The Evolution of the Atomic Model. Democritus to Rutherford

Transcription

1 The Evolution of the Atomic Model Democritus to Rutherford

2 The Atomic Theory of Matter Philosophers from the earliest times, like Democritus, described the material world as made up of tiny, indivisible particles called atoms Plato and Aristotle formulated the notion that there can be no ultimately atomic view of matter This dominated Western ideas for many centuries The notion of an atomic theory of matter reemerged during the 17 th century as a result of the work of John Dalton

3 John Dalton and the Atomic Theory of Matter Dalton s atomic theory is comprised of 4 postulates which were based on chemical observations made in the lab: Each element is composed of extremely small particles called atoms All atoms of a given element are identical, but the atoms of one element are different from the atoms of all other elements Atoms of one element cannot be changed into atoms of a different element by chemical reactions; atoms are neither created nor destroyed in chemical reactions Compounds are formed when atoms of more than one element combine; a given compound always has the same relative number and kinds of atoms

4 The Atomic Theory of Matter Dalton s theory explains several laws of chemical combination that were known during his time: Law of Constant Composition In a given compound, the relative numbs and kinds of atoms are constant Based on Law of Conservation of Mass In chemical reactions, the total mass of materials present before and after is the same Dalton used his theory to deduce the Law of Multiple Proportions which states: If two elements A and B combine to form more than one compound, the mass of B that can combine with a given mass of A are in a ratio of small whole numbers

5 The Discovery of Atomic Structure The ancient Greeks were the first to postulate that matter was indivisible Even Dalton did not have any direct evidence for the existence of atoms! Later, scientists realized that the atom consisted of subatomic, charged particles What landmark discoveries led to this conclusion?

6 Cathode rays and electrons A cathode ray tube (CRT) is a hollow vessel with an electrode at either end A high voltage is applied across the electrodes The voltage causes negative particles to move from the negative electrode to the positive electrode The path of the electrons can be altered by the presence of a magnetic field J.J. Thomson described cathode rays as streams of negatively charged particles, called corpsicles which later become known as electrons! He also determined the charge to mass ratio of an electron to be C/g

7 Cathode Ray Tube

8 Charge and Mass of the Electron Once the charge-to-mass ratio of the electron and charge of the electron was known, the mass of the electron could be deduced! In 1909, Robert Millikan succeeded in doing just that with his oil-drop experiment!

9 Charge of Mass of the Electron Consider the following experiment: Oil drops are sprayed above a positively charged plate containing a small hole As the oil drops fall through the hole, they are given a negative charge Gravity forces the drops downward The applied electric field forces the drops upward When a drop is perfectly balanced, the weight of the drop is equal to the electrostatic force of attraction between the drop and the positive plate

10 Charge and Mass of Electrons Using this experiment, Millikan determined the charge on the electron to be C Knowing the charge to mass ratio, C/g, Millikan calculated the mass of the electron as g With more accurate numbers, we get the mass of the electron to be g!

11 The Discovery of radioactivity and its Effect on Atomic Structure Consider the following experiment: A radioactive substance is placed in a shield containing a small hole so that a beam of radiation is emitted from the hole The radiation is passed between two electrically charged plates and detected Three spots are noted on the detector: A spot in the direction of the positive plate, A spot which is not affected by the electric field A spot in the direction of the negative plate

12 The Discovery of radioactivity and its Effect on Atomic Structure

13 The Discovery of radioactivity and its Effect on Atomic Structure What did these observations mean? A high deflection towards the positive plate corresponds to radiation which is negatively charged and of low mass This is called β-radiation which consists of electrons No deflection corresponds to neutral radiation This is called γ-radiation which consists of energy Small deflection towards the negatively charged plate corresponds to high mass, positively charged radiation This is called α-radiation which consists of positivelycharged helium nuclei

14 The Discovery of radioactivity and its Effect on Atomic Structure From the separation of radiation, we conclude that the atom consists of neutral, positively, and negatively charged entities Thomson assumed all these charged species were found in a sphere similar to the image at right Like a chocolate chip cookie or plum pudding

15 Rutherford and the Nuclear Atom: The Gold-Foil Experiment Ernest Rutherford carried out the following experiment: A source of a-particles was placed at the mouth of a circular detector The a -particles were shot through a piece of gold foil Most of the a-particles went straight through the foil without deflection Some a-particles were deflected at high angles

16 Rutherford and the Nuclear Atom

17 Rutherford and the Nuclear Atom If the Thomson model of the atom was correct, then Rutherford s result was impossible! So, what do these observations mean? In order to get the majority of a-particles through a piece of foil to be undeflected, the majority of the atom must consist of a low mass, diffuse negative charge - the electron! To account for the small number of high deflections of the a-particles, the center or nucleus of the atom must consist of a dense positive charge!

18 The Nuclear Atom Rutherford modified Thomson s model as follows: Assume the atom is spherical but the positive charge must be located at the center, with a diffuse negative charge surrounding it