The History of Atomic Theory - Mrs. Rhee Science - Homemrhee.weebly.com/uploads/9/8/3/2/9832831/atomic_the… · · 2014-03-03The History of Atomic Theory ... Thomson conducted

The History ofAtomic Theory

(Selected ideas from Ch. 3)

Thursday, February 27, 14

2500 years ago, Greek philosophers

believed that all matter is made of...


-Earth -Air -Water -Fire


• Greek philosopher

• 460-370 B.C

• First to propose that matter was not infinitely divisible

Democritus


Dalton's Atomic Theory(1803)


•English chemist & physicist

•1766-1844

• Considered to be the father of our modern atomic theory

John Dalton


• Proposed that matter could be explained in terms of “atoms”

from the Greek word “atomos,” meaning indivisible.


1. Matter is composed of extremely small particles called atoms.

Dalton's Atomic Theory (page 64)


2. Atoms of a given element are identical in size, mass, and other properties.


3. Atoms cannot be subdivided, created, or destroyed.


4. Atoms combine in simple whole-number ratios to form chemical compounds.


5. In chemical reactions, atoms are combined, separated, or rearranged.


1. The law of conservation of mass:Matter can neither be created nor destroyed in ordinary chemical reactions.

Dalton’s theory explain three laws of nature that were known to be true:


http://my.hrw.com/hssc_2012/hmd_na_chem/nsmedia/visualconcepts/75038.htm




2. The 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 the source of the compound.




3. The law of multiple proportions:If two or more different compounds are composed of the same elements, the elements always combine in a ratio of small whole numbers.




Although Dalton’s theory was very successful,

the question remained:

Are atoms really indivisible?



In 1898 a scientist named J.J. Thomson conducted

experiments using a Crooke’s tube

(see figure 2.1 on p68)



• all elements give off the same negatively charged particles

• He called these particles “cathode ray particles”

Thomson concluded that...


Today we call these particles...

e l e c t r o n s !



Thomson devised the short lived "plum-pudding" model of the atom...


But this model was shown to completely wrong by one of the most important experiments of all time...


The Rutherford Experiment (1911)

(see figure 3.2 on p70)



(See Fig 2.4, p 71)


Disproved the "plum-pudding" modelShowed that electrons move around a small, positively charged nucleusDetermined atoms are mostly empty space

From the results of thisexperiment, Rutherford concluded what

seemed impossible...

Atoms... matter... E V E R Y T H I N Gis composed almost entirely of...

Empty Space!99.9999999999999% nothing!!!


Rutherford believed that electrons were held in orbit by their motion

around the nucleus.

However... charged particlestraveling in a curved path

(i.e., accelerating)MUST lose energy!

The Problem



So what keeps electrons from crashing into the

nucleus???

To be continued...


Section 4.1A New Atomic Model


The Electromagnetic Spectrum(p 92)


All electromagnetic radiation results from the movement of...

Where does electromagnetic radiation come from ?

e l e c t r o n s !



The properties of waves...


Fast moving electrons produce electromagnetic radiation with: - High energy - High frequency - Short wavelengthsSlow moving electrons produce electromagnetic radiation with: - Low energy - Low frequency - Long wavelengths


Emission Line Spectra



Color-in a continuous spectruminto your notebook.

<---Blue Red--->

Lab: Atomic Emission Spectra


The Bohr Model of the Hydrogen Atom(pages 96-97)






1. Electrons orbit the nucleus only within allowed energy levels.

2. Each energy level is at a specific distance from the nucleus.

3. Within an energy level, electrons do not lose energy.

4. When electrons are in the lowest energy levels available, the atom is in its “ground state.”

In the Bohr Model:


In the Bohr Model:

5. When electrons absorb energy and move to a higher energy level, the atom is said to be in an “excited state.”

6. When an electron returns to a lower energy level it will give off electromagnetic radiation with energy exactly equal to the difference found between those energy levels.



1. How electrons could orbit the nucleus without losing energy.

2. The emission spectra of hydrogen.

The Bohr model explained:

1. The emission spectra of elements other than hydrogen.

2. The chemical properties of the elements.

But it could NOT explain:



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The History of Atomic Theory - Mrs. Rhee Science - Homemrhee.weebly.com/uploads/9/8/3/2/9832831/atomic_the… · · 2014-03-03The History of Atomic Theory ... Thomson conducted