John O'Brien - Earth Materials

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1 2.1 Atoms

2 2.2 The periodic table

3 2.3 Chemical bonds

4 2.4 Pauling's rules and coordination polyhedra

5 2.5 The chemical classification of minerals

If we zoom in on any portion of Earth, we will see that it is composed of progressively smaller entities. At very high magnification, we will be able to discern very small particles called atoms. Almost all Earth materials are composed of atoms which, in turn, strongly influence their material properties. Understanding the ways in which these basic chemical constituents combine to produce larger scale Earth materials is essential to understanding our planet.

In this chapter we will consider the fundamental chemical constituents that bond together to produce Earth materials such as minerals and rocks. We will discuss the nuclear and electron configurations of atoms and then discuss the role these play in determining both atomic and mineral properties and the conditions under which minerals form. This information will provide a basis for understanding how and why minerals, rocks and other Earth materials have the following characteristics:

1 They possess specific properties that characterize and distinguish them.

2 They provide benefits and hazards through their production, refinement and use.

3 They form in response to particular sets of environmental conditions and processes.

4 They record information about the conditions and processes that produce them.

5 They permit us to infer significant events in Earth's history.

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 atomswhose effective diameters are a few angstroms (1 Å = 10 −10m, 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.

aamu = 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.

The nucleus of atoms is composed of positively charged protons and uncharged neutrons bound together by a strong force. Ninety‐two fundamentally different kinds of atoms called elements have been discovered in the natural world. More than 25 additional elements have been created synthetically in laboratory experiments during the past century. Each elementis characterized by the number of protons in its nucleus. The number of protons in the nucleus is called the atomic number (Z). The atomic number is typically represented by a subscript number to the lower left of the element symbol. The 92 naturally occurring elements range from hydrogen (Z = 1) through uranium (Z = 92). Hydrogen ( 1H) is characterized by having one proton in its nucleus. Every atom of uranium ( 92U) contains 92 protons in its nucleus. The atomic number is the unique property that distinguishes the atoms of each element from atoms of all other elements.

Every atom also possesses mass that largely results from the protons and neutrons in its nucleus. The mass of a particular atom is called its atomic mass number, and is expressed in atomic mass units (amu). As the mass of both protons and neutrons is ~1 amu, the atomic mass number is closely related to the total number of protons plus neutrons in its nucleus. The simple formula for atomic mass number is: atomic mass number = number of protons plus number of neutrons (#p ++ #n 0). The atomic mass number is indicated by a superscript number to the upper left of the element symbol. For example, most oxygen atoms have eight protons and eight neutrons so their atomic mass number is written 16O.