The Development of the Atomic Model (SHE TASK 2)

Democritus (4th Century BC) 

Believed:

• different substances had Atomos (invisible particles) of different sizes & shapes.

• atoms couldn’t be broken down any further

• that atoms exist in empty space, and this allows them to move

John Dalton (1808)

• all matter is made of atoms

• atoms of an element are identical

• each element has different atoms

• atoms of different elements combine in constant ratios to form compounds

• atoms are rearranged in reactions

Joseph John Thomson (1897)

• Cathode Ray Tube which had a noble gas, a + & - electrodes

• gas particles attracted to + electrodes

• particles must have a - charge (electrons)

• “Plum Pudding Model” = atom was a + sphere with electrons shoved into the sides of it.

Ernest Rutherford (1910)

• Gold Foil Experiment = shot high energy beam of + alpha particles into gold foil

• Expected: the alpha particles would pass straight through the foil, but:

• few particles were deflected at small angles

• very few particles were deflected at large angles

• Proved that all atoms are mostly empty spaced

Niels Bohr (1913)

• electrons revolve around the nucleus in fixed, circular orbit

• electrons’ orbits correspond to specific energy levels or shells in the atom

• electrons only occupy fixed energy levels and can’t exist between two energy levels

• electron orbits of larger radii correspond to energy levels of higher energy

James Chadwick (1932)

• conducted experiments bombarding (forcefully hitting) beryllium atoms with alpha particles (2 protons & 2 neutrons)

• discovered neutrons (its mass is similar to protons, but it has no electrical charge)