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Unit 6

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Periodic Trends What are the Patterns in the Chemical and Physical Properties of Elements?

Truth is ever to be found in the simplicity, and not in the multiplicity and confusion of things.

Isaac Newton (1643–1727)

Engage: Can You Organize the Creatures with the Features? A. Why do scientists generally prefer to format information about elements in a table—the

periodic table—rather than as an alphabetized list?

Before Studying this Unit After Studying this Unit

UNIT

6

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High School Chemistry: An Inquiry Approach

B. Describe the main features of the shape of the periodic table, and then explain why it is

shaped as it is.

Before Studying this Unit After Studying this Unit

1. Your teacher will provide you with an envelope containing cards with illustrations of

creatures. Open the envelope and make sure you have 19 cards. If not, notify your teacher. Write a brief description of each feature that varies among the cards (e.g., number of hairs).

2. Arrange the cards into a pattern of horizontal rows and vertical columns so that the

rows and columns have a pattern in how a feature varies. Write a description of how your cards are arranged.

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Unit 6 Periodic Trends

3. Create a different arrangement of rows and columns. Describe the organizing theme of

this arrangement. 4. Your teacher will provide you with a feature-less creature cards. Can you use these it to

make better or more complete arrangements? Describe your modifications, if any. 5. If you were to extend your cards one additional vertical column to the right of your

present configuration, describe the features that you would see on the cards. 6. If you were to extend your cards one additional horizontal row below your present

configuration, describe the features that you would see on the cards.

Thinking About Your Thinking Classification

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Explore: How Can the Properties of an Undiscovered Element be Predicted? 7. Your teacher will provide you with a copy of the following table.

Achilles, Ac Atomic Mass: 9.4 u Atomic Radius: 64 pm Density: 3.1 g/cm3 Color: Yellow Melting T: 2500C Compound with Hr: Ac2Hr3 Reacts with C3 to form a yellow-red solution

Pegasus, Pg Atomic Mass: 11.8 u Atomic Radius: 59 pm Density: 4.0 g/cm3 Color: Black Melting T: 2900C Compound with Hr: None No reaction with C1, C2, C3, C4

Pandora, Pd Atomic Mass: 31.4 u Atomic Radius: 111 pm Density: 5.8 g/cm3 Color: White Melting T: 4300C Compound with Hr: None No reaction with C1, C2, C3, C4

Zeus, Zs Atomic Mass: 3.1 u Atomic Radius: 114 pm Density: 2.5 g/cm3 Color: White Melting T: 1000C Compound with Hr: Zs2Hr Reacts with C1 and C2 to form a white solid

Poseidon, Pd Atomic Mass: 16.5 u Atomic Radius: 118 pm Density: 3.5 g/cm3 Color: Turquoise Melting T: 2500C Compound with Hr: PdHr Reacts with C2 and C4 to form a colored solution

Athena, Th Atomic Mass: 29.1 u Atomic Radius: 119 pm Density: 5.0 g/cm3 Color: Red Melting T: 3800C Compound with Hr: Th2Hr3 Reacts with C3 to form a yellow-red solution

Triton, Tr Atomic mass: Atomic radius: Density: Color: Melting T: Compound with Hr:

Heracles, Hr Atomic Mass: 6.2 u Atomic Radius: 77 pm Density: 2.7 g/cm3 Color: Blue Melting T: 2000C Formula when combined with Hr: Hr–Hr Reacts with C2 and C4 to form a colored solution

Orion, Or Atomic Mass: 25.1 u Atomic Radius: 196 pm Density: 4.1 g/cm3 Color: Gray Melting T: 2500C Compound with Hr: Or2Hr Reacts with C1 and C2 to form a white solid

Jason, Js Atomic Mass: 14.1 u Atomic Radius: 157 pm Density: 3.0 g/cm3 Color: Gray Melting T: 1800C Compound with Hr: Js2Hr Reacts with C1 and C2 to form a white solid

Odysseus, Dy Atomic Mass: 20.9 u Atomic Radius: 91 pm Density: 5.0 g/cm3 Color: Gray Melting T: 3300C Compound with Hr: None No reaction with C1, C2, C3, C4

Apollo, Lo Atomic Mass: 18.7 u Atomic Radius: 94 pm Density: 4.2 g/cm3 Color: Orange Melting T: 3000C Compound with Hr: Lo2Hr3 Reacts with C3 to form a yellow-red solution

The data in this table are from a hypothetical alternate universe. Notice that the data for

the element triton is unknown. Cut out the rectangles on the handout sheet and arrange the 11 known elements into vertical columns and horizontal rows so that there is a pattern in the properties of the elements. Where does triton belong in your table? Explain.

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8. Construct the following plots: Graph 1 Atomic radius (y-axis) vs. atomic mass (x-axis) Graph 2 Density (y-axis) vs. atomic mass (x-axis) Graph 3 Melting temperature (y-axis) vs. atomic mass (x-axis) Graph 1

Graph 2

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Graph 3

9. What is the relationship between atomic radius and atomic mass of elements in the

same horizontal row? Have your teacher sign off on your answer before moving on to the next question.

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Explain 10. Based on your plot of atomic radius versus atomic mass, what do you predict for the

atomic mass of triton? Explain. 11. Based on your plot of density versus atomic mass, what do you predict for the density

of triton? Explain. 12. Based on your plot of melting temperature versus atomic mass, what do you predict for

the melting temperature of triton? Explain. 13. What color do you predict for triton? Why? 14. What compound should form as a result of the reaction of triton and heracles? Explain

your reasoning. 15. What reactivity relationship do you expect among triton and compounds C1, C2, C3,

and C4? Explain. 16. Explain, in general, how you were able to make predictions about the properties of the

element triton.

Thinking About Your Thinking Classification

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Elaborate 1: What is the Density of Eka-Silicon? 17. Obtain a container labeled silicon. Measure the mass of the container and the enclosed

pieces of the solid element, and record this mass in the table below. 18. Add water to a 25-mL graduated cylinder until it is approximately half full. Record the

volume of water to the greatest accuracy possible with your measuring device. 19. Using forceps to avoid direct skin-to-substance contact with the solid, add enough of

the solid to raise the water level about 3 mL. Record the volume. 20. Measure and record the mass of the container and remaining solid pieces. 21. Conduct two more additions of the solid, each time raising the water level by an

additional 3 mL. With each addition, measure and record the corresponding mass of the remaining solid and container.

22. When you have completed three measurements, pour off the water, being careful not to

allow any of the solid to go down the drain. Place the solid pieces on a paper towel to dry, and rinse the cylinder.

23. Repeat the procedure to obtain mass–volume measurements for tin. 24. Repeat the procedure to obtain mass–volume measurements for lead.

Element Sample Number

Initial Mass of Container

and Solid (g)

Final Mass of Container

and Solid (g)

Initial Water

Volume (mL)

Final Water Volume

(mL)

1

2

Silicon, Si

3

1

2

Tin, Sn

3

1

2

Lead, Pb

3

CAUTION! Wear splash goggles and chemical-resistant aprons.

CAUTION! Wash your hands with soap and water before leaving the lab.

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25. Calculate the density of each element by completing the table below. Fill in the appropriate units for each column within the parentheses provided.

Element Sample Number

Mass of Solid ( )

Volume of Solid ( )

Density of Solid

( )

Average Density of

Solid ( )

1

2

Silicon, Si

3

1

2

Tin, Sn

3

1

2

Lead, Pb

3

26. Silicon is in the 3rd period of the periodic table; that is, the third horizontal row, counting from the top.

Tin is in the 5th period, and lead is in the 6th. Si, Sn, and Pb are in the same group (vertical column), Group 4A/14. Plot the density of each element (y-axis) vs. its period number (x-axis).

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27. Interpolate to predict the density of an element in the same group in the periodic table

as silicon, tin, and lead. 28. What density is expected for an element in the 4th period of Group 5A/15? Explain.

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Elaborate 2: How Can Periodic Trends be Used to Predict Chemical Reactivity? 29. Obtain a well plate. If it is not already labeled, place the plate on a sheet of paper and

label the columns with numbers (1, 2, …), and label the rows with letters (A, B, …). 30. Add 20 drops of distilled water to wells A1, A2, and A3. 31. Add 20 drops of hydrochloric acid solution to wells B1, B2, and B3. 32. Add a 1-centimeter piece of magnesium ribbon to well A1. Record your observations in

the table below. 33. Add a small, loosely-rolled ball of aluminum to well A2. Record your observations. 34. Add a chip of calcium to well A3, and record what you see. 35. [Optional] Measure room temperature, and then measure the temperature of each well.

Record your observations. 36. [Optional] Litmus paper is absorbent paper that has been impregnated with a dye that

may change color, depending on the composition of a solution with which it interacts. When red litmus paper turns blue, it indicates that the tested solution belongs to a class of compounds called bases. Bases have a series of common properties such as being chemically reactive, having a bitter taste, and reacting with animal fat to form soap. If red litmus paper does not change color, the tested substance is not classified among the group of compounds known as bases. Test each well with red litmus paper, and record your observations.

37. Repeat this series of observations and tests with the three elements in dilute

hydrochloric acid, and record your data. Magnesium Aluminum Calcium Water

Hydrochloric Acid

CAUTION! Wear splash goggles and chemical-resistant aprons.

CAUTION! Wash your hands with soap and water before leaving the lab.

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38. Which is more reactive, magnesium or calcium? Justify your response with your data. 39. Which is more reactive, magnesium or aluminum? Justify your response with your data. 40. Rank the three elements from most reactive to least reactive. 41. Assume that the reactivity trends in these three metals are typical of all metal elements.

Locate magnesium, Mg, aluminum, Al, and calcium, Ca, on a periodic table. What is the trend in chemical reactivity of metal elements across a period of the periodic table? What is the trend down a group of the periodic table? Explain your logic.

42. Rank the elements sodium, Na, magnesium, Mg, and potassium, K, in order of

predicted reactivity, from least reactive to most.

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Elaborate 3: How Can Elements be Related in a Family? 43. Elements with similar chemical properties appear in the same group, or vertical column,

in the periodic table. Several of these groups form chemical families. Family trends are most apparent among the main group elements. In this text, we will consider four families: the alkali metals in Group 1A/1, the alkaline earths in Group 2A/2, the halogens in Group 7A/17, and the noble gases in Group 8A/18.

44. With the exception of hydrogen, Group 1A/1 elements are known as alkali metals. The reactivity of the element—that is, its tendency to react with other elements to form compounds—increases as you go down the column. Alkali metals do not normally look like common, everyday metals. This is because they are so reactive that they combine with oxygen in the air to form an oxide coating, which hides the bright metallic luster that can be seen in a freshly cut sample. These elements possess other common metallic properties, too. For example, they are good conductors of heat and electricity, and they are easy to form into wires and thin foils. We can see distinct trends in the physical properties of alkali metals. Their densities increase as atomic number increases. Boiling and melting points generally decrease as you go down the periodic table. The single exception is cesium (Z = 55), which boils at a temperature slightly higher than the boiling point of rubidium (Z = 37).

45. Group 2A/2 elements are called alkaline earths or alkaline earth metals. Trends like those noted with the alkali metals are also seen with the alkaline earths. Reactivity again increases as you go down the column in the periodic table. Physical property trends are less evident among the alkaline earths.

46. The elements in Group 7A/17 make up the family known as the halogens, or “salt formers.” Reactivity is greatest for fluorine at the top of the group and least for iodine at the bottom. Density, melting point, and boiling point all increase steadily with increasing atomic number among the halogens.

47. The elements of Group 8A/18 are the noble gases. In chemistry the word noble means having “a reluctance to react.” Only a small number of compounds of the noble gases are known, and none occur naturally. The noble gases provide excellent examples of periodic trends in physical properties. Without exception, the densities, melting points, and boiling points increase as you move down the column in the periodic table.

48. Look back at your answers to the Engage questions at the beginning of this unit. How have your ideas changed since beginning this unit?

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Appendix 1: The Periodic Table

As you study this section, work to achieve these learning goals: • Distinguish between groups and periods in the periodic table and identify them by

number. • Given the atomic number of an element, use a periodic table to find the symbol and

atomic mass of that element, and identify the period and group in which it is found.

During the time of early research on the atom, even before any subatomic particles were identified, some chemists searched for an order among elements. In 1869, two men found an order, independently of each other. Dmitri Mendeleev and Lothar Meyer observed that when elements are arranged according to their atomic masses, certain properties repeat at regular intervals. Mendeleev and Meyer arranged the elements in tables so that elements with similar properties were in the same column or row. These were the first periodic tables of the elements. The arrangements were not perfect. For all elements to fall into the proper groups, it was necessary to switch a few of them in a way that interrupted the orderly increase in atomic masses. Of the two reasons for this, one was anticipated at that time: There were errors in atomic weights (as they were known in 1869). The second reason was more important. About 50 years later, it was found that the correct ordering property is the atomic number, Z, rather than the atomic mass. We have seen how Dalton used his atomic theory to predict the Law of Multiple Proportions and thus gain acceptance for his theory. Mendeleev did the same with the periodic table. He noticed that there were blank spaces in the table. He reasoned that the blank spaces belonged to elements that were yet to be discovered. By averaging the properties of elements above and below or on each side of the blanks, he predicted the properties of the unknown elements. Germanium is one of the elements about which he made these predictions. Table 6.1 summarizes the predicted properties and their currently accepted values.

Table 6.1 Predicted and Observed Properties of Germanium

Property

Predicted by Mendeleev

Currently Accepted Values

Atomic weight 72 g/mol* 72.60 g/mol* Density of metal 5.5 g/cm3 5.36 g/cm3 Color of metal Dark gray Gray Formula of oxide GeO2 GeO2 Density of oxide 4.7 g/cm3 4.703 g/cm3 Formula of chloride GeCl4 GeCl4 Density of chloride 1.9 g/cm3 1.887 g/cm3 Boiling point of chloride Below 100°C 86°C Formula of ethyl compound Ge(C2H5)4 Ge(C2H5)4 Boiling point of ethyl compound 160°C 160°C Density of ethyl compound 0.96 g/cm3 Slightly less than 1.0 g/cm3

*Mol is the abbreviation for mole, the SI unit for amount of substance.

Lothar Meyer (1830-1895). From Cracolice, M. S., & Peters, E. I. (2011). Introductory Chemistry: An Active Learning Approach. Belmont: CA: Brooks/Cole Cengage Learning.

Dmitri Mendeleev (1834-1907). From Cracolice, M. S., & Peters, E. I. (2011). Introductory Chemistry: An Active Learning Approach. Belmont: CA: Brooks/Cole Cengage Learning.

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There are many ways to classify a collection of objects. Different criteria satisfy different purposes. Mendeleev and Meyer arranged their periodic tables based on two criteria: The atomic masses of the elements increased across a row and the chemical properties of the elements in a column were similar. Mendeleev is more famous than Meyer as the founder of the periodic table because he subsequently used his classification scheme to make predictions about unknown elements. His predictions were later found to be true.

When you formulate a classification scheme, its power is that it allows you to fill in gaps in your knowledge, just as Mendeleev filled in gaps in his periodic table. If you can deduce a classification pattern, you can interpolate and extrapolate to make predictions about unknown things or events in the future. (Interpolation is predicting something within the range of your data. Extrapolation is predicting something beyond the range of your data.) When you practice understanding classification schemes made up by others, such as the periodic table in chemistry, it helps you develop your skill in formulating your own classifications in all aspects of your life.

The amazing accuracy of Mendeleev’s predictions showed that the periodic table made sense, but nobody knew why; that came later. The reason for the shape of the table is explained later.

Figure 6.1 is a modern periodic table. You will find yourself referring to the periodic table throughout your study of chemistry.

Figure 6.1 Periodic Table of the elements.

Thinking About Your Thinking

Classification

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The number at the top of each box in our periodic table is the atomic number of the element (Fig. 6.2). The chemical symbol is in the middle, and the atomic mass, rounded to four significant figures, is at the bottom. The boxes are arranged in horizontal rows called periods. Periods are numbered from top to bottom, but the numbers are not usually printed. Periods vary in length: The first period has two elements; the second and third have eight elements each; and the fourth and fifth have 18. Period 6 has 32 elements, including atomic numbers 58 to 71, which are printed separately at the bottom to keep the table from becoming too wide. Period 7 also theoretically has 32 elements, but some elements in Period 7 were not yet synthesized at the time this was written. Elements with similar properties are placed in vertical columns called groups or chemical families. Groups are identified by two rows of numbers across the top of the table. The top row shows the group numbers commonly used in the United States.∗ European chemists use the same numbers, but a different arrangement of As and Bs. The International Union of Pure and Applied Chemistry (IUPAC) has approved a compromise that simply numbers the columns in order from left to right. This is the second row of numbers at the top of Figure 6.1.

We will use both sets of numbers, leaving it to your instructor to recommend which you should use. When we have occasion to refer to a group number, we will have the U.S. number first, followed by the IUPAC number after a slash. Thus, the column headed by carbon, Z = 6, is Group 4A/14.

Two other regions in the periodic table separate the elements into special classifications. Elements in the A groups (1, 2, and 13 to 18) are called main group elements. Main group elements are also known as representative elements. Similarly, elements in the B groups (3 to 12) are known as transition elements, or transition metals. The stair-step line that begins between atomic numbers 4 and 5 in Period 2 and ends between 84 and 85 in Period 6 separates the metals on the left from the nonmetals on the right.

The location of an element in the periodic table is given by its period and group numbers. Example 6.1

List the atomic number (__________), chemical symbol (__________), and atomic mass

(__________) of the third-period element in Group 6A/16. Z = 16; symbol, S; atomic mass, 32.07 amu

In Group 6A/16, the third column from the right side of the table, you find Z = 16 in Period 3. The element is sulfur. Target Check 6.1

a) How many Group 3A/13 elements are metals?

b) How many Period-4 elements are transition metals?

∗ Roman numerals are sometimes used, for example, IIIA instead of 3A.

P/Review Some atomic masses of elements in the periodic table are in parentheses. These elements are radioactive, and there is no atomic mass in the sense that we have defined it. Instead, parentheses enclose the mass number of the most stable isotope.

Figure 6.2 Sample box from the periodic table, representing sodium.

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Appendix 2: Selected Periodic Trends

Periodic trends in atomic size. From Cracolice, M. S., & Peters, E. I. (2011). Introductory Chemistry: An Active Learning Approach. Belmont: CA: Brooks/Cole Cengage Learning. 49. How does size vary with increasing atomic number within a group? How does size vary

with increasing atomic number within a period?

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Periodic trends in first ionization energy. From Cracolice, M. S., & Peters, E. I. (2011). Introductory Chemistry: An Active Learning Approach. Belmont: CA: Brooks/Cole Cengage Learning. 50. How does first ionization energy vary with increasing atomic number within a group?

How does first ionization energy vary with increasing atomic number within a period?

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Homework Questions How Can the Properties of Elements be Predicted? 1. State the element in each pair with the larger atomic size and explain your reasoning. a) Li or Cs b) C or Ge c) Al or P d) C or F 2. Rearrange the following elements in order of increasing atomic radii (smallest first,

largest last). Explain your reasoning. a) F, Li, C b) Na, Li, K 3. Rearrange the following element in order of increasing ionization energies (smallest

first, largest last). a) Mg, Si, S b) Ca, Mg, Ba c) Cl, F, Br How can Periodic Trends be Used to Predict Chemical Reactivity? 4. Which of the following is the most reactive metal: neon, boron, beryllium, lithium, or

silicon? Explain your answer. How Can Elements be Related in a Family? 5. Examine the graph in Appendix 2: Periodic Trends in First Ionization Energy. Give a

predicted value for the element rubidium (Rb) which is found under potassium on the periodic table. Explain your prediction.

6. Examine the graph in Appendix 2: Periodic Trends in First Ionization Energy. Give a

predicted value for the element xenon (Xe) which is found under krypton on the periodic table. Explain your prediction.

7. Name the members of the alkali metals. 8. Hydrogen is at the top of the Group 1A/1 but is not an alkali metal. Why? 9. Write the formulas of alkali metals Li, Na, and K when they combine with imaginary

element W (valence 1). 10. Write the formulas of alkali metals Li, Na, and K when they combine with chlorine

(valence 1). 11. Write the formulas of alkali metals Li, Na, and K when they combine with imaginary

element Y (valence 2). 12. Write the formulas of alkali metals Li, Na, and K when they combine with sulfur

(valence 2).

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13. Write the formulas of alkali metals Li, Na, and K when they combine with imaginary

element Z (valence 3). 14. Write the formulas of alkali metals Li, Na, and K when they combine with phosphorus

(valence 3). 15. Name the members of the alkaline earth metals. 16. Why is helium not placed at the top of Group 2A/2, but it placed with the noble gases,

Group 8A/18? 17. Write the formulas of alkaline earth metals Mg, Ca, and Sr when they combine with

imaginary element W (valence 1). 18. Write the formulas of alkaline earth metals Mg, Ca, and Sr when they combine with

bromine (valence 1). 19. Write the formulas of alkaline earth metals Mg, Ca, and Sr when they combine with

imaginary element Y (valence 2). 20. Write the formulas of alkaline earth metals Mg, Ca, and Sr when they combine with

oxygen (valence 2). 21. Write the formulas of alkaline earth metals Mg, Ca, and Sr when they combine with

imaginary element Z (valence 3). 22. Write the formulas of alkaline earth metals Mg, Ca, and Sr when they combine with

nitrogen (valence 3). 23. In Groups 1A/1 and 2A/2 (alkali and alkaline earth metals), reactivity increases as you

go down the column. Is that the same pattern in the halogens, Group 7A/17? 24. Write the expected formula of the noble gas Ne combined with chlorine, sulfur, and

phosphorus. Explain your answer. 25. Iodine reacts with the unknown element X to form the compound XI2. Would element

could X likely be: Na, O, Al, or Mg? 26. The formula of ytterbium oxide is Yb2O3. What is the likely formula for ytterbium

chloride?

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