Atoms are not the smallest thing

1. Atoms are not the smallest thing Growing evidence for the divisibility of the indivisible

2. Atoms are not the smallest thing Growing evidence for the divisibility of the indivisible

3. Learning Objectives • Describe the three particles in the atom • Define atomic number and mass number • Describe isotopes • Write symbols for elements • Determine the numbers of particles in any atom from the element symbol

4. Electrostatics and electricity • Static electricity was observed by Thales (300 BC). Some “charged” objects repel and others attract • The voltaic cell (Volta, 18th century) generated electrical current from chemical reactions • Mechanical electrical generation was achieved in 1825 • The point: Atoms are neutral. If indivisible, where do electrical charges come from?

5. Faraday’s prescience • “Although we know nothing of what an atom is, we cannot resist forming some idea of a small particle; and though we are in equal ignorance of electricity, there is an immensity of facts which justify us in believing that the atoms of matter are associated with electrical powers to which they owe their most striking qualities, and amongst them their chemical affinity.”

6. Ray of hope • 1858 • Discovery of cathode rays by Julius Plucker • Application of a large voltage across an evacuated tube causes a current to flow. The current flow is accompanied by radiation from the excited gas molecules • How does the neutral and indivisible atom create a charge?

7. Cathode “rays” are electrons1897. J.J. Thomson demonstrates that cathode rays are negatively charged particles, which have a much smaller mass than an atom. The first sighting of the electron.

8. The Thomson :Plum Pudding” model "I regard the atom as containing a large number of smaller bodies which I will call corpuscles, these corpuscles are equal to each other.... In the normal atom, this assemblage of corpuscles forms a system which is electrically neutral. Though the individual corpuscles behave like negative ions, yet when they are assembled in a neutral atom the negative effect is balanced by something which causes the space through which the corpuscles are spread to act as if it had a charge of positive electricity equal in amount to the sum of the negative charges of the corpuscles…”

9. X-rays and atoms’ invisible rays • 1895 • Wilhelm Conrad Roentgen discovers X-rays while doing experiments with cathode rays

10. Radioactivity: invisible rays and unstable atoms • 1896 • Henri Becquerel discovered radioactivity, which suggested that some atoms were capable of decomposing to give smaller particles. • 1903 • Frederick Soddy and William Ramsey demonstrated that uranium decayed to give helium. Direct proof that atoms were divisible.

11. 1909 Ernest Rutherford’s gold foil experiment • The atom’s inner secrets exposed by its own offspring

12. Hierarchy of discovery • The path to knowledge occurs in steps, each depending on a previous advance

13. The nucleus • Tiny • Incredibly dense – contains all the mass of the atom • Positively charged • Contains protons (charged) and neutrons (neutral) – not discovered until much later

14. Summary of Atom Pictures • Dalton: Indivisible atom • Thomson: Electrons • Rutherford: Nucleus

15. Comparison of subatomic particles

16. Atoms are neutral: # electrons = # protons • Atomic number (Z) = number of protons in nucleus • Each element has unique atomic number • For neutral atom, atomic number equals number of electrons around nucleus.

17. Isotopes and the mass number • Mass number = protons plus neutrons • Isotopes have same atomic number, different mass number

18. Element notation: Atomic number and mass number Mass number: number of protons + neutrons Element symbol Atomic number: number of protons • Counting particles: • Number of electrons = number of protons = 6 • Number of neutrons = mass number – atomic number (13 – 6 = 7)

19. Ions: losing and gaining electrons • Atoms can lose or gain electrons • Atomic number remains the same • Loss: Positive ions have fewer electrons than protons • Gain: Negative ions have more electrons than protons

20. What of the electrons? • We now understand atom contains tiny positively charged massive nucleus surrounded by vast empty space containing electrons • When atoms combine the electrons must interact • We need to understand the arrangement of electrons in the atom