1

Atoms are divisible!

• By the 1850s, scientists began to realize that the atom was made up of subatomic particles

• Thought to be positive and negative

• How would we know this if we can’t see it or touch it?

2

Cathode Rays and Electrons

• Mid-1800’s scientists began to study electrical discharge through cathode-ray tubes. Ex: neon signs

• Cathode Ray Tubes (CRT)• Partially evacuated tube in which a current passes• Forms a beam of electrons which move from cathode to

anode• Electrons cause the screen to fluoresce.

J.J. Thompson concluded that the “ray” was made up of electrons.

Did the electrons have mass?

Thompson found that the electrondid have mass using differentCRTs. He found that the chargeof the electron was negative.Since an atom has no charge, thenthere must be a positive to balance it out.

So, JJ Thomson’s Model was…

Called the PlumPudding Model

Discovery of Radioactivity

• 1896 – Henri Becquerel

• Originates from the nucleus

•

Rutherford’s Au Foil

If he could have seen the atoms…

A More clear picture develops…

Millikan’s Oil Drop

Analysis of data

• JJ had calculated charge/mass ratio

• Now, Millikan can calculate the charge

Discovery of Neutron!!!J. Chadwick

Implications of discovery!!

• New model of atom

J. J. Thomson also …

• Discovered isotopes!!

• Isotopes are atoms of the same element with different numbers of neutrons.

• For example, Carbon – 12 & Carbon – 14

• Carbon – 14 has 2 more neutrons than Carbon – 12.