Geology and Mineralogy of Gemstones. David Turner R.Читать онлайн книгу.
places. For example, silver has an atomic number of 47 and an atomic mass of 107.868. This is not to say that silver has 47 protons and 107.868 neutrons. Rather, 107.868 represents the average atomic mass of a sample of gold that includes gold atoms with different numbers of neutrons. Material comprised of high atomic mass elements will generally be of higher density, such as in the case of the native metals in Table 1.1.
1.3.3 Atomic Structure, Electrical Charges, and Ions
Protons and neutrons are roughly the same size and are located in the nucleus or core of an atom. Outside this nucleus are the electrons that orbit the atomic core in an unpredictable but organized electron cloud much larger than the size of the nucleus itself (Figure 1.1). Because the mass of neutrons and protons is so much greater than that of the electrons, nearly all the mass of an atom exists at its nucleus. Strong atomic forces keep the neutrons and protons tightly packed in a dense cluster.
Table 1.1 Atomic masses and physical properties of selected elements and their native metal mineral.
| Element | Atomic number | Average atomic mass | Density (g/cm3) of native metal mineral |
|---|---|---|---|
| Copper, Cu | 29 | 63.55 | 8.9 |
| Silver, Ag | 47 | 107.87 | 10.5 |
| Gold, Au | 79 | 196.97 | 19.3 |
Protons have a positive electrical charge, neutrons have no electrical charge, and electrons have a negative electrical charge. The sum of their charges denotes overall ionic or atomic charge. In a basic atom of a given element with all of its allotted electrons, an atom is neutral. This means that all of the negative charges of the electrons are balanced by all of the positive charges of the protons.
Some atoms are prone to gaining electrons from outside sources, which results in them having a net negative charge. Other atoms are prone to losing electrons, resulting in a net positive charge. The resulting charge, positive or negative, is called the valence state (or valence charge) of an atom. Charged atoms are called ions; specifically, positively charged ions are called cations while negative ones are called anions (Figure 1.2). The exchange (gain or loss) of electrons almost always occurs within the outermost portion of the electron cloud.
Figure 1.1 An atomic model of the element helium (He), with two protons, two neutrons, and two electrons.
The electron cloud of an ion can be estimated to be in the shape of a sphere and its size is defined by the distance from the center of the nucleus to the limit of the cloud. This is called the ionic radius, which is measured in units called Angstroms or Å. An Angstrom unit is very short – it is equal to one tenth of a nanometer. Note that a nanometer is 0.000000001 meter or 10–9 meter!
1.3.4 Elements
There are 92 naturally occurring elements out of a total 118 identified, each with its own symbol that acts as a shorthand notation. Some familiar elements and symbols are Au for gold, C for carbon, Ag for silver, and Pt for platinum. Element abbreviations start with a capital letter and if a second letter is present it will always be lowercase. Many elements are already part of our everyday vocabulary, such as oxygen (O), carbon (C), nitrogen (N), and potassium (K) but others are much more obscure, such as beryllium (Be), scandium (Sc), and rhodium (Rh).
Figure 1.2 An atom can lose electrons and become a cation, a positively charged ion. If it gain electrons, it becomes an anion, a negatively charged ion.
1.3.5 Element Groups
A group is a column of elements in the periodic table (Figure 1.3). Elements within a group have similar chemical behavior because of the similarity in the distribution of their electrons, especially in the valence (outermost) shell. The elements of the first group are called the alkali metals and tend to give up an electron, resulting in a characteristic +1 valence charge. A familiar element in this group is sodium (Na), part of the NaCl (table salt) molecule. Although hydrogen sits at the top of the column, it does not actually belong to the alkali metals group.
The elements of the second group are collectively called the alkaline earth metals. These elements usually lose two electrons, resulting in a characteristic +2 valence charge. Calcium (Ca) and magnesium (Mg), two of the important bone‐forming ingredients, are elements of this group.
The middle block of elements (ranging from Sc down and across to element 112, Cn) are called the transition metals. These elements can have variable valence charges, usually up to +4 but sometimes as high as +6. Note how the precious metals Cu (copper), Ag (silver), and Au (gold) are all Group 1B transition metals and thus share similar physical properties. The metals Ni (nickel), Pd (palladium), and Pt (platinum) are similarly related as Group VII elements. The transition elements often endow gemstones with their striking colors.
Elements classified as semi‐metals or other metals include aluminum (Al) and lead (Pb). The next group are the metalloids, including silicon (Si) and arsenic (As). Nonmetals include the biologically important elements carbon (C), nitrogen (N), oxygen (O), phosphorus (P), and sulfur (S).
The halogens occupy the seventeenth column and will almost always have a –1 charge. Familiar elements in this group are chlorine (Cl) and iodine (I). Elements on the far right are the noble gases, which do not combine with other elements. Notable gases in this group are helium (He) and neon (Ne). The two large blocks below the table are the Lanthanide and Actinide series elements.
Figure 1.3 Periodic table of the elements with atomic numbers and chemical abbreviations. Dashed and dotted borders indicate the element is not naturally occurring.
1.3.6 Elemental Abundance in the Earth’s Crust
Although the periodic table appears to suggest that the elements are equally abundant and distributed proportionally on Earth, this is far from the case. The chemical composition of the Earth’s crust is in fact made up of eight dominant elements that comprise ~98.5%; all other elements combined make up the remaining ~1.5%. This distribution is shown in Table 1.2. Consequently, the bulk of the minerals commonly encountered have their base chemical formula closely associated with these eight elements.
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