The Evolution of Atomic Theory: From Ancient Ideas to Modern Discoveries

1. Chapter 4 review

2. Whose work is credited with being the beginning of the modern atomic theory? • Distinguish between the ancient ideas and modern ideas of an atom. • John Dalton • Ancient: atoms were indivisible, solid piece of matter • Modern: subatomic particles, can be divided

3. State the main points of Dalton’s atomic theory. • All matter is composed of atoms • Atoms of an element are identical • Atoms cannot be subdivided, created or destroyed • Atoms combine in simple whole-number ratios to form chemical compounds • In chemical reactions, atoms are combined, separated, or rearranged

4. Define matter and give 2 examples • Term for substance • Ex. Pen, paper, book, elements, etc • What is found in the nucleus of an atom? • Neutrons, protons • What is the charge of the nucleus • Positive (because of the protons)

5. Who developed the “plum pudding” model of the atom? • J.J. Thompson • Describe the positive charge distributed in the plum pudding model. • Evenly distributed throughout atom • “positively charged pudding”

6. Arrange the subatomic particles in order of increasing mass: neutron, electron, proton • Electron is smallest, neutron is largest • Explain why atoms are electrically neutral. • The number of protons = number of electrons • Which particles account for most of an atom’s mass? • Protons and neutrons – they are the largest subatomic particles

7. Which subatomic particle was discovered by researchers working with cathode-ray tubes? • Electrons • What happened to the cathode-ray stream when a negatively charged plate was beside the tube? • The stream moved away from the negatively charged plate

8. What caused the deflection of the alpha particles in Rutherford's gold foil experiment? • The nucleus, which is very dense • What kind of particles were used in the gold foil experiment? • Alpha particles

9. How is an atom’s atomic number related to its number of protons? To electrons? • How do isotopes differ? How are they similar? • How is the mass number related to the number of protons and neutrons an atom has? • Atomic number = # protons, # electrons • Different # of neutrons, same # protons • Mass number = # protons + neutrons

10. Define atomic mass unit. • What are the benefits of developing the atomic mass unit as a standard unit of mass? • Does the existence of isotopes contradict part of Dalton’s original atomic theory? Explain. • Standard unit for subatomic particle • Easier to work with compared to grams • Yes – Dalton’s theory stated that atoms were identical in mass

11. Magnesium has 3 isotopes. Calculate its average atomic mass. 18.95 + 2.499 + 2.859 = 24.31 amu

12. Silver has 2 isotopes. Ag-107 has a percent abundance of 52% and Ag-109 has a percent abundance of 48%. What is the average atomic mass? 55.64 + 52.32 = 107.96 amu