1. two compounds form
2. precipitate forms
3. ions exchanged between the two reactants
Answer: The atomic number
The accepted model of an atom was made by Schrodinger, which is known as the quantum mechanical model. Unlike Bohr's model, electrons are represented as a cloud and not small balls orbiting the nucleus. There is no define path of electrons in Schrodinger's model, but it can predict the odds of where an electron is located.
The answer to your question is then Electrons form a cloud around the nucleus.
