Chemistry - 1 Atomic Structure - 1.5 History of the Atom & 1.6 Structure of the Atom

John Dalton's model of the atom:

tiny, hard, indivisible spheres - each element's atom has a different mass

Who discovered the electron?

JJ Thomson

How did JJ Thomson discover the electron?

cathode ray tube experiment

cathode ray tube

evacuated glass tube in which a stream of electrons emitted by a cathode strikes a fluorescent material, causing it to glow

Cathode ray tube experiment:

JJ Thompson put positively and negatively charged plates around the cathode ray tube and saw that the ray curved towards the positive plate. He deduced that there must be electrons

JJ Thomson's model of the atom

plum pudding model

Who discovered the nucleus?

Ernest Rutherford

How was the nucleus discovered?

Gold foil experiment

Gold foil experiment [4]:

- gold foil placed in the middle of a fluorescent screen
- alpha particles shot at gold foil
- most particles went through
- some were deflected

conclusions from gold foil experiment [3]:

- atom is mostly empty space
- nucleus has a positive charge
- electrons float around nucleus

Ernest Rutherford's model of the atom:

Nuclear/planetary model

Who discovered that electrons are in shells?

Niels Bohr

Who discovered the neutron?

James Chadwick

How was the neutron discovered?

- alpha particles shot at materials
- emitted rays with no charge
- rays with no charge dislodged protons from paraffin
- deduced that they were neutrons

atomic number

number of protons in an atom

mass number

number of protons + number of neutrons