periodic table

The periodic table is the chart of the elements, ordered by atomic number — the count of protons in the nucleus — and laid out so that elements with similar behavior line up in the same vertical column. The rows are called periods and the columns are called groups or families. The metals fill the left and the middle of the table, the non-metals sit in the upper right, and the far-right column holds the noble gases.

Reading it answers most of the questions. Moving across a period the elements gain protons and their character shifts from metal toward non-metal. Moving down a group the atoms get larger and a family's behavior becomes more pronounced: the alkali metals of group 1 react more vigorously as you go down, while the noble gases in group 18 form almost no compounds at all. Metals are shiny, are good conductors of heat and electricity, and can be bent into shape; non-metals generally are none of those things.

Each square carries a symbol, an atomic number and an atomic mass. A symbol of one capital letter, or of one capital with a lower-case letter after it (Na, Cl, Fe), is a single element, while a formula built from two or more symbols joined together (H2O, CO2) is a compound. Question writers here ask which family an element belongs to, or whether a named material is a metal or a non-metal.