Answer the following question given this paragraph: 

All matter is made of tiny particles. Protons, neutrons, and electrons form atoms that bond together to create molecules. Atoms are the smallest units that have the properties of an element. Molecules are the smallest units of a compound. Chemical bonds hold molecules together. Molecules form the different types of minerals. The silicates make up most of Earth's crust. Minerals come together to create the three major rock types. They are igneous, sedimentary, and metamorphic. Igneous rocks form from cooled magma. Sedimentary rocks form from compacted or cemented sediments. Metamorphic rocks are those that have been altered by heat and pressure. These three rock types are the material part of the rock cycle. They are connected by different processes. Different processes convert any type of rock into any other type of rock. These processes are weathering and erosion, crystallization, and burial and pressure, among others. Each rock contains a story of how it formed. For most rocks it is possible to know what it formed from.


Q: The smallest units of matter that retain the unique properties of an element are known as what?


A:
atoms