1.1 Atomic Structure
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Atoms at the start of the 19th century
John Dalton described them as solid spheres and different solid spheres made up the different elements
Who devised the plum pudding model
JJ Thompson
When was the Plum Pudding model discovered
1897
Why did JJ Thompson devise the plum pudding model
His measurements of charge and mass showed that an atom must contain even smaller negatively charged particles (electrons)
Plum pudding model
An atom is a ball of positive charge with electrons stuck in it
Who devised the nuclear model
Ernest Rutherford
When was the nuclear model discovered
1909
How was the nuclear model discovered
Hans Geiger and Ernest Marsden fired positively charged alpha particles at an extremely thin sheet of gold
Gold foil experiment expectations from the plum pudding model
Most particles should pass straight through the sheet or be slightly deflected because the positive charge of each atom was thought to be spread out
What actually happened in the gold foil experiment
Most alpha particles went through the gold foil, some were deflected more than expected but some were deflected backwards so he plum pudding model couldn’t be right
Why does the nuclear model work with the gold foil experiment
When alpha particles came near the concentrated positive charge of the nucleus they were deflected, when they were fired directly at the nucleus they were deflected backwards
Nuclear model
A tiny positively charged nucleus where most of the atoms mass is concentrated and a ‘cloud’ of negative electrons surround the nucleus so the atom is mostly empty space
Who devised the Bohr model
Niels Bohr
Bohr model
-Electrons orbit the nucleus in fixed shells -Each shell is a fixed distance from the nucleus and has a fixed energy
Why was the nuclear model incorrect
If the electrons were in a cloud, they would be attracted to the nucleus and the atom would collapse
Proton relative mass
1
Neutron relative mass
1
Electron relative mass
0.0005
Proton relative charge
+1
Neutron relative charge
0
Electron relative charge
-1
Nucleus position
Centre of the atom
What does the nucleus contain
Protons and neutrons
What is the charge of the nucleus
Positive