Geology For Dummies. Alecia M. Spooner

The number of protons in an atom’s nucleus determines which element the atom is. For example, an atom with six protons is an atom of the element carbon. An atom with seven protons is an atom of nitrogen.

      Getting to know the periodic table

The periodic table of elements lists the known elements in order of their atomic number, which is their number of protons. Each square on the periodic table provides you with all the information you need to know about that element and how it will interact with other elements. Figure 5-2 illustrates what the different numbers for each element on the periodic table represent:

      FIGURE 5-2: The parts of one square of the periodic table of elements.

       Atomic mass number: The atomic mass number of an element is the total number of protons and neutrons in its nucleus.

       Atomic number: The atomic number of an element is the number of protons in its nucleus.

       Group number: The group number tells you how many electrons in the atom are located in the outermost orbital shell and are, therefore, available to bond it to other atoms. For example, elements in Group I have one electron in the outer electron shell, and Group II elements have two electrons in the outer electron shell. The group number for each element may help you understand why some elements, such as Magnesium (Mg) and Calcium (Ca), which are both in Group II, react in similar ways during rock formation and other geologic processes.

       Symbol: The letters on the periodic table are the symbols for each element. These symbols are a shorthand so that when combinations of elements 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.