Atomic Structure – The Nucleus!

The Important Idea

• Matter is discontinuous – it cannot be subdivided infinitely.

• Credited to Democritus of Abdera (ca. 400 BC) – matter is composed of tiny pieces (atomos – “indivisible”) that move in a void space.– Atoms bounce off each other when they collide– Have tiny velcro-like hooks on their surfaces that

allow them to stick together sometimes– Change is the result of atoms recombining, not new

atoms being formed

The Modern Version• John Dalton

(1766-1844)• Dalton describes behavior

and nature of atoms in his atomic theory:

1. Atoms are fundamental, indivisible, and invisible.

2. Atoms of the same element are identical.

3. Atoms of different elements are unique.4. Atoms are not created or destroyed but

can recombine. When they combine they do so in whole number ratios.

The Atom is Cracked• Pure gases + electricity looks cool:

Evacuated and filled with a low pressure gas.

Slits.

“cathode”

“anode”

The Atom is Cracked, part I• Pure gases + electricity looks cool:

“cathode”

“anode”

•The green beam was known as a “cathode” ray•Cathode rays can be bent by magnetic fields•The cylinder at the lower right contained an “electrometer”, a primitive device that showed the presence of negative charge.• In this first experiment, Thomson showed that cathode rays were negatively charged

The Atom is Cracked, part II

• Electricity and magnetism are related.• Thomson tries to use electric fields to bend the

beam (he was the first to do this successfully).• The point is to show that the beam itself carries

the charge, and not that the beam and the charge are two separate phenomena.

cathodeElectric Plates

Thomson’s Discovery (1898)

• Experiment 1 says cathode rays are negatively charged

• Experiment 2 says the rays are made of matter that is negatively charged

• Experiment 3 allows him to measure the mass to charge ratio (m/z) of the particles that make up the beam. They are 1000x smaller than the m/z for hydrogen.

• Conclusion:– There is a subatomic particle which has a low mass

and is negatively charged. It is common to all elements: the “electron”

The Plum Pudding Model (1904)

JJ Thomson (1899)

Everyone eats So everyone knows it looks like

Slices of Plum Pudding, showing the fruits randomly distributed

Electrons are like fruits, positive cloud is “pudding”

New Zealand – Not Just Sheep and “Lord of the Rings”

• Ernest Rutherford, former student of Thomson

• Noticed that a beam of alpha particles became “fuzzy” when directed at a piece of Mica

αFuzzy.Fuzzy.

The Gold Foil Experiment

• Rutherford gives (undergraduate) student Ernest Marsden a little project: see if alpha particles could be reflected by metals.

Why alphas were deflected.

+

Gold Foil Results/Implications

• Atom is mostly empty space: most alphas were neither scattered nor deflected.

• Atom has a small, massive, positively charged object likely at its center (why not the center?). This is inferred from the number, charge, and angle of the particles deflected.

• Why gold? Other metals exhibited the same effect, but gold worked nicely.

• Why alphas and not betas? Betas scattered too much (electron cloud); beta scattering was well-known but alpha scattering was not.

Models

1. Democritus and the Velcro Ball

2. Thomson’s Plum Pudding

3. The Rutherford Atom