1. S ECTION 5–3: R EPRESENTATIVE G ROUPS Physical Science Coach Kelsoe Pages 139–145

2. O BJECTIVES Relate the number of valence electrons to groups in the periodic table and to properties of elements in those groups. Predict the reactivity of some elements based on their locations within a group. Identify some properties of common A group elements.

3. V ALENCE E LECTRONS Notice the periodic table in the back of your book. Certain columns (or groups) are numbered with the letter “A” and others are numbered with the letter “B.” The number of an “A” group matches the number of valence electrons in an electron configuration for an element in that group. A valence electron is an electron that is in the highest occupied energy level of an atom, or otherwise known as the outermost electrons in an atom.

4. V ALENCE E LECTRONS Elements in a group have similar properties because they have the same number of valence electrons. These properties will not be identical because the valence electrons are in different energy levels. Hydrogen is the exception to most rules, but it does have one valence electron, and that is why it is grouped with the alkali metals.

5. T HE A LKALI M ETALS The elements in Group 1A are called alkali metals. These metals have a single valence electron and are extremely reactive. Because these elements are extremely reactive, alkali metals are found in nature only as compounds like sodium chloride (salt).

6. T HE A LKALI M ETALS Not all the elements in a group are equally reactive. The reactivity of alkali metals increases from the top of Group 1A to the bottom. Many of the alkali metals react violently with water.

7. T HE A LKALINE E ARTH M ETALS The elements in Group 2A are called alkaline earth metals. All alkaline earth metals have two valence electrons. Alkaline earth metals are harder than alkali metals. Differences in reactivity among the alkaline earth metals are shown by the way they react with water. Beryllium does not react with water, but barium reacts easily with cold water.

8. T HE A LKALINE E ARTH M ETALS Magnesium Plays a key role in photosynthesis. It is at the center of chlorophyll. Frames of bicycles and backpacks contain it because it can be as strong as steel without weighing as much. Calcium Used in keeping bones and teeth strong. Calcium carbonate is the main ingredient in chalk, limestone, coral, and pearls. Plaster casts use calcium sulfate.

9. T HE B ORON F AMILY Group 3A contains the metalloid boron, and the metals aluminum, gallium, indium, and thallium. All these elements have 3 valence electrons. Aluminum is the most abundant metal in Earth’s crust, found in a mineral called bauxite. Boron is used to make a stronger type of glass, particularly that used in labs.

10. T HE C ARBON F AMILY Group 4A contains a metal (carbon), two metalloids (silicon and germanium), and two metals (tin and lead). Each has four valence electrons. Except for water, most of the compounds in your body contain carbon. Silicon is the second most abundant element in Earth’s crust.

11. T HE N ITROGEN F AMILY Group 5A contains two nonmetals (nitrogen and phosphorus), two metalloids (arsenic and antimony), and one metal (bismuth). Much of the nitrogen obtained from the air is used to produce fertilizers. Besides nitrogen, fertilizers often contain phosphorus. Phosphorus exists in several forms, and many are flammable.

12. T HE O XYGEN F AMILY Group 6A has three nonmetals (oxygen, sulfur, and selenium) and two metalloids (tellurium and polonium). All these elements have 6 valence electrons. Oxygen is the most abundant ELEMENT in Earth’s crust. Complex life forms need oxygen to release the energy stored in food. Oxygen can be stored as a liquid under pressure and is highly flammable.

13. T HE O XYGEN F AMILY Ozone is another form of oxygen. At ground level, ozone can irritate your eyes and lungs, but in the upper levels of the atmosphere, ozone absorbs harmful radiation. Sulfur was one of the first elements discovered. Its main purpose is in the production of sulfuric acid, which is used to make fertilizers.

14. T HE H ALOGENS The elements in Group 7A are called halogens. Each halogen has 7 valence electrons. Despite their physical differences (two are gases, one liquid, two solids), the halogens have similar chemical properties. They are highly reactive nonmetals, with fluorine being the most reactive. Halogens are used in toothpaste, Teflon, bleach, pools, and in your body.

15. T HE N OBLE G ASES The elements in Group 8A are called noble gases. All but helium have 8 valence electrons; helium has 2. The noble gases are colorless and odorless and extremely unreactive. Argon is used in light bulbs and producing computer chips.

16. C OLORS OF N EON S IGNS When electric current passes through noble gases, they emit different colors. Helium Pink Neon Orange-Red Argon Lavender Krypton White Xenon Blue

17. V OCABULARY Valence electron Alkali metals Alkaline earth metals Halogens Nobel gases