Elements on Earth and in Living Systems

The majority of elements are natiiraUy occurring. How are these elements distributed on Earth, and which are essential to living systems  

Earth s crust extends from the surface to a depth of about 40 km. Because of technical difficulties, scientists have not been able to study the irmer portions of Earth as easily as the crust. Nevertheless, it is believed that there is a solid core consisting mostly of iron at the center of Earth. Sirrrounding the core is a layer called the mantle, which consists of hot fluid containing iron, carbon, silicon, and sulfur. 

The accompanying table lists the essential elements in the human body. Of special interest are the trace elements, such as iron (Fe), copper (Cu), zinc (Zn), iodine (1), and cobalt (Co), which together make up about 0.1 percent of the mass of the body. These elements are necessary for biological functions such as growth, transport of oxygen for metabolism, and defense against disease. There is a delicate balance in the amounts of these elements in our bodies. Too much or too little over an extended period can lead to serious illness, retardation, or even death. 

Element Percent by Mass Element Percent by Mass  

The periodic table is a handy tool that correlates the properties of the elements in a systematic way and helps us to predict chemical behavior. We will take a closer look at this keystone of chemistry in Chapter 2. 

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