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target="_blank" rel="nofollow" href="#fb3_img_img_4899a4e7-1b98-51c7-9253-0ae424141df2.png" alt="Remember"/> 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

Schematic illustration of the parts of one square of the periodic table of elements.

      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).

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

      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.

      

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.

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