Materials Included In Kit

Teacher Tips

• Element Card Arrangement Handout: Make enough copies of this handout (see Teacher PDF) on regular paper to give each student an individual copy. After students have completed their own element arrangement on the worksheet, they will work from this handout to complete the questions and graphs on the “It’s in the Cards” Worksheet.
• Although this is a “dry-lab” activity, it is best to schedule enough time for it in class so that students can complete steps 1–6 in the Procedure section. This will ensure that students really do treat the lab as a discovery activity. Do not give students the Element Card Arrangement Handout until they have completed the Table of the Elements on the Data Sheet.
• Keeping in mind the spirit of discovery, remind students that there is no single correct answer for the arrangement of the elements. Remember that in Mendeleev’s first published table of the elements (1869), elements were listed in order of increasing atomic mass in columns, so that similar elements (e.g., Cl, Br, I) were found in horizontal rows rather than vertical columns. The vertical arrangement of similar elements into element groups appeared two years later, in 1871.
• Although Dmitri Mendeleev is often given top billing in textbook discussions of the development of the periodic table, Lothar Meyer in Germany is credited with independent discovery of the periodic law. In an 1870 paper, Meyer printed a graph of atomic volume versus atomic weight that revealed the characteristic repeating pattern in the properties of elements. In the same paper, Meyer also displayed an arrangement of the elements into a table of rows and columns on the basis of increasing atomic weight. Meyer did not, however, predict the existence of missing or undiscovered elements.

Answers to Prelab Questions

1. Ionization energy—Energy needed to remove one of the electrons from an atom (specifically, from an isolated atom in the gas phase). The energy needed to remove successive electrons is called the first ionization energy, second ionization energy, etc. Units are kilojoules per mole (kJ/mole).
2. Atomic radius—For a neutral atom, the distance from the center of an atom’s nucleus to its outermost electron. Since the electron cloud that surrounds the nucleus does not have an exact boundary, the atomic radius of an atom is not a precise number. The atomic radius is usually estimated as one-half the distance between two atoms in a form of the element—for example, one-half the distance between chlorine atoms in the diatomic chlorine molecule. Units are nanometers (1 nm = 1 x 10^–9 m).
3. Density—The ratio of mass to unit volume for the standard form of the element. Units are grams per cubic centimeter (g/cm³) for a solid or liquid and grams per liter (g/L) for a gas. Density depends on temperature.
4. Electronegativity—Assigned numerical value for an element that reflects the ability of an atom to attract electron density in a covalent bond. Electronegativity is not an amount of energy, nor can it be directly measured. There are no units for electronegativity; it is assigned on a relative scale from 0.5 to 4.0.
5. Atomic mass—Also called the average atomic mass; the average mass of an element’s atoms. Since the mass of an individual atom is too small to be directly measured, atomic mass is defined relative to that of a reference atom, an arbitrary standard. Thus, the mass of the carbon-12 isotope is defined as exactly equal to 12 atomic mass units. The atomic mass of a particular atom is very nearly, but not exactly, equal to its mass number (number of protons and neutrons). The average atomic mass of an element is computed as a weighted average of the masses of the isotopes that make up the element and their fractional (natural) abundance. The units of atomic mass are atomic mass units (amu).
6. Melting point—Temperature at which the solid form of an element is in equilibrium with the liquid phase at atmospheric pressure. Usually given in units of kelvin (K).

Sample Data

“Vertical” Table of the Elements

“Horizontal” Table of the Elements Properties of the Missing Element

These are the predicted, not actual, properties. Student predictions will, of course, vary.

Answers to Questions

1. Mendeleev’s Periodic Law can be stated: “The physical and chemical properties of elements are periodic functions of their atomic masses.” Looking at your arrangement of the element cards, describe in your own words what the term “periodic function” means.

   “Periodic function” means that the properties of the elements repeat in a regular manner (are periodic) and that the cycle for the repeating pattern depends on (is a function of) the atomic masses of the elements.

2. Some of the properties listed on each card are periodic properties, others are not. Name one property that is periodic and one that is not.

   Ionization energy, electronegativity and atomic radius reveal distinct periodic trends when the elements are arranged in order of increasing atomic mass. Density and melting point show trends, but they are not as obvious. Atomic mass is not a periodic property.

3. The elements in the modern periodic table are arranged in order of increasing atomic number (instead of increasing atomic mass). Why didn’t Mendeleev use atomic number to arrange the elements?

   The concept of atomic number did not exist at the time of Mendeleev. Note: In 1913, the English chemist Henry Moseley, working in Ernest Rutherford’s laboratory on the properties of the nucleus, hypothesized that every element had a different amount of positive charge and that the amount of positive charge occurred in whole-number increments. Moseley introduced the concept of atomic number to account for the positive charge of the nucleus.

4. From your instructor, obtain a handout showing one possible arrangement of the element cards. Identify each elements on the handout with its atomic number and chemical symbol. Use your textbook to obtain this information.

   The elements are listed in order from left to right on the Teacher PDF, in rows according to their periodic number. The transition metals are not included. The atomic numbers of the elements listed are 3–20, 31–38 and 49–54.

5. Using the possible arrangement of the element cards obtained from your instructor, pick two of the numerical properties of the elements that are periodic and plot their values on the graphs below. Give each graph a descriptive title and label the axes.
   {12047_Answers_Figure_3}
   {12047_Answers_Figure_4}
   {12047_Answers_Figure_5}
6. There are certain trends in the properties of the elements, both within a column (from top to bottom) and across a row (from left to right) in the periodic table. On the arrow for each property, write the word increases or decreases to describe how that property changes.
   {12047_Answers_Figure_6}
7. On the outline of the periodic table shown, locate the following: metals, nonmetals and metalloids (or semimetals).
   {12047_Answers_Figure_7}
8. On the outline of the periodic table shown below, locate the following: groups or families of elements, periods or series of elements, noble gases, alkali metals, alkaline earth metals and halogens.
   {12047_Answers_Figure_8}
9. On the outline of the Periodic Table shown, locate the following: transition elements, inner transition elements, representative elements.
   {12047_Answers_Figure_9}

Introduction

Dmitri Mendeleev’s discovery of the Periodic Law ranks as one of the greatest achievements in the history of science. It has survived the test of time and stands to this day as the single most important tool to understand the chemistry of the elements. As we try to understand the essence of this discovery, it is worthwhile to go back in time and look at how it was achieved.

Concepts

• Periodic law
• Periodic table

Background

In the years 1868–1870, Dmitri Mendeleev, a professor of chemistry at the University of St. Petersburg in Russia, was writing a new textbook called Principles of Chemistry. More than 60 individual elements were known, along with a great many facts about their properties and compounds. Mendeleev knew the atomic masses of the elements, their densities, boiling points and melting points as well as the formulas of their compounds with hydrogen, oxygen and chlorine. What was missing was a way to organize these facts, a way to understand how individual facts related to each other—in short, a way to classify the elements. The following quote from Mendeleev reveals his thoughts at the time:

“I wished to establish some sort of system of elements in which their distribution is not guided by chance... but by some sort of definite and exact principle.”

Mendeleev decided to arrange the elements according to their atomic mass. He wrote out the exact atomic masses (as they were known at the time) in the margin of a list of the elements, then wrote out separate cards for each of the elements, with their atomic mass and other chemical and physical properties. Using these cards, Mendeleev played “chemical solitaire” for several hours, finally copying to a sheet of paper the arrangement he had worked out with the cards. With slight modification, this became Mendeleev’s first Periodic Table of the Elements.

Experiment Overview

The purpose of this activity is to re-create Mendeleev’s discovery of the classification of the elements and the periodic law using a special deck of element cards. The real properties of the elements, but not their names or symbols, are written on these cards. As the cards are arranged and rearranged based on logical trends in some of these properties, the nature of the periodic law should reveal itself.

Materials

Prelab Questions

Define each property and give its typical units, if appropriate.

1. Ionization energy
2. Atomic radius
3. Density
4. Electronegativity
5. Atomic mass
6. Melting point

Procedure

1. Form a group with three other students. Obtain a deck of element cards and spread the cards out on the table.
2. Each card lists the properties of a single element (X), as shown:
   {12047_Procedure_Figure_1}

   * Density values are in units of g/cm³ for solids and liquids, g/L for gases.
   † Dashed lines for a property indicate that no data is available. Some elements, for example, may not form a compound with hydrogen.

3. Working together, discuss the possibilities for arrangement of the element cards with all members of the group and look for a logical arrangement of the cards. Consider the similarities and differences among the elements as well as possible numerical or logical trends in their properties.
4. It is NOT within the rules of this game of chemical solitaire to look up information in a textbook or to use a modern periodic table as a guide!

Mendeleev’s greatest insight in creating the periodic table was in recognizing there were some gaps when the elements were arranged in logical order. He had the ingenuity not only to leave blanks in his table for the missing elements, but also to predict their properties.

5. One of the element cards is also missing in your deck of cards. Decide where the missing element belongs in the arrangement of the elements and rearrange the cards if necessary to accommodate the missing element.
6. On the worksheet, fill in the Table of the Elements to illustrate a logical arrangement of the element cards. To do this, write down only the atomic mass of each element, as shown on its card. Leave a blank space for the missing element. Note: The table is 8 x 8 and contains 32 more squares than are needed—plenty of room to arrange the elements many different ways!
7. Predict the properties of the missing element by averaging the properties of its nearest neighbors. On the worksheet, complete the card for the missing element by entering its predicted properties alongside the name of each property.
8. Answer Questions 1–9 on the It’s in the Cards Worksheet.

Student Worksheet PDF

12047_Student1.pdf