Lesson Notes By Weeks and Term v3 - Senior Secondary 3
Models of the atom
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Models of the atom
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Subject: Physics
Class: Senior Secondary 3
Term: 1st Term
Week: 1
Theme: Energy Quantization And Duality Of Matter
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Students should be able to:- state and discuss what chemical evidence the re is for the existence of at om. State and discuss what experimental evidence for believing that matter is electrical in nature Describe Bohr- Rutherford models of the at om. Explain nucleon number and the ir relationship.
Energy Quantization And Duality Of Matter of each element, and these particles combine in simple numerical relationships (e.g., 1:1, 1:2, 2:3, etc.).
Example (Carbon and Oxygen): Carbon and oxygen can form carbon monoxide (CO) and carbon dioxide (CO2). In CO, 12g of carbon combines with 16g of oxygen. In CO2, 12g of carbon combines with 32g of oxygen. For a fixed mass of carbon (12g), the masses of oxygen combining are 16g (for CO) and 32g (for CO2).
The ratio of these oxygen masses is 32g : 16g = 2:1, which is a simple whole-number ratio. This ratio reflects that CO2 has twice as many oxygen atoms as CO for the same number of carbon atoms.
2. Question: Describe Rutherford's gold foil experiment, stating its key observations and the conclusions drawn about the atomic structure. What was a major limitation of this model?
Solution: Experiment: Alpha particles (positively charged helium nuclei) emitted from a radioactive source were directed as a narrow beam at a very thin gold foil. A detector screen (coated with zinc sulfide) was placed around the foil to observe the scattering of the alpha particles.
Observations:
1. Most alpha particles passed straight through the gold foil with little or no deflection.
2. A small percentage of alpha particles were deflected at large angles.
3. A very tiny fraction (about 1 in 8000) of alpha particles were deflected back towards the source (i.e., rebounded at angles greater than 90°).
Conclusions:
1. Atom is mostly empty space: The fact that most alpha particles passed straight through indicated that atoms are not solid spheres but largely empty space.
2. Dense, positively charged nucleus: The large-angle deflections and rebounds suggested that the positive charge and most of the atom's mass are concentrated in a very small, dense central region called the nucleus, which repelled the positively charged alpha particles.
3. Electrons orbit the nucleus: The electrons, being much lighter and negatively charged, were assumed to orbit this nucleus, occupying the vast empty space.
Major Limitation: According to classical electromagnetic theory, electrons orbiting the nucleus should continuously lose energy by emitting radiation and spiral into the nucleus, causing the atom to collapse. Rutherford's model could not explain the observed stability of atoms. It also could not explain the discrete line spectra of elements.
3. Question: An atom of a common Nigerian resource, Iron (Fe), has an atomic number of 26 and a mass number of 56. a) Calculate the number of protons, neutrons, and electrons in a neutral iron atom. b) If this iron atom loses two electrons to form an ion ($Fe^{2+}$), how many electrons will it have?
Solution: a) For a neutral Iron (Fe) atom with Z=26 and A=56: Number of protons (Z) =
2
6. Number of electrons (in a neutral atom) = Number of protons =
2
6. Number of neutrons (N) = A - Z = 56 - 26 =
3
0. Answer: Protons = 26, Neutrons = 30, Electrons = 26. b) If the iron atom loses two electrons to form an $Fe^{2+}$ ion: The number of protons remains unchanged =
2
6. The number of neutrons remains unchanged =
3
0. Initial number of electrons =
2
6. After losing two electrons, the number of electrons = 26 - 2 =
2
4. Answer: The $Fe^{2+}$ ion will have 24 electrons.
4. Question: Distinguish between isotopes and isobars, providing one example for each.
Solution: Isotopes: Isotopes are atoms of the same element (meaning they have the same proton number, Z) but have different numbers of neutrons (N), leading to different mass numbers (A). Chemically, they are very similar due to the same electron configuration, but their physical properties (like mass) differ.
Example: Uranium-235 ($^{235}_{92} U$) and Uranium-238 ($^{238}_{92} U$) are isotopes of Uranium. Both have 92 protons, but U-235 has 143 neutrons (235-92), while U-238 has 146 neutrons (238-92).
Isobars: Isobars are atoms of different elements (meaning they have different proton numbers, Z) but possess the same mass number (A). They have different chemical properties because they are different elements.
Example:* Potassium-40 ($^{40}_{19} K$) and Calcium-40 ($^{40}_{20} or air). For instance, the atomic composition of crude oil or minerals found in Nigeria influences their processing and economic value.
8. Differentiation, Remediation and Extension Differentiation: Visual Learners: Utilize detailed diagrams (Rutherford's setup, Bohr's energy levels), animated simulations if available, and provide clear visual aids on atomic notation. Encourage drawing.
Auditory Learners: Facilitate group discussions, encourage verbal explanations of concepts, and use a storytelling approach for historical context.
Kinesthetic Learners: Provide opportunities for hands-on activities, such as drawing and labeling atomic models, or using physical models if available (e.g., balls and sticks to represent nuclei and electrons).
Varied Questioning: Use a range of question types, from recall-based to application-based, ensuring all learning styles and cognitive levels are addressed.
Remediation (for struggling learners): Simplified Concepts: Break down complex concepts into smaller, more manageable parts. Focus on the core ideas of each atomic model.
Concept Reinforcement: Revisit the basic definitions of protons, neutrons, and electrons. Use simpler analogies (e.g., comparing the atom to a small village with a central market (nucleus) and people moving around (electrons)).
One-on-One Support: Provide individualized attention or pair struggling learners with high-achieving peers for guided practice and explanation.
Targeted Practice: Assign specific, simpler problems on nucleon number calculations.
Visual Aids and Summaries: Provide simplified summary notes or flowcharts of the historical development and key features of each atomic model.
Extension (for high-achieving learners): Research Project: Assign a short research project on the next evolution of atomic models (e.g., the quantum mechanical model, wave-particle duality of electrons, Schrödinger's equation, Heisenberg's uncertainty principle). Students could present their findings to the class.
Advanced Applications: Task them with exploring more complex applications of atomic structure, such as how spectroscopy is used in astrophysics or in chemical analysis (e.g., in forensic science in Nigeria).
Critical Analysis: Challenge them to critically evaluate the limitations of Bohr's model in explaining phenomena like the fine structure of hydrogen spectra or the spectra of multi-electron atoms. * Problem-Solving: Present more complex problems involving isotopic abundance calculations or nuclear reactions (if covered later in the curriculum). based on these developments.
Marking Scheme: Dalton (2 marks): Mention indivisible atoms, chemical laws.
Thomson/Electrical Nature (2 marks): Mention discovery of electron, matter is electrical.
Rutherford (4 marks): Describe gold foil experiment (observations & conclusions), planetary model, and its limitation (stability/spectra).
Bohr (5 marks): State key postulates (quantized orbits, energy transitions, angular momentum), how it addressed Rutherford's limitations (stability, hydrogen spectra), and its own limitations.
Modern Concept (2 marks): Emphasize nucleus with protons/neutrons, electrons in quantized energy levels, atom mostly empty space, and the idea that current models go beyond Bohr to quantum mechanics (wave nature of electrons, probability clouds – can be briefly mentioned for completeness). (Total: 15 marks)
2. Evaluation Guide Objective 2: Students to solve problems involving nucleon number, proton and number of atoms.
Question: The element Silicon (Si), used extensively in electronics and solar panels in Nigeria, has an atomic number of
1
4. Its most common isotope has a mass number of 28. a) For a neutral atom of this isotope ($^{28}_{14} Si$): i) How many protons does it have? ii) How many neutrons does it have? iii) How many electrons does it have? b) If this Silicon atom forms an ion with a charge of +4 ($Si^{4+}$), determine the number of electrons in this ion. c) Another isotope of Silicon has 16 neutrons. Write its atomic notation. d) An atom of Aluminium (Al) has a mass number of 28 and an atomic number of
1
3. Is this Aluminium atom an isotope, isobar, or isotone with the Silicon-28 atom from part (a)? Explain your answer.
Marking Scheme: a) i) 14 protons (1 mark) a) ii) 14 neutrons (28-14) (1 mark) a) iii) 14 electrons (1 mark) b) Electrons = 14 - 4 = 10 (2 marks) c) Atomic number is
1
4. If N=16, then A = 14+16 =
3
0. Notation: $^{30}_{14} Si$ (2 marks) d) Isobar (1 mark).
Explanation: They have different atomic numbers (Si=14, Al=13) but the same mass number (A=28 for both) (2 marks). (Total: 10 marks)
7. Real-life Applications / Integration
1. Material Science and Industry (e.g., Nigerian Manufacturing): Understanding atomic models is crucial for material scientists and engineers in Nigeria. For example, the arrangement of electrons determines the chemical bonding and properties of materials. This knowledge allows for the development of new alloys for construction (e.g., steel reinforcement for buildings and bridges), semiconductors for electronic gadgets assembled in Nigeria, or polymers for plastics packaging and components. Knowing the atomic structure helps in designing materials with specific strength, conductivity, or insulating properties suitable for various industrial applications.
2. Energy Sector and Health (e.g., Nuclear Technology in Nigeria): The concepts of nucleon number, isotopes, and atomic stability are foundational to nuclear physics and its applications. While Nigeria does not yet have operational nuclear power plants, there is ongoing research and interest. Understanding isotopes like Uranium-235 (used in nuclear reactors) or Cobalt-60 (used in radiotherapy) helps in appreciating the potential for nuclear energy generation and medical applications like cancer treatment available in some specialized Nigerian hospitals.
Furthermore, radioactive isotopes are used in agriculture for crop improvement and pest control, and in oil and gas exploration (e.g., well logging) - critical sectors in the Nigerian economy.
3. Chemistry, Biology, and Environmental Science: Atomic models underpin the entire fields of chemistry and biochemistry. Understanding how atoms combine to form molecules is essential for drug discovery (e.g., for malaria or sickle cell anemia research), food science (e.g., nutrient analysis), and environmental monitoring (e.g., detecting pollutants at the atomic level in Nigerian rivers or air). For instance, the atomic composition of crude oil or minerals found in Nigeria influences their processing and economic value.
8. Differentiation, Remediation and Extension Differentiation: Visual Learners: Utilize detailed diagrams (Rutherford's setup, Bohr's energy levels), animated simulations if available, and provide clear visual aids on atomic notation. Encourage drawing.
Auditory Learners: Facilitate group discussions, encourage verbal explanations of concepts, and use a storytelling approach for historical context. * Kinesthetic Learners: Provide opportunities for hands-on activities, such as drawing and labeling atomic models, or using physical models if available
Example 1:
An atom of Sodium (Na) has an atomic number of 11 and a mass number of
2
3. Determine the number of protons, neutrons, and electrons in a neutral sodium atom.
Solution:
Proton number (Z) =
1
1. Therefore, number of protons =
1
1.
In a neutral atom, number of electrons = number of protons =
1
1.
Mass number (A) =
2
3.
Number of neutrons (N) = A - Z = 23 - 11 =
1
2. Answer:* Protons = 11, Electrons = 11, Neutrons =
1
2. Example 2:
Consider an ion of Oxygen ($^{16}_8 O^{2-}$). How many protons, neutrons, and electrons does it contain?
Solution:
Proton number (Z) =
8. Therefore, number of protons =
8.
Mass number (A) =
1
6.
Number of neutrons (N) = A - Z = 16 - 8 =
8.
The charge is 2-, meaning it has gained 2 electrons.
In a neutral oxygen atom, electrons =
8.
For O²⁻ ion, electrons = 8 + 2 =
1
0. Answer:* Protons = 8, Neutrons = 8, Electrons =
1
0. Example 3:
An element X has two common isotopes. Isotope 1 has 17 protons and 18 neutrons. Isotope 2 has 17 protons and 20 neutrons.
a) State the atomic number (Z) and mass number (A) for each isotope.
b) Identify the element X.
c) Discuss one application of an isotope in Nigeria.
Solution:
a)
Isotope 1: Z = 17, N =
1
8. So, A = Z + N = 17 + 18 =
3
5. Notation: $^{35}_{17} X$.
Isotope 2: Z = 17, N =
2
0. So, A = Z + N = 17 + 20 =
3
7. Notation: $^{37}_{17} X$.
b) The element with atomic number 17 is Chlorine (Cl). So, X is Chlorine.
c)
Application:* In medicine, radioactive isotopes like Iodine-131 are used for diagnosing and treating thyroid conditions, or Cobalt-60 for cancer therapy in some Nigerian hospitals. In agriculture, isotopes are used to trace nutrient uptake in plants or to sterilize pests. In industry, isotopes are used in gauging equipment (e.g., measuring the thickness of sheet metal or paper produced in Nigerian factories) or for non-destructive testing of welds and materials.
Teaching and Learning Activities
Teacher Activities:
Introduction (10 minutes):
Initiate a brief review of elementary concepts: matter, elements, compounds, and the smallest particle of matter (atom).
Pose questions to students: "How do we know atoms exist if we can't see them directly?" "What are atoms made of?"
Introduce the topic "Models of the Atom" and its learning objectives.
Discussing Chemical Evidence (15 minutes):
Explain Dalton's Atomic Theory and its postulates.
Lead a discussion on the Law of Conservation of Mass, Law of Definite Proportions, and Law of Multiple Proportions, using simple, relatable examples (e.g., forming salt from sodium and chlorine; making water).
Emphasize how these laws necessitate the existence of discrete, indivisible units (atoms).
Explaining Electrical Nature of Matter (20 minutes):
Describe J.J. Thomson's cathode ray experiment and the discovery of the electron. Use a simple diagram of a CRT.
Briefly mention anode rays and the proton.
Explain Millikan's oil drop experiment conceptually, highlighting the quantization of charge.
Discuss the implications of radioactivity in revealing subatomic particles.
Presenting Rutherford's Model (15 minutes):
Describe Rutherford's gold foil experiment, including the experimental setup, observations, and conclusions. Use a clear diagram.
Present Rutherford's postulates (dense nucleus, orbiting electrons, empty space).
Lead a discussion on the limitations of Rutherford's model (stability, continuous spectrum).
Presenting Bohr's Model (20 minutes):
Introduce Bohr's attempt to resolve Rutherford's limitations.
Explain Bohr's four postulates with emphasis on quantized energy levels and electron transitions.
Use energy level diagrams to illustrate electron excitation and de-excitation, showing emission/absorption of photons.
Highlight the successes (hydrogen spectrum, stability) and remaining limitations of Bohr's model.
Explaining Nucleon Number (15 minutes):
Define proton number (Z), neutron number (N), and nucleon number (A).
Explain the relationship A = Z + N.
Introduce standard atomic notation ($^A_Z X$).
Define and differentiate isotopes, isobars, and isotones with clear examples relevant to common elements.
Work through guided examples on calculating protons, neutrons, and electrons in neutral atoms and ions.
Guided Practice and Consolidation (20 minutes):
Facilitate students in solving structured problems related to nucleon number on the board or in groups.
Address misconceptions and provide feedback.
Conclusion (5 minutes):
Summarize the key takeaways from the historical progression of atomic models.
Assign independent practice questions.