Atomic Models Flashcards
What did JJ Thompson’s series of experiments in 1897 conclude?
He concluded that atoms weren’t solid and indivisible. His measurements of charge and mass showed than an atom must contain even smaller, negatively charged particles. He called these particles ‘corpuscles’- we call them electrons. The ‘solid sphere’ idea of atomic structure had to be changed. The new model was known as the ‘plum pudding model’- a positively charged sphere with negative electrons embedded into it.
What was the gold foil experiment?
Rutherford designed an experiment to test the plum pudding model. It was carried out by his assistants Geiger and Marsden. A beam of alpha particles was aimed at very thin gold foil and their passage through the foil detected. The scientists expected the alpha particles to pass straight through the foil but something else also happened.
What results did they get from the gold foil experiment?
Some of the alpha particles emerged from the foil at different angles and some even came straight back. The scientists realised that the positively charged alpha particles were being repelled and deflected by a tiny concentration of positive charge in the atom. As a result of this experiment, the plum pudding model was replaced by the nuclear model of the atom.
What was Bohr’s model?
The Bohr model was an improvement to Rutherford’s model. The four basic principles of this model were that…
Electrons only exist in fixed orbits (shells) and not anywhere in between.
Each shell has a fixed energy.
When an electron moves between shells electromagnetic radiation is emitted or absorbed.
Because the energy of shells is fixed, the radiation will have a fixed frequency.
How did Bohr’s model explain why some elements are inert?
Bohr said that the shells of an atom can only hold fixed numbers of electrons, and that an element’s reactivity is due to its electrons. So, when an atom has a full shell of electrons it’s stable and does not react.
What is the most accurate model of the atom we have today?
It involves complicated quantum mechanics. Basically, you never know where an electron is or which direction its going in at any moment, but you can say how likely it is to be at a certain point of the atom.
Describe how JJ Thompson’s model of the atom was different from Dalton’s model.
Dalton described atoms as solid spheres. JJ Thompson suggested that they war not that but in fact they contained small negatively charged particles (electrons) in a charged “pudding”
What name was given to JJ Thompson’s model of the atom?
The plum pudding model.
Name the scientists who conducted the gold foil experiment in 1909.
Ernest Rutherford, Hans Geiger and Ernest Marsden.
Explain how the gold foil experiment provided evidence that Thomson’s model was wrong.
If Thomson’s model was correct the alpha particles fired at the sheet of gold should have been deflected very slightly by the positive “pudding” that made up most of the atom. Instead, most of the alpha particles passed straight through the gold atoms, and a very small number were deflected backwards. So the plum pudding model couldn’t be right.
Describe Rutherford’s model of the atom.
Rutherford’s model has a tiny positively charged nucleus at the centre surrounded by a cloud of negative electrons. Most of the atom is empty space.
Describe the main features of Bohr’s model of the atom.
In Bohr’s model the electrons only exist in fixed shells and not anywhere in-between. Each shell has a fixed energy. When an electron moves between shells electromagnetic radiation is emitted or absorbed. Because the energy of the shells is fixed, the radiation will have a fixed frequency.
