TEKS 8.8 A and B
A Tiny Big Discovery!
TAKS Objective 3 – The student will demonstrate an understanding of the structures and properties of matter.
Learned Science Concepts:
- Matter is composed of atoms.
- Substances have chemical and physical properties.
- Complex interactions occur between matter and energy.
TEKS Science Concepts 8.8
The student knows that matter is composed of atoms. The student is expected to:
(A)describe the structure and parts of an atom; and
(B)identify the properties of an atom including mass and electrical charge.
Overview
As this lesson is taught focus on the following concepts. An atom is very small and composed mostly of empty space. The proton is positively charged, located in the nucleus, contains most of the atom’s mass, and the number of protons identifies the element. The neutron’s mass is almost the same as a proton’s mass; it has no charge. The electron has almost no mass, is negatively charged, and is mostly likely located somewhere in a 3-dimensional configuration called an electron cloud.
Instructional Strategies
These activities are built around models and visuals to help students construct understanding of particles that are not visible. Care must be taken when comparing the atom to a model so that misconceptions about the atom are not developed (See TAKS Objective 1 unit on Models). Students should understand that the atom is still a mystery.
The lesson titled Who Knew? helps students understand how scientists build knowledge from what previous scientists have discovered. This knowledge is based on the best information of the time.
Students will construct their understanding of atoms and atomic particles through the use of models and analogies. Beginning with a “black box” investigation, students will discover that scientific knowledge is sometimes dependent upon combining bits of information from many sources.
The activities are all centered on answering the question, “What is an atom?” Students will learn how scientists through history learned bits of information that were constructed into the model of the atom.
Lesson Objectives
- The learner will be able to describe the parts of an atom.
- The learner will be able to compare mass and charges of the electron, proton, and neutron.
- The learner will use appropriate models and analogies to describe the size of an atom and its particles.
- The learner will explain how scientists are able to discover the properties of an atom.
For Teacher’s Eyes Only
The discovery and understanding of atoms has occurred over many centuries. Atoms are so small that they cannot be seen using visible light which means they cannot be viewed under a conventional microscope. Scientists find evidence and test their ideas to discover what these small building blocks of matter called atoms are. If a scientist’s idea holds up under much scrutiny and no contrary information negates the idea, then he or she uses the idea to build new or enhance old theories.
Atoms are often perceived to be miniature solar systems with the nucleus in the center and spheres of electrons orbiting in given paths around the center. This thinking has been perpetuated by textbook diagrams of electrons orbiting in circular paths around a nucleus. Such an electron would be constantly accelerating and thus giving off radiation. As the electron emitted radiation, it would loose energy and fall toward the nucleus. The frequency of radiation would increase (exactly opposite of a smaller orbital) and the atom would collapse. Radiation emitted would be continuous rather than discrete spectrum. This model does not hold up under any experiment.
In the early 1900s, Niels Bohr proposed a quantum mechanical model of the atom. The Bohr Model assigned discrete orbiting paths for the single electron in the hydrogen atom. The strength of this model is that it correctly predicted the major spectral lines emitted and absorbed by the electron in the hydrogen atom. Quantum amounts of energy are absorbed when an electron moves from the ground state to an excited state. Likewise, quantum amounts of energy are emitted when the electron moves from an excited state back to the ground state.
The weakness of the Bohr Model lies with the energy levels. There is no explanation for the fine line spectrum that appears along with the strong spectral lines. The Bohr model does not account for the existence of more than one electron, thus does not explain the configuration of atoms other than the hydrogen atom. The model also fails to explain molecular bonding. The Bohr model has some limited value, but is more inconsistent than consistent with experimental information collected on atoms.
According to modern theories the atom can best be described mathematically. Models and analogies can still be used, however, on a limited basis. Exact positions and motions of subatomic particles cannot be measured and are thus unknown. An electron orbital is a volume configuration that denotes where there is the probability of finding an electron. These orbitals appear as fuzzy electron clouds.
Most of an atom is empty space. The nucleus, commonly composed of protons and neutrons, is the massive part of the atom. A proton is about 1800 times as massive as an electron. The electron does not have a size in the way that particles are thought of as taking up space. Electrons occupy space according to a probability configuration identified by the energy level. In other words, the first energy level is shaped like a solid sphere. The second energy level looks like a dumbbell and so on. The electron is most likely found somewhere inside this area, but not always. There is a small chance it could be found elsewhere. Think of this example: During a school day, a second grade teacher (s electron) is most like to be found somewhere in her classroom. She may also be found in the hallway, playground, or even at home. The likelihood, however, is that she will be found at various points in her classroom at different times of the day. The classroom is her probability configuration. The principal (p electron) on the other hand has the whole school as a probability configuration. A principal who spend most time roaming and interacting is most like found somewhere within the school building during a normal school day. The principal could even be found in the second grade teacher’s classroom. Again, there is some possibility that the principal is found outside of the school, but that is small.
The atom’s properties are for the most part determined by the nucleus or the number of protons. Stripped of electrons, it is called a positive ion, but still is much like the original atom. Electrons move randomly around like Bingo balls in a turning cage. They can be shared between atoms or flowing freely as in electric current. Electrons have a negative charge and are responsible for bonding in chemical reactions.
Atoms are too small to be seen with visible light. In the latter part of the 20th century, the Scanning Tunneling Microscope (STM) was developed. The microscope has a very sharp needle with one single atom at its tip. As the needle is passed over a surface, a very small current is sent through the needle. The electrical field surrounding electron clouds of an atom cause the current to increase and decrease as the needle gets farther or closer to the electron cloud. The result is a three dimensional image that can be constructed to show the shapes of the electron clouds.
Lesson Layout
Student Misconceptions
Misconception
Electrons are little spheres that orbit the nucleus of an atom.
Science Concept
Electrons have no known shape and almost no mass. They do not orbit. They are most probably found somewhere inside a defined shape, but could be anywhere outside it, too.
Rebuild Concept
Use analogies and compare an electron to a popcorn popper or Bingo cage. The electron could pop up at any spot or even pop out. Just like the popcorn and the Bingo ball, no two electrons can be in the same spot at the same time.
Misconception
Atom’s are like miniature solar systems.
Science Concept
The only thing an atom and a solar system have in common is both are mostly empty space.
Rebuild Concept
Perform a kinesthetic activity where students move around and act like the parts of an atom. They will get a “feel” of how unpredictable the motions of the atom’s parts are.
Misconception
The electron is about the same size as a proton or neutron.
Science Concept
The electron has no size and practically no mass compared to the proton or neutron.
Rebuild Concept
Compare masses of the subatomic particles to known objects. For example: a proton is to an electron as an automobile is to a pound bag of chips. Size cannot be compared since the electron has no size.
Student Prior Knowledge
Students should have some knowledge of what is meant by positive and negative charge. They should also know that empty space is void. Students should know that atoms are the smallest particles of matter.
5 E’s
ENGAGE
Activity: Cut it Up
Class Time: 5 minutes
Materials:
One piece of 8 ½ by 11 inches paper per student
One pair of scissors per students
Students will discover that an atom is small. More specifically, an atom is the smallest division of a material that still retains all of the characteristics of the material. Encourage students to cut a piece of notebook paper in half again and again always cutting in the same direction as many times as they can. Check out to see how many cuts to be the size of an atom. As students cut the paper into smaller and smaller strips, they will begin to have a kinesthetic understanding of how small an atom is. This lesson will help students understand the physical significance of size.
EXPLORE
Exploration 1
Activity: What and Where is it?
Class Time: 20 minutes
Objective: The learner will use appropriate models and analogies to describe the size of an atom and its particles.
Process Skills:
TEKS 8.2 (A) – The student is expected to plan and implement investigative procedures including asking questions, formulating testable hypothesis, and selecting and using equipment and technology,
TEKS 8.2 (B) – The student is expected to collect data by observing and measuring.
TEKS 8.2 (C) – The student is expected to organize, analyze, evaluate, make inferences, and predict trends from direct and indirect evidence.
Materials
Opaque boxes (shoe box) for each group
Solid objects to glue in each box
Marble
This activity is designed to get students thinking about how scientists describe things they cannot “see”. The activity is a spin-off of the Rutherford experiment to find the nucleus of an atom.
Build the Box Glue an object inside an opaque box. Insert a single marble into the box and seal the box. A different object can be used for each group.
Characterize the Object Present one of the boxes to the students and ask them what is in the box. Ask them how they could find out what is in the box without opening it. Give each group a box. Let them roll the marble around inside the box and try to determine the size, shape, and location of the object inside the box. The box can then be given to another group. See if the group’s results agree. This activity can be expounded upon or left as a short exercise.
Exploration 2
Activity: Who knew?
Class Time: 45 minutes
Objective: The learner will explain how scientists are able to discover the properties of an atom.
Process Skills:
TEKS 8.3 (B) – The student is expected to connect Grade 8 science concepts with the history of science and contributions of scientists.
Materials:
Poster board
Marking pens
Internet
Procedure: This activity is designed to help students think and reason how scientists depend on their predecessors and build upon prior knowledge. Students may work individually or in groups of two to complete this activity. Assign the name of a scientist involved in atomic theory discoveries. A partial list is included in this procedure. Allow students to research their scientist and then draw a graphic representation of what they have learned on the poster board. Arrange the poster pictures in chronological order. Allow students to tell about their scientist. Discuss how each scientific discovery depended upon previous discoveries.
Scientist List:
Atoms were first suggested in 440 B.C. by Democritus, a famous Greek philosopher. He actually coined the term “atom” which came from the Greek word “atomos” meaning “indivisible.” Aristotle (384 – 322 B.C) was another Greek philosopher who followed Democritus but was much more popular and believed that all matter was composed of only four elements: earth, water, wind, and fire.
Society believed Aristotle and the idea of atoms did not come back on the scene until 1803 when an English school teacher named John Dalton introduced his idea of the atomic theory.
Dalton’s atomic theory contained three parts:
- Everything is made up of atoms and the atoms cannot be created, divided, or destroyed.
- Atoms of the same element are exactly alike while atoms of different elements are different.
- Atoms join with other atoms to form new substances.
Dalton performed many experiments showing that compounds of different elements always combine in definite proportions. He believed this was because the elements were composed of individual atoms. His theory was revised as more was discovered about the atom.
The events that led to the development of the atomic theory as we know it today are as follows:
440 B.C.Democritus coined the term “atom”
1803John Dalton proposed that all atoms of a substance are alike and that they can join other atoms to form a new substance.
1897J.J. Thomson conducted a famous experiment with a cathode ray tube. He shot an electric current through a tube that had a negatively charged plate attached to the top and a positively charged plate attached to the bottom. When the plates were not charged, the beam went straight through the tube and made a small glowing spot at the end of the tube. When the plates were charged, the glowing spot was considerable lower, showing that the beam was bending toward the positively charged plate. This showed that the beam must contain negative particles because it was attracted to the positively charged plate. From this experiment he came up with the “plum pudding” model of the atom. He proposed that the atom was like plum pudding in that it was mostly a positive material (the pudding) with negative particles scattered throughout (the plums).
1869Dmitri Mendeleev arranged elements into seven groups according to their atomic weights. Each group had similar chemical properties. He predicted that there were still unknown elements.
1898Ernest Rutherford studied the radioactive decay of uranium and thorium. He named the particles alpha and beta. Later it was discovered the alpha particles were helium nuclei and beta particles were electrons.
1903Hantaro Nagaoka proposed a model of the atom. This model resembled Saturn with flat rings of negative particles circling the positive particle.
1909Ernest Rutherford conducted his famous gold foil experiment. He bombarded a thing piece of gold foil with alpha particles (+) and deducted that atoms are mostly empty space. The beam went straight through the foil as if it were not there most of the time rather than being continuously deflected. Once in a while, it would bounce back with great force showing that the center of the atom must be positive and massive because + repels +.
1913Neils Bohr suggested that electrons travel around the nucleus in definite paths and that they could jump from a path in one level to a path in another level. His model became known as the planetary model.
1914H.G.J. Moseley used cathode rays to bombard atoms. He photographed the resulting x-rays. Using the information he interpreted from the photographic plates, he was able to determine the positive charge on the nucleus of the atom. From this he was able to reorganize the Periodic Table.
1919Francis William Aston developed the mass spectrograph, a device that separates atoms or molecular fragments of different mass and measures those masses with remarkable accuracy.
1924Louis de Broglie proposed that matter has wave properties.
1926Erwin Schrödinger developed wave mechanics to describe the behaviors of quantum systems of subatomic particles.
1027Werner Heisenberg formulated the uncertainty principle that states the position and speed of a subatomic particle cannot both be determined exactly.
1932James Chadwick discovered neutrons.
EXPLAIN
Show the video “The Atomic Field” narrated by Dr. Sam Matteson. The video will provide students with a vivid picture analogy of the comparative sizes of subatomic particles and space. Let students discuss the idea of an atom being mostly empty space.