Chemical Elements

Fundamentals of Chemistry DOE-HDBK-1015/1-93 CHARACTERISTICS OF ATOMS Rev. 0 CH-01 Page 3 The particles that orbit the nucleus are electrons. They are very small, with a mass only 1/1835 the mass of a proton or neutron. Each electron is negatively charged, and the charge of one electron is equal in magnitude (but opposite in sign) to the charge of one proton. The number of electrons orbiting a nucleus is exactly equal to the number of protons contained in that nucleus. The equal and opposite charges cancel each other, and the atom as a whole is neutral. The electrons are bound in the atom by electrostatic attraction. The atom remains neutral unless some external force causes a change in the number of electrons. The diameter of the atom is determined by the range of the electrons in their travels around the nucleus and is approximately 10 cm. The diameter of the nucleus is roughly 10,000 times -8 smaller, approximately 10 to 10 cm. Because the nucleus is composed of neutrons and -13 -12 protons that are about 1835 times heavier than an electron, the nucleus contains practically all the mass of the atom, but constitutes a very small fraction of the volume. Although electrons are individually very small, the space in which they orbit the nucleus constitutes the largest part of the atomic volume. Figure 1 illustrates these size relationships, but not to scale. If the nucleus were the size shown, the electrons would be several hundred feet away. Some of the properties of the atom and its component parts are summarized in Table 1. The masses listed in Table 1 are measured in atomic mass units (amu), which is a relative scale in which the mass of a proton is about 1.0. TABLE 1 Properties of the Atom and its Fundamental Particles Particle Name Relative Mass Relative Charge (amu) (based on charge of proton) Electron 0.00055 or 1/1835 -1 Proton 1.0 1 Neutron 1.0 0 Chemical Elements An atom is classified chemically by the number of protons in its nucleus. Atoms that have the same number of protons in their nuclei have the same chemical behavior. Atoms that have the same number of protons are grouped together and constitute a chemical element.