Geology For Dummies. Alecia M. Spooner

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or chemical reactions are described you don’t have to write each element’s entire name. The symbols of the periodic table are the same all over the world to make it easier for scientists to communicate. In many cases, the elemental symbol is based on the name of an element in a different language and may not make sense in your native language. For example, the symbol for gold is Au because in Latin the word for gold is Aurum, which means yellow. And the symbol for tungsten is W based on its name in German: wolfram.

       Element name: Some periodic tables also list the name of the element below the symbol (see Figure 5-3).

Table 5-1 lists the most common elements in Earth’s crust and their approximate percentage. (This list does not represent the proportion of elements in the mantle, nor does it include the iron and nickel that are found in the earth’s core, as I describe in Chapter 4.) These elements are the ones that compose nearly all the rocks on Earth’s surface. You see them often in this book, so it’s a good idea to get familiar with their atomic symbols.

TABLE 5-1 Common Elements in Earth’s Crust

Element	Atomic Symbol	% of Crustal Material
Oxygen	O	46.6
Silicon	Si	28
Aluminum	Al	8.1
Iron	Fe	5
Calcium	Ca	3.6
Sodium	Na	2.8
Potassium	K	2.6
Magnesium	Mg	2.1

      FIGURE 5-3: The periodic table of the elements.

      Interpreting isotopes

      Most elements exist as atoms of different atomic mass number, indicating different numbers of neutrons in the nucleus. As long as the number of protons stays the same (the atomic number), you have the same element, but its atomic mass changes with the addition or subtraction of neutrons. These various atoms of the same element with different atomic mass numbers are called isotopes.

      Take, for example, the element carbon, which has three common isotopes:

       Carbon-12 has six protons and six neutrons.

       Carbon-13 has six protons and seven neutrons.

       Carbon-14 has six protons and eight neutrons.

      

Isotopes are very useful because although the element is the same (such as Carbon-12, Carbon-13, and Carbon-14), the heavier isotope reacts differently in chemical reactions. This means the isotopes can be counted or measured to interpret conditions of temperature or pressure when a chemical reaction occurred in the past. Also, some isotopes change or decay over time at a measurable and constant rate, which makes them useful for measuring time. You find details about how isotopes are used to determine the age of rocks in later chapters.

      Charging particles: Ions

      Each subatomic particle in an atom has a charge, similar to the way opposite ends of a battery or magnet are charged: positive or negative. In an atom, the protons are positive, the neutrons are neutral (no charge), and the electrons are negative. Most atoms have the same number of protons and electrons, which means the atom itself has no charge; it’s neutral.

When an atom with only one electron in its outer shell is near an atom with seven electrons in its shell, the single electron will jump over to join and complete the almost-full shell. This action results in the first atom having one more proton than electrons and, therefore, a positive (or +1) charge. Meanwhile, the second atom has one more electron than protons and, therefore, a negative (or –1) charge. (Later in the chapter, Figure 5-4 illustrates this fact.)

      

Atoms or molecules (more than one atom joined together) with positive or negative charge are called ions. The charge of the ion is determined by how the electrons in its outer shell move to and from nearby atomic shells. An atom with a positive charge is called a cation, and an atom with a negative charge is called an anion. Atoms, and even compounds, can have negative charges of 1, 2, 3, and even 4 (though 4 is rare) and positive charges up to +8. The interaction of ions with one another is one way that atoms form bonds; keep reading to find out the details.

Chemically Bonding

      Very few atoms exist in nature all by themselves. Multiple atoms joined together are called molecules. Some atoms of the same element pair up with each other to form molecules. An example of this is oxygen gas, which is composed of two oxygen atoms, written as O₂. (The small 2 indicates how many atoms of oxygen are in the molecule.)

      In other cases, atoms of two or more different elements combine to form a compound. The compound is held together by a chemical bond. In this section, I explain the three most common types of chemical bonding between atoms.

      

How two atoms bond together is determined by the number of electrons in their outer orbital shells. For example, an atom with 13 total electrons such as aluminum (Al) will have two electrons in the first orbital shell, eight electrons in the second orbital shell, and three electrons in the outermost orbital shell. The three electrons in the outermost shell are the ones that participate in bonds with other atoms.

      Donating electrons (ionic bonds)

      When two atoms trade electrons between their outer orbital shells, becoming a cation and an anion, they form an ionic bond. The result of an ionic bond is that the positively charged cation and negatively charged anion combine into a compound that has a neutral charge. All ionic bonds create compounds called salts. Of these compounds, you are most familiar with table salt, or NaCl. In this molecule, an atom of sodium (Na) and an atom of chlorine (Cl) have bonded together. They are held together because the single electron in the

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