 Atomic and Nuclear Physics Properties of the three subatomic particles are listed in Table 1. TABLE 1 Properties of Subatomic Particles Particle Location Charge Mass Neutron Nucleus none 1.008665 amu Proton Nucleus +1 1.007277 amu Electron Shells around nucleus -1 0.0005486 amu Measuring Units on the Atomic Scale The size and mass of atoms are so small that the use of normal measuring units, while possible, is often inconvenient. Units of measure have been defined for mass and energy on the atomic scale to make measurements more convenient to express. The unit of measure for mass is the atomic mass unit (amu). One atomic mass unit is equal to 1.66 x 10^-24 grams. The reason for this particular value for the atomic mass unit will be discussed in a later chapter. Note from Table 1 that the mass of a neutron and a proton are both about 1 amu. The unit for energy is the electron volt (eV). The electron volt is the amount of energy acquired by a single electron when it falls through a potential difference of one volt. One electron volt is equivalent to 1.602 x 10^-19 joules or 1.18 x 10^-19 foot-pounds. Nuclides The total number of protons in the nucleus of an atom is called the atomic number of the atom and is given the symbol Z. The number of electrons in an electrically-neutral atom is the same as the number of protons in the nucleus. The number of neutrons in a nucleus is known as the neutron number and is given the symbol N. The mass number of the nucleus is the total number of nucleons, that is, protons and neutrons in the nucleus. The mass number is given the symbol A and can be found by the equation Z + N = A. Each of the chemical elements has a unique atomic number because the atoms of different elements contain a different number of protons. The atomic number of an atom identifies the particular element. NP-01 Page 4 Rev. 0