Particles

• These particles can be deflected by magnets or  charge

• These particles must be negatively charged

• Thompson called them electrons

(1856-1940)

• Cathode Ray Tube

Ernest Rytherford

“I did an experiment that proved the plum pudding idea was wrong. I called it the ‘scattering experiment”

In the 20th Century the work of Thompson, Millikan, Rutherford and Chadwick led to an understanding of the structure of atoms and the existence of subatomic particles.

Rutherford’s atom model:

• Electron- negative, mass nearly nothing

• Nucleus= positively charged (can deflect alpha particles)

• Most of the mass concentrated in the nucleus

• Most alpha particles passed straight through the atom, so most of the atom is empty

Elements are chemicals that can’t be broken down into simpler substances. They are building blocks to make other chemicals. Scientists wanted to know how each of the elements were related to each other so that they could understand the rules of how new chemicals could be made.

Mendeleev grouped elements according to their properties and mass to make the periodic table. He used his table to predict the existence of undiscovered elements.

Isotopes

It is possible for the nuclei of the same element to have different numbers of neutrons in the nucleus (but it must have the same number of protons).

Three types of Hydrogen (istopes)

• Hydrogen

• Deuterium

• Tritium

Dalton's Atomic Theory (1808)

1. All matter is composed of atoms. Atoms are indivisible and indestructible I.E. Democritus’ idea (no knowledge of protons, neutrons, electrons, quarks etc.)
2. All atoms of given elements have the same mass (no knowledge of isotopes)
3. Atoms combine to form compounds in simple whole-number ratios
4. An atom of one element cannot change into an atom of another element-  chemical reactions only involve rearrangement of atoms

Discovery of Ions:

In 1830, Faraday observed that substances can move when an electric field is placed over the water. He called these substances ‘ions’ (greek for wanderer). In 1884, Arrhenius reasoned that ions are atoms that carry charge.