There are only about 100 ___ in the universe. Elements on Earth are the same as elements on other planetes. Elements contain one type of ____. The atoms of an element are identical. For example, iron (Fe) atoms on Earth are the same as iron atoms found on Mars. Atoms have a ___ in middle, which contains protons and neutrons. Protons are ____ charged and neutrons are neutral. Electrons are ____ charged and surround the nucles. Elements in the periodic table ale listed in order of their atomic number. The atomic number is the number of ____ in the nucleus.