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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.