Читать книгу Earth Materials - John O'Brien - Страница 38

Earth materials are composed of smaller entities. At very small scales, such as viewed through a tunneling electron microscope, we are able to discern particles called atoms whose effective diameters are a few angstroms (1 Å = 10⁻¹⁰ m, about a million times smaller than the width of a human hair). These tiny atoms, in turn, consist of three main particles – electrons, protons, and neutrons – which were discovered between 1895 and 1902. The major properties of electrons, protons, and neutrons are summarized in Table 2.1.

Protons (p⁺) and neutrons (n0) each have a mass of ~1 atomic mass unit (amu) and are clustered together in a small, positively charged, central region of the atom called the nucleus (Figure 2.1). Protons possess a positive electric charge and neutrons are electrically neutral. The nucleus is surrounded by a vastly larger, mostly “empty”, region called the electron cloud. The electron cloud represents the area in which the electrons (e⁻) in the atom move about the nucleus in patterns called orbitals (Figure 2.1). Electrons have a negative electric charge and an almost negligible mass of 0.000 005 amu. Knowledge of these three fundamental particles in atoms is essential to understanding how minerals and other materials form, how they can be used as resources and how we can deal with their hazardous effects that can be seen occasionally.

Table 2.1 Major properties of electrons, protons, and neutrons.

Particle type	Electric charge	Atomic mass (amu)a
Proton (p+)	+1	1.00 728
Neutron (n0)	0	1.00 867
Electron (e−)	−1	0.0 000 054

^a amu = atomic mass unit = 1/12 mass of an average carbon atom.

Figure 2.1 Simplified model atom with nucleus that contains positively charged protons (dark blue) and electrically neutral neutrons (light blue) surrounded by an electron cloud (red shades) in which negatively charged electrons move in orbitals about the nucleus.