Structure of the Atom

This chapter explains that atoms are not indivisible, as Dalton thought, but are made of smaller sub-atomic particles. You will learn how J.J. Thomson identified the negatively charged electron, how E. Goldstein discovered canal rays that led to the positively charged proton, and how James Chadwick discovered the neutral neutron in 1932. You will study the models of the atom: Thomson's model, which pictured the atom like a watermelon with electrons embedded in positive charge; Rutherford's alpha particle scattering experiment with thin gold foil, which showed that an atom has a tiny, dense, positively charged nucleus; and Bohr's model, in which electrons revolve in discrete orbits or shells without radiating energy. You will learn the Bohr-Bury rules for distributing electrons in shells K, L, M and N, with a maximum of 2n^2 electrons in a shell, and how valence electrons decide valency. You will define atomic number and mass number, and study isotopes, such as protium, deuterium and tritium, and isobars, such as calcium-40 and argon-40, along with uses of isotopes in medicine and nuclear reactors.

Detailed Worksheet: Structure of the Atom Section A - Definitions (10 marks) 1. Name the three sub-atomic particles and give the charge and relative mass of each. (2 marks) 2. What are canal rays? Who discovered them? (2 marks) 3. Define atomic number and mass number. (2 marks) 4. What are isotopes? Give one example. (2 marks) 5. What are isobars? Give one example. (2 marks) Section B - Calculations and Applications (15 marks) 6. Find the number of protons, electrons and neutrons in the atoms of: oxygen (atomic number 8, mass number 16), sodium (11, 23), chlorine (17, 35) and potassium (19, 39). (3 marks) 7. Write the electronic configuration and valency of the following elements: carbon (6), sodium (11), magnesium (12), sulphur (16), chlorine (17), argon (18). (3 marks) 8. Chlorine occurs in nature as two isotopes, chlorine-35 and chlorine-37, in the ratio 3 : 1. Calculate the average atomic mass of chlorine. (3 marks) 9. If bromine atom is available in the form of two isotopes, bromine-79 (49.7%) and bromine-81 (50.3%), calculate the average atomic mass of bromine to two decimal places. (3 marks) 10. The average atomic mass of a sample of an element X is 16.2 u. What are the percentages of isotopes X-16 and X-18 in the sample? (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of Rutherford's alpha particle scattering experiment showing the paths of alpha particles through the gold foil. (4 marks) 12. Draw Bohr's model of the atoms of sodium and chlorine showing the distribution of electrons in shells. (3 marks) 13. Draw diagrams showing the composition of the nuclei of the three isotopes of hydrogen. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. Describe Rutherford's alpha particle scattering experiment. State the observations and the conclusions drawn from them about the structure of the atom. What were the drawbacks of his model? (5 marks) 15. State the postulates of Bohr's model of the atom. How did it overcome the drawback of Rutherford's model? (5 marks) 16. Give three uses of isotopes in medicine, industry and energy production. Why do isotopes of an element have the same chemical properties but different physical properties? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show all calculation steps and write symbols with mass number and atomic number where needed.