A Brief History leading up to Rutherford

Atomic Models

• Do things come from nothing?• Everything is made up of atoms.• from the Greek adjective atomos or atomon,

‘indivisible,’• All changes in the visible objects of the world of

appearance are brought about by relocations of these atoms:

• Could matter be divided into smaller and smaller pieces forever, or was there a limit to the number of times a piece of matter could be divided?

• Atoms are the smallest pieces.

Democritus Model (> 2400 years ago)

• Indivisible and indestructible

• An infinite number of atoms and kinds of atoms, which differ in shape, and size

• Separated by empty space

• Perfectly solid, with no internal gaps.

• Have always been, and always will be, in motion

• Repel one another when they collide or combine into clusters by means of tiny hooks and barbs on their surfaces

Democritus Model (> 2400 years ago)

• All atoms of a given element are identical in mass and properties.

• Compounds are formed by a combination of two or more different kinds of atoms.

• A chemical reaction is a rearrangement of atoms.

• Atoms can be neither created nor destroyed – in a chemical reaction, or any time!

Dalton Model (> 200 years ago)

• Atom is not indivisible (IS divisible)• Made of smaller pieces - electrons and protons

• “Plum Pudding Model”A sphere of positive charge with negatively charged electrons embedded in it.

• Same number of positive and negative charges, so it is electrically neutral.

Thomson Model (>110 years ago)

Rutherford Model (100 years ago)

• Tiny, dense, positively charged core – nucleus• Nearly all the mass is concentrated here

• Light, negative electrons circulate at some distance, much like planets revolving around the Sun.

• Vast majority of the atom consisting of empty space

• Also called the planetary model

SHOW PHET

Rutherford: How he did it!GOLD FOIL EXPERIMENT

Alpha (α) particles:• Positively charged• Helium atoms minus 2

electrons

1 – Aimed a beam of + particles at gold foil

2 – Measured where the particles hit after they went through the gold

Rutherford: What he saw

Alpha (α) particles:• Positively charged

• Nucleus is positively charged

• Nucleus contains virtually all the mass BUT…

• Nucleus takes up only one-billionth of the volume of the atom

• 0.000000001 of the total volume (1 x 10 -9)

• Electrons – much smaller particles – orbit the nucleus at a great distance, relatively speaking.

Rutherford: Conclusions

Rutherford: Summary

Reasoning: the solidness of the material corresponded to the shape of the atoms involved. Thus:

• Iron atoms are solid and strong with hooks that lock them into a solid

• Water atoms are smooth and slippery

• Salt atoms, because of their taste, are sharp and pointed

• Air atoms are light and whirling

Democritus Model (> 2400 years ago)

Show: Phet Simulation

• All matter is made of atoms. Atoms are indivisible and indestructible.

• All atoms of a given element are identical in mass and properties.

• Compounds are formed by a combination of two or more different kinds of atoms.

• A chemical reaction is a rearrangement of atoms.

• Atoms can be neither created nor destroyed – in a chemical reaction, or any time!

Dalton Model (> 200 years ago)