Elements are pure chemical substances consisting of one type of atom, while compounds consist of two or more elements chemically bonded together. Atoms are the basic units of matter, containing a nucleus of protons and neutrons surrounded by electrons. Metals readily lose electrons to form positive ions surrounded by delocalized electrons, giving them high conductivity. Non-metals have more varied properties. The periodic table organizes the elements and displays recurring trends in their properties based on atomic structure. Chemical reactions lead to the transformation of substances and can be identified by types such as synthesis, where elements combine to form compounds, or decomposition, where compounds break down.