Atom Model.pptx

Revaluation of Atom

1. Development of the Atomic Theory

2. Atomic Theory Timeline Here is a timeline of some of the major ideas. Dalton Thomson Ruthe r fo r d Chadwick Bohr Modern

3. Democritus (440 BCE) Greek philosopher In 440 BC, Democritus, a Greek Philosopher theorized that if you cut an object in half, then cut that one in half, you would eventually end up with a particle that could not be cut. He called this an atom. From the Greek word Atomo s, meaning “not able to be divided.”

4. Atoms are small hard particles made of a single material that can’t be divided. Developed his ideas by thinking. Problem : no evidence and not as popular

5. All substances are made of atoms Atoms cannot be created, divided or destroyed Atoms of same element are exactly alike Atoms join together to make new substances John Dalton (1803) British chemist and schoolteacher (John Dalton created the very first Atomic theory by his observations from experiment) (He viewed atoms as tiny, smooth solid balls that could not be broken into smaller pieces )

6. Thomson discovered that atoms were not simple, solid spheres Atoms contained subatomic particles Very small, negatively charged particle. Called them electrons. J.J. Thomson (1897) British scientist He was the first scientist to show that the atom was made of even smaller things .

7. Cathode-Ray tube Experiment

8. Also knew that atoms were electrically neutral Must contain enough positive charge to balance negative charge of electrons Thompson proposed a model where electrons were stuck into a positively charged sphere Like chocolate chips in cookie dough

9. D r a wba c k s He explained that hydrogen can give rise only to a single spectral lines. He couldn't explain the fine spectra

10. Millikan’s Oil Drop Experiments Robert Milikan (1909) Oil Drop Experiment Measured the electrical charge on the electron Mass can be calculated (Thomson determined the e/m ratio) electron has a mass of 9.11 x 10 -28 g So, at this point we know: Electrons are negatively charged Atoms are neutral The mass of an electron is very small Where is the mass of the atom?

11. Ernest Rutherford (1911) Scientist, former student of Thomson Gold Foil Experiment

12. Alpha particles (+2 charge) beamed at thin gold foil When alpha particles (+2) hit screen, the screen lights up Most particles went through Some particles bounced to the sides and at 180º In process: discovered the proton Proton is positively charged

13. Atom is mostly empty space Tiny part is highly dense matter that is positively charged Named this dense part the nucleus Based on his experimental results, Rutherford made the following conclusions about the structure of the atom: Like the pit in a peach

14. 17 Rutherford’s Contributions Discovered the nucleus. Proposed the NUCLEAR MODEL of the atom also called the PLANETARY ATOM Proposed the existence of the neutron Determined the size of the nucleus (1x10 -15 m) and the atom (1 x 10 -11 m for hydrogen atom, larger for other atoms) But, Did not know where to place the electrons

15. (Studied the way atoms react to light) Niels Bohr (1913) Danish scientist who worked with Rutherford Modified Rutherford’s mode l Proposed that each electron has a certain amount of energy Helped electron move around nucleus Electrons move around nucleus in region called energy levels Energy levels surround nucleus in rings, like layers of onion

16. It Has been called planetary model Energy levels occupied by electrons are like orbits of planets at different distances from the sun (nucleus)

17. Electrons move around nucleus in certain paths These paths are energy levels Electrons can jump from one level to another.

18. D r a wba c k s Spectrograph of high resolving showed that lines are not single. Each spectral lines actually consisted of several very close line packed together. This is called fine structure of spectral lines. Bohr theory could not explain this fine structure.

19. Sommerfeld’s atom model Sommerfeld introduced two main modification in Bohr’s model: (1)The path of an electron around the nucleus, in general ,is an ellipse with the nucleus at one of the foci. (2)The velocity of the electron moving in an elliptical orbit varies considerably at different parts of the orbit.

20. N electron r Elliptical orbit for hydrogen atom Drawbacks Sommer f e l d ’ s t heor y w as able to give an explanation of the fine structure of the spectral l i n e of hy d r o g en a t om . B u t h e c o ul d no t p r ed i c t the correct number of spectral lines.

21. Particle in nucleus with protons Mass about the same as a proton No charge Particle is called Neutron James Chadwick(1932) physicist, former student of Rutherfod

22. Electron Cloud Model Model accepted today Electrons dart around in an energy level Rapid, random motion creates a “cloud” of negative charge around nucleus Electron cloud gives atom its size and shape

23. Modern Atomic Model In 1932, Chadwick discovered another particle in the nucleus of an atom. This new particle is called a neutron. Neutrons have no electrical charge. According to this theory, “At the center of the atom is a tiny, massive nucleus containing protons and neutrons. Surrounding the nucleus is a cloudlike region of moving electrons.”