Elements are one type of pure substance. In an element, every atom is the same as every other atom. It has the same properties throughout. All of the known elements on Earth are arranged in the periodic table in order of atomic number. That's the number of protons its atoms have. Hydrogen is an element with atomic number 2,oxygen is atomic number 8 and chlorine is 17. Each element has a symbol on the periodic table. Hydrogen is H, oxygen is O and chlorine is CI.Because every atom of an element is the same,you cannot break an element down into any smaller unit of matter.