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Hydrogen and Alkali Metals
Characteristics of Group 1 of the periodic table.
You are watching: Alkali metals are extremely reactive because they
· Identify theelementsin group 1 of the periodic table.
· Explain why group 1elementsare very reactive.
· List other properties of alkalimetals.
You probably think ofwateras a substance that can put out fires. But someelementsare so reactive that they burn in water. In fact, they virtually explode in water. That’s what is happening in the photo above. About 3 pounds of sodium were added to water, and the result was this explosive reaction. Why is sodium such a reactive element? In this lesson you will find out.
The First Group
Sodium (Na) is an element in group 1 of the periodic table of the elements. This group (column) of the table is shown below. It includes the nonmetal hydrogen (H) and sixmetalsthat are calledalkali metals. Elements in the same group of the periodic table have the same number of valenceelectrons. These are the electrons in their outerenergy levelthat can be involved in chemical reactions. Valence electrons determine many of the properties of an element, so elements in the same group have similar properties. All the elements in group 1 have just one valence electron. This makes them very reactive.
Q:Why does having just one valence electron make group 1 elements very reactive?
A:With just one valence electron, group 1 elements are “eager” to lose that electron. Doing so allows them to achieve a full outerenergy leveland maximum stability.
Reactivity of Group 1 Elements
Hydrogen is a very reactive gas, and the alkalimetalsare even more reactive. In fact, they are the most reactive metals and, along with the elements in group 17, are the most reactive of all elements. The reactivity of alkali metals increases from the top to the bottom of the group, so lithium (Li) is the least reactive alkali metal and francium (Fr) is the most reactive. Because alkali metals are so reactive, they are found in nature only in combination with other elements. They often combine with group 17 elements, which are very “eager” to gain an electron. At the URL below, you can watch a video that shows just how reactive the alkali metals are.
Other Properties of Alkali Metals
Besides being very reactive, alkali metals share a number of other properties.
· Alkali metals are allsolidsat roomtemperature.
· Alkali metals are low in density, and some of them float onwater.
· Alkali metals are relatively soft. Some are even soft enough to cut with a knife, like the sodium pictured in theFigurebelow.
A Closer Look
Although all group 1 elements share certain properties, such as being very reactive, they are not alike in every way. Three different group 1 elements are described in more detail below (Figuresbelow,below, andbelow). Notice the ways in which they differ from one another.
Hydrogen has the smallest, lightest atoms of all elements. Pure hydrogen is a colorless, odorless, tasteless gas that is nontoxic but highly flammable. Hydrogen gas exists mainly as diatomic (“two-atom”) molecules (H2), as shown in the diagram on the right. Hydrogen is the most abundant element in the universe and the third most abundant element on Earth, occurring mainly in compounds such as water.
Q:Why do you think hydrogen gas usually exists as diatomic molecules?
A:Each hydrogen atom has just one electron. When two hydrogenatomsbond together, they share a pair ofelectrons. The shared electrons fill their onlyenergy level, giving them the most stable arrangement of electrons.
Potassium is a soft, silvery metal that ignites explosively in water. It easily loses its one valence electron to form positive potassium ions (K+), which are needed by all living cells. Potassium is so important for plants that it is found in almost all fertilizers, like the one shown here. Potassium is abundant in Earth’s crust in minerals such as feldspar.
Francium has one of the largest, heaviest atoms of all elements. Its one valence electron is far removed from the nucleus, as you can see in the atomic model on the right, so it is easily removed from the atom. Francium is radioactive and quickly decays to form other elements such as radium. This is why francium is extremely rare in nature. Less than an ounce of francium is present on Earth at any given time.
Q:Francium decays too quickly to formcompoundswith other elements. Which elements to you think it would bond with if it could?
A:With one valence electron, francium would bond with a halogen element in group 17, which has seven valenceelectronsand needs one more to fill its outer energy level. Elements in group 17 include fluorine and chlorine.
· Group 1 of the periodic table includes hydrogen and the alkali metals.
· Because they have just one valence electron, group 1 elements are very reactive. As a result, they are found in nature only in combination with other elements.
· Alkali metals are allsolidsat roomtemperature. They are relatively soft and low in density.
· From the top to the bottom of group 1, the elements have heavier, more reactiveatoms.
At the following URL, click on “Group 1 data and graphs.” Use the data in the group 1 table to fill in the blanks below with eitherincreasesordecreases.
From the top to the bottom of group 1, properties of the alkali metals change as follows:
1. Melting point __________.
2. Boiling point __________.
3. Density __________.
4. Atomic radius __________.
5. Number of electrons __________.
1. What are alkali metals?
2. Why is hydrogen, a nonmetal, placed in the same group as the alkali metals?
3. Explain why group 1 elements often formcompoundswith elements in group 17.
4. Compare and contrast hydrogen and francium.
Alkaline Earth Metals
Characteristics of Group 2 of the periodic table.
· Identify alkaline Earthmetals.
· List properties of alkaline Earthmetals.
· Explain why alkaline Earthmetalsare very reactive.
Sparklers like the one this girl is holding make festive additions to many celebrations. You may use them yourself. But watch out if you do because their flames are really hot! The bright white flames are produced when magnesium burns. Magnesium is a light- weight metal that burns at a very hightemperature. Other uses of magnesium include flash photography, flares, and fireworks. Magnesium is a metal in group 2 of the periodic table, which you will read about in this concept.
The Second Group
Barium (Ba) is one of sixelementsin group 2 of the periodic table, which is shown below. Elements in this group are calledalkaline Earth metals. These metals are silver or gray incolor. They are relatively soft and low in density, although not as soft and lightweight as alkali metals. You can watch a brief video introduction to the alkaline Earth metals at this URL:http://www.youtube.com/watch?v=DFQPnHkQlZM.
Reactivity of Alkaline Earth Metals
All alkaline Earth metals have similar properties because they all have two valenceelectrons. They readily give up their two valence electrons to achieve a full outerenergy level, which is the most stable arrangement of electrons. As a result, they are very reactive, although not quite as reactive as the alkali metals in group 1. For example, alkaline Earth metals will react with coldwater, but not explosively as alkali metals do. Because of their reactivity, alkaline Earth metals never exist as puresubstancesin nature. Instead, they are always found combined with otherelements.
The reactivity of alkaline Earth metals increases from the top to the bottom of the group. That’s because theatomsget bigger from the top to the bottom, so the valenceelectronsare farther from the nucleus. When valence electrons are farther from the nucleus, they are attracted less strongly by the nucleus and more easily removed from the atom. This makes the atom more reactive.
Q:Alkali metals have just one valence electron. Why are alkaline Earth metals less reactive than alkali metals?
A:It takes more energy to remove two valenceelectronsfrom an atom than one valence electron. This makes alkaline Earth metals with their two valence electrons less reactive than alkali metals with their one valence electron.
Examples of Alkaline Earth Metals
For a better understanding of alkaline Earth metals, let’s take a closer look at two of them: calcium (Ca) and strontium (Sr). Calcium is a soft, gray, nontoxic alkaline Earth metal. Although pure calcium doesn’t exist in nature, calciumcompoundsare very common in Earth’s crust and in seawater. Calcium is also the most abundant metal in the human body, occurring as calcium compounds such as calcium phosphate and calcium carbonate. These calcium compounds are found in bones and make them hard and strong. The skeleton of the average adult contains about a kilogram of calcium. Because calcium—like barium—absorbs x-rays, bones show up white in x-ray images. Calcium is an important component of a healthy human diet. Good food sources of calcium are pictured inFigurebelow.
Q:What health problems might result from a diet low in calcium?
A:Children who don’t get enough calcium while their bones are forming may develop a deficiency disease called rickets, in which their bones are softer than normal and become bent and stunted. Adults who don’t get enough calcium may develop a condition called osteoporosis, in which the bones lose calcium and become weak and brittle. People with osteoporosis are at high risk of bone fractures.
Strontium is a silver-colored alkaline Earth metal that is even softer than calcium. Strontiumcompoundsare quite common and have a variety of uses—from fireworks to cement to toothpaste. In fireworks, strontium compounds produce deep red explosions. In toothpaste, like the one pictured in theFigurebelow, the compound strontium chloride reduces tooth sensitivity.
· Elements in group 2 of the periodic table are called alkaline Earth metals. They are silvery or gray incolor. They are also relatively soft and low in density.
· Alkaline Earth metals are very reactive because they readily give up their two valence electrons to achieve a full outerenergy level, which is the most stable arrangement of electrons. Reactivity increases from the top to the bottom of the group.
· Examples of alkaline Earth metals include calcium, which is needed for strong bones, and strontium, which is used for making cement and other products.
At the following URL, observe how four different alkaline Earth metals react withwater. After you watch the video, answer the questions below.
1. Observe the reactions in the video, and then rank the alkaline Earth metals from most to least reactive with water.
2. What explains the differences in reactivity?
3. Predict the reactivity of beryllium with water. Where would it fit in your ranking?
4. Whatsubstancesare produced in each reaction that you observed in the video?
1. What are alkaline Earth metals? What are their physical properties?
2. Why are alkaline Earth metals very reactive?
3. Compare and contrast the reactivity of beryllium (Be) and barium (Ba).
Characteristics of Group 18 of the periodic table.
· Identify the noblegases.
· List properties of noblegases.
· Explain the nonreactivity of noblegasesand how it is related to theoctet rule.
Okay, helium balloons are light, but they’re not that light! This fanciful picture serves to make the point that helium is one of the lightestelements. Helium belongs to a group of elements called the noble gases.
What Are Noble Gases?
Noble gasesare nonreactive, nonmetallicelementsin group 18 of the periodic table. As you can see in the periodic table inFigurebelow, noble gases include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). All noble gases are colorless and odorless. They also have lowboilingpoints, explaining why they are gases at roomtemperature. Radon, at the bottom of the group, is radioactive, so it constantly decays to otherelements. For an excellent overview of the noble gases, watch the video at this URL:
· To learn how the noble gases were discovered, go to this URL:http://www.youtube.com/watch?v=ceQMs30D16E.
· To explore specific noble gases, click on their symbols in the periodic table at this URL:http://www.ptable.com/.
Q:Based on their position in the periodic table (Figureabove), how many valenceelectronsdo you think noble gases have?
A:The number of valenceelectronsstarts at one for elements in group 1. It then increases by one from left to right across each period (row) of the periodic table for groups 1–2 and 13–18. Therefore, noble gases have eight valence electrons.
Chemical Properties of Noble Gases
Noble gases are the least reactive of all known elements. That’s because with eight valenceelectrons, their outer energy levels are full. The only exception is helium, which has just two electrons. But helium also has a full outerenergy level, because its only energy level (energy level 1) can hold a maximum of two electrons. A full outer energy level is the most stable arrangement of electrons. As a result, noble gases cannot become more stable by reacting with other elements and gaining or losing valence electrons. Therefore, noble gases are rarely involved inchemical reactionsand almost never formcompoundswith other elements.
Noble Gases and theOctet Rule
Because the noble gases are the least reactive of all elements, their eight valence electrons are used as the standard for nonreactivity and to explain how other elements interact. This is stated as the octet (“group of eight”) rule. According to this rule,atomsreact to formcompoundsthat allow them to have a group of eight valence electrons like the noble gases. For example, sodium (with one valence electron) reacts with chlorine (with seven valence electrons) to form the stable compound sodium chloride (table salt). In this reaction, sodium donates an electron and chlorine accepts it, giving each element an octet of valence electrons.
Some Uses of Noble Gases
Did you ever get helium balloons like those in the opening picture? Unlike a balloon filled with air, a balloon filled with helium needs to be weighted down so it won’t float away — although you don’t have to use an elephant!.
Q:Why does a helium balloon float away if it’s not weighted down?
A:Heliumatomshave just twoprotons, twoneutrons, and two electrons, so they have less mass than any other atoms except hydrogen. As a result, helium is lighter than air, explaining why a helium balloon floats up into the air unless weighted down.
Early incandescent light bulbs, like the one pictured in theFigurebelow, didn’t last very long. The filaments quickly burned out. Although air was pumped out of the bulb, it wasn’t a complete vacuum. Oxygen in the small amount of air remaining inside the light bulb reacted with the metal filament. This corroded the filament and caused dark deposits on the glass. Filling a light bulb with argon gas prevents these problems. That’s why modern light bulbs are filled with argon.
Q:How does argon prevent the problems of early light bulbs?
A:As a noble gas with eight electrons, argon doesn’t react with the metal in the filament. This protects the filament and keeps the glass blub free of deposits.
Noble gases are also used to fill the glass tubes of lighted signs like the one in theFigurebelow. Although noble gases are chemically nonreactive, their electrons can be energized by sending anelectric currentthrough them. When this happens, the electrons jump to a higherenergy level. When the electrons return to their original energy level, they give off energy as light. Different noble gases give off light of different colors. Neon gives off reddish-orange light, like the word “Open” in the sign below. Krypton gives off violet light and xenon gives off blue light.
· Noble gases are nonreactive, nonmetallic elements in group 18 of the periodic table.
· Noble gases are the least reactive of all elements. That’s because they have eight valence electrons, which fill their outerenergy level. This is the most stable arrangement of electrons, so noble gases rarely react with other elements and formcompounds.
· Theoctet rulestates thatatomsreact to form compounds that allow them to have eight valence electrons like the noble gases, which are the least reactive elements.
· Noble gases are used for balloons, light bulbs, and lighted signs.
Watch the video about noble gases at the following URL, and then answer the questions below.
1. Compare and contrast the reactivity of the elements helium, neon, argon, krypton, and xenon.
2. Compare the density of these noble gases to the density of air.
3. Describe how the mass of noble gases changes from the top to the bottom of group 18.
1. What are noble gases?
2. Explain why noble gases are almost completely nonreactive.
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3. What is the octet rule? How is it related to noble gases?
4. Hydrogen (H) atoms have one electron and exist as diatomic (“two-atom”) molecules (H2). Helium atoms have two electrons and exist only as single helium atoms. Explain why hydrogen and helium differ in this way.