| A | B |
|---|
| periodic | having a regular, repeating pattern |
| periodic law | the law that states that the chemical and physical properties of elements are periodic functions of their atomic number |
| period | a horizontal row of elements on the periodic table |
| group | a column of elements on the periodic table |
| alkali metal | the element in group 1 of the periodic table; they are the most reactive metals, and their atoms have one electron in the outer level |
| alkaline-earth metals | the elements in group 2 of the periodic table; they are reactive metals but less reactive than alkali metals; their atoms have two electrons in their atoms have two electrons in their outer level |
| halogens | the elements in group 17 of the periodic table; they are very reactive nonmetals, and their atoms have seven electrons in their outer level |
| noble gasses | the unractive elements in group 18 of the periodic table; their atoms have eight electrons in their outer level (except for helium, which has two electrons) |
