Atom ModelsFrom Democritus to Bohr’s

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•Learning Goals

Explain how different atomic models evolved as a result of experimental evidence.

Describe the characteristics of neutrons, protons and electrons (charge, location and relative mass)

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•Questions to ask...

•How did we find out that gold was gold and silver was something different?•How did we find out what part of a particle makes a substance unique?•How can models of the atom help explain matter?

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• Democritus (400 BCE)

- Greek philosopher- All matter can be divided into smaller and smaller pieces- He called the last piece that “cannot be cut” an ATOM

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•Democritus’ Atomic Theory

•atoms are different sizes•atoms are in constant motion•atoms are separated by empty spaces

•Democritus' Atomic Theory

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•Aristotle (~340 BCE)

• Rejected the idea of the atom

• All matter is made up of 4 substances• Earth, Water, Fire and Air

•John Dalton (1807)

Billiard ball model- each atom of one material is the same- atoms cannot be divided

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•Dalton’s Atomic Model

•All mater is made up of tiny, indivisible particles called atoms•all atoms of an element are identical•atoms of different elements are different •atoms are rearranged to form new substances in chemical reactions, but they are never created or destroyed

• J.J. Thomson (1897)Discovered electrons- negatively charged particles

“Plum Pudding” model- negative electrons embedded in a positive atom - electrons are evenly distributed

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• Ernest Rutherford (1909)

•Discovered the nucleus• is very small• contains most the mass• surrounded by a cloud of negatively charged electrons•most of the atom is empty space

• Rutherford’s Gold Foil Experiment

See page 230

• Niels Bohr (1913)Bohr refined Rutherford's model: - Electrons orbit the nucleus (like planets)- Electrons cannot be between orbits / they can jump between orbits

- Each orbit can hold a maximum number of electrons

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• James Chadwick (1932)

• Student of E. Rutherford• The nucleus contains 2 particles

• Positive Protons (discovered by Rutherford in 1920)•Neutral NEUTRONS

•A neutral atom contains the same number of protons and electrons

•Type of Subatomic Particles

Proton Neutron Electron

Charge + O -

Location in nucleus in nucleusorbiting nucleus

Relative Mass 1 1 1/2000

The Bohr-Rutherford Model is what we will use (for the first 20 elements).

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• Have we met our Learning Goals?

Can YOU explain how different atomic models evolved as a result of experimental evidence?

Can YOU describe the characteristics of neutrons, protons and electrons (charge, location and relative mass)?

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• Group Task

1. Create a TIMELINE of the history of the atom.

OR

2. Create a mini poster of the 1 you think is MOST important