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The periodic table is a way to list the elements based on the structure of their atoms. From left to right and top to bottom the elements are listed in the order of their atomic number, which is the number of protons in each atom. In general the metals are to the left and the nonmetals are to the right. The rows are called periods and columns are called groups. Dmitry Mendeleev was a Russian chemist and the first to publish a recognizable periodic table in 1869. All of the elements from the atomic number 1, hydrogen, through 118 have been discovered completing the first seven rows of the periodic table. The first 98 elements exist in nature. Elements 99 to 118 have only been created in labs or in nuclear reactors.
Atoms are the basis for everything in the universe because all matter is composed of atoms. Solids are made up of densely packed atoms and gases have atoms that are more spread out. There are three basic parts of an atom: electrons, protons and neutrons. Electrons are the smallest of the three particles that make up atoms. Electrons are found in the shells, or orbitals, that surround the nucleus of an atom. Protons and neutrons are found in the nucleus. They group together in the center of the atom. Each unique element has an atomic number equal to the number of protons it contains. Atoms can join together to form molecules which in turn form most of the objects around you. Different elements like oxygen and carbon contain different kinds of atoms. The atomic mass number is equal to the number of protons plus neutrons together.
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