Chemistry 2

All matter in the world is composed of elements.  Each element is made up of its own kind of atom.  All of the elements on the earth are known.  In 1867 Dmitri Mendeleev made the Periodic Table of Elements.  He lined up elements in rows from left to right, according to their atomic number.  The atomic number of an element is equal to the number of protons in its atom.  Elements in the vertical columns have similar chemical properties.  For example, the alkali metals give electrons, but the noble gases do nothing.   

The symbol of an element represents its name.  For example, C stands for carbon, but some of the elements’ symbols come from other languages.  Tungsten’s symbol is W, which comes from the Latin word, wolfram.   The atomic weight of an atom is the sum of its protons and neutrons.  For example, hydrogen has the atomic weight of 1, which means it has one proton and no neutrons.  Another element, Uranium, weighs 238 atomic mass units, which means that it has 92 protons, and 146 neutrons.