Earth Science: Chemistry – Inorganic Chemistry – The Periodic Table
A copy of the periodic table hangs in every chemistry classroom around the world. Today it includes 111 confirmed elements, arranged in groups and periods according to their atomic number and properties.
Chemists had long speculated about an order for the elements when, in 1829, the German chemist Johan Dobereiner found several groups of three elements (“triads”) with similar chemical properties and noticed that the atomic mass of the element in the middle is approximately the same as the average of the mass of the other elements.
19th-century discoveries
Many elements were discovered in the mid-19th century. In 1869 Russian scientist Dimitri Mendeleyev and German scientist Lothar Meyer independently developed a periodic table. Mendeleyev classified the elements in ascending atomic weight order and placed related elements under each other in vertical columns.
He also left blank spaces under aluminum and silicon to accommodate elements yet to be found. About a century later, gallium and germanium were discovered and had the properties predicted by Mendeleyev. This contributed greatly to the scientific acceptance of the periodic table.
The periodic table today
The periodic table shows the elements with their chemical symbols arranged in rows in the order of their atomic number. The atomic number (also called the proton number) is the number of positive elementary particles (protons) in the nucleus of the element’s atom. It uniquely identifies a chemical element. In an atom of neutral charge, atomic number is equal to the number of electrons.
Hydrogen atoms (symbol: H) have the simplest structure since they have only one proton. Uranium atoms contain 92 protons—these are the largest atoms found in nature. The element’s chemical properties are determined by the number of electrons in its outermost shell.
Elements with similar properties are arranged in 18 vertical groups. For example, all the elements in the eighth main group (the “noble gases”) have eight electrons in the outer shell —except for helium, which has two—and do not react with other elements. The horizontal rows are the periods (1-7). The atomic mass, measured in unified mass units, is calculated as the combined mass of protons and neutrons, because the weight of electrons is so small as to be insignificant. The atomic mass differs between isotopes of one element.
ISSUES TO SOLVE
NEW ELEMENTS Today, scientists can use nuclear fusion to make elements with a short half life. In the future, man made elements may become more stable