Periodic trends and properties of elements lab

Chemistry Periodic Trends

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PERIODIC TRENDS
In the nineteenth century, Mendeleev used the chemical and physical properties of the elements to develop an arrangement of the elements that today we call the periodic table. In this lab activity, you will investigate how three properties of the elements—atomic radius, ionization energy, and electronegativity—vary according to atomic number. You will be making a 3-dimensional model of these properties for some of the representative elements, and will use it to describe and explain the trends in these properties.

Objectives

When you have completed this activity, you should be able to:

1. Use a model to describe the trends in several physical properties of the elements.

2. Relate these trends to the electron configuration of the elements.

Materials

· small plate

· scissors

· ruler

· plastic straws, 3 colors

· calculator

· ruler

· forceps

· plastic wrap

· periodic table

Procedure

1. You will prepare a 3-dimensional model for each of the properties of the elements.

2. In order to make the 3-D model, you need to cut the straws to scale so their heights represent the relative values for each element. On the Data Table provided, convert the values for your assigned property to a scale in centimeters by using the conversion factors given below. Round your answers to the nearest tenth of a centimeter. An example is given for each property.

Property

Conversion Factor

Example

(For Hydrogen)

Atomic radius

1 cm = 30 pm

32 pm x 1 cm = 1.07 cm (1.1 cm)

30 pm

Ionization Energy

1 cm = 400 kJ

1312 kJ x 1 cm = 3.28 cm (3.3 cm)
400 kJ
Electronegativity

1 cm = 0.8 unit

2.2 units x 1 cm = 2.75 cm (2.8 cm)
0.8 unit

3. Use the diagram of the well plate provided below to plan the placement of the elements. Your well plate should be oriented to correlate with the elements of the periodic table. Row 1 represents the first period, with well H1 as Hydrogen and well A1 as Helium. Row 2 represents the second period, with well H2 as Lithium and well A2 as Neon. Rows 3 and 4 contain the elements in periods 3 and 4, omitting the transition elements.

Write the symbols for the elements in the circles.

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4. Select colored straws to represent one of the three properties.

5. Place the well plate template with tape onto the bottom of the plate. Wrap the plastic wrap over the top of the plate. Make sure it is secure on the bottom and is tight across the top of the plate.

6. Carefully measure and cut a piece of straw for each element. Following the diagram you completed in Procedure 3, insert each piece into the plastic wrap.

7. Repeat steps 5 and 6 for each of the remaining two properties.

Analysis

1. Examine your tray. How do the properties change as you look from top to bottom within a group or family?

Atomic radius

Ionization energy

Electronegativity

2. As you look from left to right across the periodic table, how do the properties change?

Atomic radius

Ionization energy

Electronegativity

3. Write the electron configuration for the following alkali metals (Group 1).

Li

Na

K

In what way are the electron configurations for group 1 similar?

How do the electron configurations differ as you move down group 1?

4. Write the electron configuration for the elements in period 2, from Li to Ne.

Li

Be

B

C

N

O

F

Ne

What do all the electron configurations above have in common?

How do these electron configurations differ? How does the number of protons differ?

5. In terms of electron configuration, explain the trend in atomic radius.

Down a group

Across the period from left to right

6. In terms of electron configuration, explain the trend in ionization energy.

Down a group

Across the period from left to right

7. In terms of electron configuration, explain the trend in electronegativity.

Down a group

Across the period from left to right

8. Look at your model again. Why do you think we say that the properties of the elements are “periodic”?

9. On the diagram of the periodic table below, draw and label arrows that show the direction in which the three properties (atomic radius, ionization energy, and electronegativity) increase.

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10. Some of the elements show exceptions to the trends. Look at your model and select an element that seems to have an unusual value for one of these properties. Try to explain why this happens by writing the electron configuration for this element and looking for a definite change from the element before it.

11. Write a paragraph explaining why atomic radius, electronegativity, and ionization energy show periodic trends.

PROPERTIES OF REPRESENTATIVE ELEMENTS

Element

Atomic number

Atomic radius

(pm)

Straw

Length

(cm)

Ionization Energy

(kJ/mol)

Straw

Length (cm)

Electronegativity
(units)

Straw

length (cm)

H

1

32

1312

2.2

He

2

50

2372

-

Li

3

155

520

1.0

Be

4

112

900

1.6

B

5

98

801

2.0

C

6

91

1086

2.5

N

7

92

1402

3.0

O

8

73

1314

3.5

F

9

72

1681

4.0

Ne

10

71

2081

-

Na

11

190

496

0.9

Mg

12

160

738

1.3

Al

13

143

578

1.6

Si

14

132

786

1.9

P

15

128

1012

2.2

S

16

127

1000

2.6

Cl

17

99

1251

3.2

Ar

18

98

1520

-

K

19

235

419

0.8

Ca

20

197

590

1.0

Ga

31

141

579

1.4

Ge

32

137

762

2.0

As

33

139

944

2.2

Se

34

140

941

2.5

Br

35

114

1140

3.0

Kr

36

112

1351

-

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PRE-LAB: PERIODIC TRENDS

1. Which groups of the periodic table contain the “representative elements”?

Why do you think these elements are called “representative”?

2. What is a periodic trend?

3. Consult a chemistry textbook to find the definitions for the following properties, then restate the definition in your own words.

Atomic radius

Ionization energy

Electronegativity

4. Read the procedure for this activity. What is the purpose for including drinking straws in the materials list?

5. Follow the directions in Procedure Step 2 to calculate the straw length needed to represent:

The atomic radius of oxygen

The ionization energy of neon

The electronegativity of arsenic