NSES Content Standard B  
Physical Science: Properties and changes of properties in matter 
Grades 5-8, page 154 

Chemical elements do not break down by normal laboratory reactions such as heating, exposure to electric current, or reaction with acids. There are more than 100 known elements that combine in a multitude of ways to produce compounds, which account for the living and nonliving substances that we encounter. 

Benchmark 4D The Physical Setting: Structure of Matter
Grades 6-8, page 78
Scientific ideas about elements were borrowed from some Greek philosophers of 2,000 years earlier, who believed that everything was made from four basic substances: air, earth, fire, and water. It was the combinations of these "elements" in different proportions that gave other substances their observable properties. The Greeks were wrong about those four, but now over 100 different elements have been identified, some rare and some plentiful, out of which everything is made. Because most elements tend to combine with others, few elements are found in their pure form.