1. Period: horizontal row of elements in the periodic table whose properties change gradually and predictably

2. Group: family of elements in the periodic table that have similar physical or chemical properties.

3. Representative element: elements in groups 1 and 2 and 13-18 in the periodic table that include metals, metalloids, and nonmetals.

4. Transition element: elements in groups 3-12 in the periodic table, all of which are metals.

5. Metal: element that has luster, is malleable, ductile, and a good conductor of heat and electricity.

6. Nonmetal: element that is usually a gas or brittle solid at room temperature and is a poor conductor of heat and electricity.

7. Metalloid: elements that shares some properties with both metals and nonmetals.

8. Alkali metal: elements in group 1 of the periodic table.  All are silvery solids with low densities and low melting points.

9. Alkaline earth metals: elements in group 2 of the periodic table.  Denser and harder and has a higher melting point than the alkali metal in the same period.

10. Semiconductor: element that does not conduct electricity as well as a metal but conducts it better than a nonmetal.

11. Halogen: elements in group 17 of the periodic table.  Elements form salts with sodium an with other alkali metals

12. Noble gas: elements in group 18 of the periodic table.  Rarely combine with other elements and found only as uncombined elements in nature.

13. Catalyst: a substance that can make something happen faster but is not changed itself.

14. Lanthanide: the first series of inner transition elements which goes from cerium to lutetium.  Also called rare Earth elements, due to scarcity.  Soft, malleable, shiny, and high conductivity; used to make various alloys.

15. Actinide: the second series of inner transition elements which goes from thorium to lawrencium.  All are radioactive, very unstable and break apart quickly.

16. Synthetic elements: elements that are made in laboratories and nuclear reactors.