Atomic Theory

The Atom Recall the atom is the smallest

particle making up an element

Evolution of the AtomOur ideas about the nature of atoms have progressed over the last two centuries (and continue to develop today).

There are a few scientists who’s works have contributed to our current knowledge of the atom.

JJ Thomson

Year 1900

Discovered the electron

Thomson proposed raisin bun model…

Described the atom as positively charged sphere of matter with negative electrons dispersed throughout

electrons=raisins, positive mass=dough

MODEL THOMSON (1900)

DIAGRAM

Raisin Bun

Ernest Rutherford

Year 1910

Rutherford proposed the nuclear model….

Described the atom as having a central positive nucleus (core) surrounded by negative orbiting electrons

MODEL RUTHERFORD (1910)

DIAGRAM

Nuclear Model

Proof?

Rutherford’s Famous Gold Foil Experiment

A beam of positively charged alpha particles are aimed at a thin sheet of gold foil (source radioactive radium)

Rutherford expected that the positive particles would pass through the sheet of gold foil since years earlier Thomson proposed that mass of the atom was evenly dispersed throughout the atom.

MOST of the alpha particles did go straight through the gold foil with NO deflection.

1.

However a small hand full of particles were deflected from original path.

This could only happen if the the particles hit something that had substantial mass and size and then bounced off at an angle.

Rutherford’s experiment PROVED the presence of a dense, positively charged core called the nucleus which was surrounded by mostly empty space and orbiting electrons.

Niels Bohr

Year 1911

Bohr proposed the planetary model….

Nucleus (core) contains protons and neutrons but electrons are located in specific regions around the nucleus

MODEL BOHR (1911)

DIAGRAM

1.

Solar System

Like planets are at a defined distance from Sun

In each of the 3 boxes on the lower half of your sheet, make the following headings:

location charge symbol mass

Location Charge Symbol Mass

 

orbitals 

negative 

e- 

1/1800 amu

 

Electron

Location Charge Symbol Mass

 

nucleus 

positive 

p+ 

1 amu 

Proton

1 atomic mass unit (amu) = 1.67 x -24 g

1 amu = 0.00000000000000000000000000000167 g

Location Charge Symbol Mass

 

nucleus 

neutron 

n0 

1 amu 

Proton

1 atomic mass unit (amu) = 1.67 x -24 g