3. A pure substance consisting of two or more elements is said to be a(n) a. ion b. isotope c. compound d. mixture 4. A(n) is the smallest particle of an element that retains the characteristics of the element. a. atom b. electron c. proton d. neutron 5. Anything that has mass and occupies space is known as a. energy b. work c. calorie d. matter is the SI standard unit of length. 6. a. centimeter b. meter c. foot d. inch