Structure of Atom

Atoms were once thought indivisible. This chapter describes the discovery of the electron through cathode ray experiments, J.J. Thomson's measurement of its charge to mass ratio, and Millikan's oil drop experiment, which gave the charge as 1.602 x 10^-19 C. It also covers the discovery of the proton and of the neutron by Chadwick in 1932. You will compare Thomson's plum pudding model with Rutherford's nuclear model, based on the alpha particle scattering experiment, and learn about atomic number, mass number, isotopes and isobars. You will study the wave nature of electromagnetic radiation (c = frequency x wavelength), Planck's quantum theory (E = h x frequency), the photoelectric effect and the line spectrum of hydrogen. Bohr's model explains the hydrogen spectrum with quantised energy levels, En = -2.18 x 10^-18 (1/n^2) J. De Broglie's dual behaviour of matter and Heisenberg's uncertainty principle lead to the quantum mechanical model with its quantum numbers, the shapes of s, p and d orbitals, and the rules for filling electrons: the Aufbau principle, the Pauli exclusion principle and Hund's rule.

Detailed Worksheet: Structure of Atom Section A - Definitions (10 marks) 1. What are isotopes and isobars? Give one example of each. (2 marks) 2. State Heisenberg's uncertainty principle. (2 marks) 3. What is the photoelectric effect? Define threshold frequency. (2 marks) 4. State the Pauli exclusion principle and Hund's rule. (2 marks) 5. What are the four quantum numbers? What does each describe? (2 marks) Section B - Calculations and Applications (15 marks) 6. Yellow light emitted from a sodium lamp has a wavelength of 580 nm. Calculate its frequency and wavenumber (c = 3.0 x 10^8 m/s). Calculate the energy of one photon of this light (h = 6.626 x 10^-34 J s). (3 marks) 7. Using Bohr's model, calculate the energy of the photon emitted when an electron in a hydrogen atom falls from n = 5 to n = 2. Calculate the wavelength of this radiation and name the series to which it belongs (En = -2.18 x 10^-18 (1/n^2) J). (3 marks) 8. Light of wavelength 300 nm falls on a metal surface whose work function is 4.0 x 10^-19 J. Calculate the energy of the incident photon and the maximum kinetic energy of the emitted electrons. (3 marks) 9. Calculate the de Broglie wavelength of an electron (mass 9.1 x 10^-31 kg) moving at 2.19 x 10^6 m/s, and of a cricket ball of mass 0.1 kg moving at 10 m/s. Why are wave properties not noticed for the ball? (3 marks) 10. The uncertainty in the position of an electron is 1.0 x 10^-10 m. Calculate the minimum uncertainty in its velocity (h = 6.626 x 10^-34 J s, m = 9.11 x 10^-31 kg, take pi = 3.1416). (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of the Rutherford alpha particle scattering experiment and show the paths of alpha particles. (4 marks) 12. Draw the shapes of the s, p (px, py, pz) and d orbitals. (3 marks) 13. Draw an energy level diagram of the hydrogen atom showing the transitions of the Lyman, Balmer and Paschen series. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Explain Rutherford's alpha scattering experiment, its observations and conclusions. What were the drawbacks of his model? (5 marks) 15. Explain Bohr's model of the hydrogen atom and how it explains the line spectrum. What are its limitations? (5 marks) 16. Explain the Aufbau principle and the (n + l) rule. Write the electronic configurations of Cr (Z = 24) and Cu (Z = 29), and explain why they are exceptions. (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show all calculations with units and significant figures. Draw diagrams neatly with labels.