Materials Included In Kit

Additional Materials Required

Safety Precautions

The materials in this kit are considered nonhazardous and are reusable. Follow all classroom or laboratory safety guidelines.

Disposal

The atomic structure puzzle may be stored for reuse. After determining if the puzzle sheets will be cut, instruct students on proper storage. If the sheets will be cut, use resealable bags, such as Ziploc^® bags, or envelopes to keep the puzzle pieces together and tidy.

Lab Hints

• Enough materials are provided in this kit for 30 students working in pairs or for 15 groups of students. Both parts of this laboratory activity can reasonably be completed in one 50-minute class period. The prelaboratory assignment may be completed before coming to lab or at the beginning of the class.
• After students have completed the atomic structure puzzle questions, they need to organize the information on the reverse side. The answers must be arranged in numerical order from lowest to highest to obtain the correct clues.
• The easiest way to do this is for students to cut out the squares, arrange them in numerical order, and then flip them over to get the proper arrangement of letters, spaces and punctuation.
• The students may also systematically work through the puzzle clues and then note the answers lowest to highest, writing down the information as they go. This eliminates the cutting and physical manipulation and the extra step of ordering the puzzle clues 1–20.
• The instructor should let students know how each group will relay their information to the rest of the class. For example are students going to cut out each square and tape them onto a board or wall in the proper order to arrive at the quote, or are students going to write their section on the board and then each subsequent group will do the same?
• Before beginning the activity, copy the Atomic Structure Puzzle Sheets so you will have extra copies in case of loss.

Teacher Tips

• This activity is appropriate for studying the concepts of atomic structure, isotopes, and ions.
• The Atomic Structure Puzzle Sheets can be used while learning the material or as a review.
• Each puzzle sheet follows the same basic template, thus all are the same level of difficulty.

Answers to Prelab Questions

Possible Answers: a) mass number, b) atomic number, c) atomic mass, d) proton, e) electron, f) neutron

1. Which term(s) are found on the periodic table? b, c
2. Which terms do not change for a given element? b, c, d
3. Which terms change for different isotopes of an element? a, f
4. For an ion, either cation or anion, which term changes? e
5. True or false. Mass number is not on the periodic table; it must be given or calculated. True
6. Write three mathematical expressions using the words mass number, protons, neutrons and atomic number.

   Protons + Neutrons = Mass Number OR Neutrons + Protons = Mass Number (These do not count as two equations since they are not mathematically different—addition is a communitive property.)
   Atomic Number + Neutrons = Mass Number
   Mass Number – Protons = Neutrons
   Mass Number – Atomic Number = Neutrons
   Mass Number – Neutrons = Protons
   Mass Number – Neutrons = Atomic Number

Answers to Questions

Atomic Structure Quote

“Do not think for a moment that you know the real atom. The atom is an idea, a theory, a hypothesis, a human construction. It is whatever you need to account for the facts of experience. As our ideas about the atom have changed in the past, so will they continue to change in the future. An idea in science, remember, lasts only as long as it is useful.” —Alfred Romer

Each Atomic Structure Puzzle Sheet has 20 letters, blank spaces or punctuation marks. Set 1 is the first 20 characters of this quote, set 2 and those after continue, with set 15 having the last 20 characters of this quote. Blanks correspond to spaces after periods.

Puzzles

Teacher Handouts

12838_Teacher1.pdf

12838_Teacher2.pdf

References

Special thanks to Fran Zakutansky, Pascack Valley High School, Hillsdale, NJ, for sharing this activity with Flinn Scientific.

Introduction

What is an atom? Over time, as new knowledge has been obtained, the definition of an atom has evolved or changed to accommodate the information. Clue into atomic structure with this puzzle activity and then join efforts with the whole class to decode a message about the atom.

Concepts

• Atomic number
• Mass number
• Atomic mass
• Isotopes
• Subatomic particles
• Protons
• Neutrons
• Electrons
• Ions

Background

Atomic structure is a fundamental concept. There are some very specific definitions about the parts of an atom. Once the definitions are understood, figuring out how much of one subatomic particle there is relative to other subatomic particles is somewhat like doing Sudoku puzzles. All the clues are there and the rules are in place, but it is a matter of putting the correct pieces of information together to get the answer.

All atoms consist of protons, neutrons and electrons. Protons and neutrons are arranged together to form a nucleus in the center of the atom. The electrons are very mobile and can be located anywhere from the center of the atom to its outer edge. (The electrons cannot be found in the nucleus, however.) Two of these subatomic particles have electrical charge. The proton has a positive (+) charge and the electron has a negative (–) charge. An atom is identified by its number of protons and, for a specific element, the number of protons does not change. Atoms of the same element, however, may have different numbers of neutrons or electrons. First, let’s look at atoms where the neutrons vary in amount within the same element.

At the beginning of the 19th century, John Dalton (1766–1844) proposed his new atomic theory—all atoms of the same element are identical and equal in mass. It was a simple yet revolutionary theory. It was also not quite right. The discovery of radioactivity in the 20th century made it possible to study the actual structure and mass of atoms. Gradually, evidence was obtained that atoms of the same element could have different masses. These atoms were called isotopes.

Two experiments in the early 20th century suggested the possible existence of isotopes. The first was work by J. J. Thomson (1856–1940) with positively charged atoms in gas discharge tubes. When the positively charged atoms were bent by electric and magnetic fields and then allowed to strike a photographic film, they left curved “spots” on the film at an angle that depended on the mass and charge of the atoms. In 1912, Thomson found that when the gas in the tube was neon, he obtained two curves or spots. The major spot corresponded to neon atoms with a mass of about 20 atomic mass units (amu). There was also a much fainter spot, however, corresponding to atoms with a mass of about 22 amu.

The second experiment suggesting the existence of isotopes came from studies of radioactivity. One of the products obtained from the radioactive decay of uranium is lead. When the atomic mass of lead deposits in radioactive uranium minerals was analyzed, it was found to be different from the atomic mass of lead in lead ore. In 1913, Frederick Soddy (1877–1956), professor of chemistry at the University of Glasgow in Scotland, used the term isotope to define atoms of the same element that have the same properties but different atomic masses. The word isotope was derived from Greek words meaning “same place”—isotopes occupy the same place in the periodic table (they are the same element) even though they have different masses. Soddy was awarded the Nobel Prize in Chemistry in 1921 for his work on isotopes.

Final proof for the existence of isotopes came from the work of Francis W. Aston (1877–1945) at Cambridge University. Aston built a more accurate version of the apparatus that Thomson had earlier used to study charged atoms. In 1919, Aston measured the precise masses of the two isotopes of neon. Aston received the Nobel Prize in Chemistry in 1922 for his discovery of more than 200 isotopes.

The modern definition of isotopes depends on the number of subatomic particles in atoms. Isotopes have the same number of protons but different numbers of neutrons. Since the identity of an element depends on the number of protons (the atomic number), isotopes have the same chemical properties. Isotopes are thus chemically indistinguishable from one another—they undergo the same reactions, form the same compounds, etc. The only difference is that one isotope of an element has a different number of neutrons than another isotope of the same element, with same number of protons (atomic number). Isotopes have different mass numbers.

The mass number of an isotope is the total number of protons and neutrons in an atom. Mass numbers are given or calculated and are not located on the periodic table. Protons and neutrons are counted as whole numbers (integers) so the addition of the two would also be a whole number, making mass number always a whole number. Chlorine, for example, occurs in the form of two isotopes, chlorine-35 and chlorine-37, where 35 and 37 are the mass numbers. Since the atomic number of chlorine is 17, chlorine-35 has 17 protons and 18 neutrons (35 – 17 = 18) and chlorine-37 has 17 protons and 20 neutrons (37 –1 7 = 20). Mass number and atomic mass are different. The atomic mass of an element represents the weighted average of the masses of the isotopes, this is why the majority of the atomic masses are decimals. The ratios used to calculate the weighted average are the ratios of the isotopes as they occur in nature. Chlorine consists of 75.8% chlorine-35 atoms and 24.2% chlorine-37 atoms. Equation 1 shows the atomic mass calculation for the element chlorine. The mass of each isotope is equal to its mass number. 

The atomic mass can be found on the periodic table and is a calculated number given the ratio of isotopes as they occur in nature.

The mass number is usually conveyed in one of two ways, either after the element name or symbol followed by a dash and then the mass number, or using isotopic notation. Isotopic notation is a type of shorthand, as shown: In these examples, Chlorine-35 means that 35 is the mass number. It can also be written as Cl-35. The isotopic notation would look like For chlorine-37 or Cl-37 the isotopic notation is The 17 in both cases is the atomic number (number of protons) for chlorine which can be found on the periodic table.

Now let’s look at atoms of the same element when the electrons vary. An uncharged atom has equal numbers of positively charged protons and negatively charged electrons. This makes the charge on the atom zero—the atom is said to be neutral. Elements may also gain and lose electrons due to interactions with other elements. Ions are atoms or groups of atoms that have lost or gained electrons and thus have a net positive or negative charge due to the change in the amount of electrons. Remember, the number of protons defines the element and does not change, so the electrons have to be the subatomic particle that varies in number of ions. If the overall change is negative then there must be MORE electrons because there is MORE negative charge. If the overall charge is positive then there must be LESS electrons because there is LESS negative charge. Cations are atoms or groups of atoms with a net positive charge, such as Cu²⁺ or NH₄⁺, while anions are atoms or groups of atoms with a net negative charge, such as O^2– or NO₃^–. Atoms of metallic elements tend to form cations while atoms of nonmetallic elements tend to form anions.

In summary, the number of protons for an element is always constant. (If the number of protons changes, the element changes.) Neutrons and electrons can vary in amount. If the number of neutrons changes and the protons do not, these atoms are called isotopes. If the electrons change in amount but the protons do not, these atoms are called ions and they have a charge. An ion with more electrons than protons = negative charge, and an ion with fewer electrons than protons has a positive charge.

Experiment Overview

The purpose of this cooperative class activity is to answer a series of atomic structure questions on a puzzle sheet and use the answers to decode a puzzle about our knowledge of the atom. There are 15 different puzzle sheets, sets 1–15, each with 20 unique puzzle pieces (questions) that have whole number answers. After each group has verified their answers, the puzzle pieces may be cut out and arranged in numerical order from lowest to highest. The reverse side of each puzzle piece has a clue, which may be a letter, punctuation mark, or blank. (Blanks correspond to spaces after periods.) The whole class works together to enter their clues in the correct order and sequence and to solve the puzzle!

Materials

Prelab Questions

Possible Answers: a) mass number, b) atomic number, c) atomic mass, d) proton, e) electron, f) neutron

1. Which term(s) are found on the periodic table?
2. Which terms do not change for a given element?
3. Which terms change for different isotopes of an element?
4. For an ion, either cation or anion, which term changes?
5. True or false. Mass number is not on the periodic table; it must be given or calculated.
6. Write three mathematical expressions using the words mass number, protons, neutrons and atomic number.

Safety Precautions

This activity is considered nonhazardous. Please follow all classroom or laboratory safety guidelines.

Procedure

1. Divide the class into 15 groups.
2. Each group receives a unique Atomic Structure Puzzle Sheet.
3. Working within your group, answer all 20 puzzle questions on the puzzle sheet. The answers are all whole numbers.
4. After verifying the answers with your teammates, cut out the individual puzzle pieces and arrange them in order from lowest to highest numerical values based on the answers to the puzzle questions.
5. Each box on the reverse side of the puzzle sheets has a letter, punctuation mark or is blank. Turn over the pieces in order and write the clues in the spaces provided on the Puzzle Quote Sheet (see the Set No. for your puzzle sheet).
6. All 15 groups should report their clues in order, from set 1 through set 15, to the whole class. The result is a profound quote about the atom!
7. (Optional) After verifying the answers with your teammates, rank each puzzle, 1–20, in order from lowest to highest numerical values based on the puzzle answers. Transfer the corresponding clues on the Atomic Structure Puzzle Quote Sheet without cutting.

Student Worksheet PDF

12838_Student1.pdf