periodic table

The periodic table arranges all the elements in order of increasing atomic number, in rows and columns, so that elements with similar chemistry line up. A group is a vertical column, and its main-group elements share the same number of valence electrons, so they behave alike. A period is a horizontal row, and its elements share the same number of electron shells. The table is also split by kind: metals on the left and center lose electrons and conduct, nonmetals on the upper right gain or share electrons, and metalloids along the staircase between them — several of them are semiconductors. Two trends are worth knowing: across a period the atoms get smaller and less metallic, and down a group they get larger and more metallic. Reading an element's box and its position gives you the atomic number, the decimal average atomic mass, and — for a named isotope — the mass number.