The History of the Atom

1. The History of the Atom

2. From Philosophical Ideal to Scientific Theory • The particle theory of matter was supported as early as 400 BCE by certain Greek thinkers, such as Democritus. • He called nature’s basic particle an atom • Based on the Greek word meaning “indivisible”

3. From Philosophical Ideal to Scientific Theory • Democritus’s Theory: • Matter could not be divided into smaller and smaller pieces forever, eventually the smallest possible piece would be obtained. • He believed, atoms were small, hard particles that were all made of the same material but were different shapes and sizes.

4. From Philosophical Ideal to Scientific Theory Virtually all chemists in the late 1700s accepted the modern definition of an element as a substance that cannot be further broken down by ordinary means. Chemists also assumed that these elements combined to form compounds that have different physical and chemical properties than those of the elements that make them.

5. From Philosophical Ideal to Scientific Theory • In the 1790s, the study of matter was revolutionized by a new emphasis on quantitative analysis of chemical reactions which led to the discovery of several basic laws: • Law of conservation of mass • Law of definite proportions • Law of multiple proportions

6. Law of Conservation of Mass Mass is neither created nor destroyed during ordinary chemical reactions or physical changes

7. Law of Definite Proportions A chemical compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of the compound.

8. Law of Multiple Proportions If two or more different compounds are composed of the same two elements, then the ration of the masses of the second element combined with a certain mass of the first element is always a ration of small whole numbers.

9. John Dalton In 1808, an English school teacher named John Dalton proposed an explanation that encompassed all these laws. He proposed the Atomic Theory of matter based on his experimental observations

10. John Dalton • His theory can be summed up by the following: • Elements are composed of tiny particles called atoms, which are indestructible. • All atoms of a single element are identical, and all atoms of different elements are different from each other. • Atoms are neither created nor destroyed in chemical reactions. • Compounds are formed when more that one element combine; a given compound always has the same number and types of atoms.

11. John Dalton • Today we know that atoms are divisible into even smaller particles . • We also know that a given element can have atoms with different masses. • Is Dalton’s atomic theory discarded? • No, only modified!

12. J.J Thomson’s Atomic Theory • In 1897, Joseph John Thomson totally changed the view of an atom by discovering electrons! • Thomson’s atomic theory suggested that the atom was made of smaller subatomic particles. • Electrons and protons

13. J.J Thomson’s Atomic Theory • Thomson believed: • An atom consists of a sphere of positive charges with negatively charged electrons embedded in it. • The positive and negative charges in an atom are equal in magnitude (it has not over all negative charge).

14. J.J Thomson’s Atomic Theory Thomson discovered that atoms emit negative particles when zapped with electricity. In an experiment he found that a magnet changed the path of cathode ray.

15. J.J Thomson’s Atomic Theory Thomson proposed a model of the atom which is known as the plum pudding model.

16. Ernest Rutherford • In 1911, Ernest Rutherford presented a paper that accurately described the structure of the atom. • Based on an experiment he had performed, he realized that the atom must have almost all of its mass concentrated at its center, with a vast majority of the atom consisting of empty space.

17. Ernest Rutherford • In 1909, Rutherford conducted an experiment where he studied the deflection angles of “alpha particles” that they shot through a microscopically thing layer of gold. • Alpha particles are just helium atoms stripped to their 2 electrons. • While the nucleus contains virtually all of the mass of an atom, it only takes up one-billionth of the volume of the atom. • Much smaller particles (electrons) orbit the nucleus at a great distance.

18. Ernest Rutherford • Gold Foil Experiment • Shot thin gold foil with alpha particles (big, positive charged). • If the plum pudding model was accurate than the alpha particles should go right through. • However, some alpha particles were deflected or even bounced back! • This led Rutherford to believe: • Atoms have a dense, positively charged nucleus • Which balances out the electron’s (-) charge.

19. Ernest Rutherford

20. Neils Bohr • Neils Bohr refined Rutherford’s model in 1913 by proposing that electrons • Orbit the nucleus without losing energy • Could move only in fixed orbits of specific energies • Electrons with low energy would orbit closer to the nucleus while electrons with high energy orbit further from the nucleus.

21. James Schrödinger • In 1926, Schrödinger suggested, electrons were found in “probability clouds”, not orbits • The densest area of the cloud is where you have the greatest probability of finding the electron.

22. James Chadwick • In 1932, Chadwick identified the neutron. • Shot alpha particles at beryllium foil • The electrically neutral particles ejected • Provided evidence for the existence of neutrons in the nucleus.

23. History of The Atom Timeline • You must create a detailed timeline showing the history of the discovery of the atom. • You must make a point on your time line for each of the scientists we talked about so far. • Democritus, Dalton, Thomson, Rutherford, Bohr, Schrödinger, Chadwick • For each scientist you also must present the following information • Their name • The date of their discovery • What they discovered • How they discovered it • You also must show a diagram of the plum pudding model, planetary model, and the gold foil experiment.