DEVELOPMENT OF THE ATOMIC MODEL From Democritus to Rutherford

1. DEVELOPMENT OF THE ATOMIC MODELFrom Democritus to Rutherford Dr, Anilkumar R. Koppalkar S.S.Margol College Shahabad

2. c.400 B.C.Ancient Greek Philosopy • Everything in the universe is made of one or more of the basic “elements:” • Earth, Fire, Water, Air

3. ARISTOTLE Matter is infinitely divisible; no matter how small a piece is, it can always be divided into smaller pieces DEMOCRITUS There exists a “smallest piece” of matter, which cannot be divided any further. These pieces are called “ατομοσ,” or “atoms” Democritus Aristotle What makes up the elements?

4. Whose Argument Prevails? • Aristotle’s viewpoint enjoyed the support of most of the world because he was more well-known and because Democritus had no evidence to back up his claim since these “atoms” would be too small to see.

5. 1803: John Dalton • Dalton was an English schoolteacher • Began teaching mathematics and chemistry at the age of 12 • Revived the idea of Democritus’ “smallest piece” of matter

6. Dalton’s Atomic Model • All matter is made of tiny particles called “atoms” • Atoms are indivisible and indestructible • Atoms of the same element are identical • Atoms of different elements differ in some fundamental way • Atoms combine in simple whole number ratios to form compounds

7. Why did people believe Dalton? • He had EVIDENCE! • Dalton found that compounds always contained the same mass ratio of one element to another • This is now known as the “Law of Definite Proportions” • Elements could combine in different ratios, but when they did, they would make different compounds • This is now known as the “Law of Multiple Proportions”

8. So what? • These results could only be explained by assuming that matter was made of atoms – tiny building blocks – and that these atoms only came in certain sizes. Dalton’s View of an atom

9. Hooray, Dalton! • Besides giving us his Atomic Theory, Dalton did much more in the newly-emerging field of chemistry: • He worked with mixtures of gases and determined how the pressure was related to their proportions • He made a list of all the known elements from lightest to heaviest

10. 1869: Dmitri Mendeleev • Russian chemist • Arranged elements in tabular form so that elements with similar properties were in the same column • When listed in order by mass, elements generally repeat properties in groups of 8 (Law of Octaves)

11. The First Periodic Table • Most tables at the time listed elements by mass • Mendeleev also arranged elements by mass, but left several “holes” in his table and occasionally reversed the order of elements to fit the properties of others in that column • The “holes” were later filled in with newly discovered elements that had the properties predicted by Mendeleev’s table. • The reason for the reversal of elements was explained later by Henry Moseley, who noted that the elements were in order by atomic number (number of protons) rather than by mass

12. 1897: J.J. Thomson • English physicist • Worked with Cathode-Ray Tubes (CRTs) • Credited with the discovery of the electron

13. CRTs • Mysterious particles emanated from the cathode end • These particles were deflected by magnetic and electric fields • They were very small and negatively charged These particles were called “electrons” and were assumed to be a part of all matter

14. Thomson’s Atomic Model Electrons Positively charged “goo” “Plum-Pudding Model”

15. 1909-11: Robert Millikan • Set out to discover the charge of a single electron • Famous experiment called the “oil-drop experiment” • Using his results and the charge-to-mass ratio from Thomson, the mass of an electron was found

16. The Oil Drop Experiment Scheme of the experimental cell: 1 – glass plate; 2 – oil droplet  1 l; 3 – glass holders; 4 – surfactant solution; 5 – syringe; 6 – cuvette. Procedure: (1) Oil drop is placed on the dry glass substrate; (2) Solution of surfactant + NaCl is poured in the cuvette; (3) The process of oil drop detachment is recorded by horizontal microscope and video-camera.

17. 1910: Ernest Rutherford • Expert in radiation • Famous “Gold-Foil Experiment” • Discovered the presence of the nucleus by firing alpha particles at a sheet of gold foil

18. Zinc sulfide screen Thin gold foil Lead block Radioactive substance path of invisible -particles Ernest Rutherford (movie: 10 min.) • Rutherford shot alpha () particles at gold foil. Most particles passed through. So, atoms are mostly empty. Some positive -particles deflected or bounced back! Thus, a “nucleus” is positive & holds most of an atom’s mass.

19. Rutherford’s Atomic Model Electrons Empty Space Nucleus Positively charged Made of “protons”

20. 1932: James Chadwick • A fellow researcher with Rutherford, Chadwick discovered years later that the nucleus was not made of only one particle – the proton – but of two particles. • This second particle was called the “neutron” because it had no electrical charge

21. HISTORY OF THE ATOM Niels Bohr 1913 studied under Rutherford at the Victoria University in Manchester. Bohr refined Rutherford's idea by adding that the electrons were in orbits. Rather like planets orbiting the sun. With each orbit only able to contain a set number of electrons.

22. Bohr’s Atom electrons in orbits nucleus

23. HELIUM ATOM Shell proton N + - + N - neutron electron What do these particles consist of?

24. ATOMIC STRUCTURE Particle Charge Mass proton + ve charge 1 neutron No charge 1 electron -ve charge nil

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