Which of the following figures do not represent Bohr's model of an atom correctly?

Define the term valency. How is valency different from valence electrons? Find the valency of oxygen and fluorine.

Atomic models have been improved over the years. Arrange the following atomic models in their chronological order:
$(i)$ Rutherford's atomic model
$(ii)$ Thomson's atomic model
$(iii)$ Bohr's atomic model

Show diagrammatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.

Identify the isotopes out of $A, B, C$ and $D$ given the following representations:
${ }_{17}^{35} A, { }_{20}^{40} B, { }_{17}^{37} C, { }_{19}^{38} D$

$(a)$ Why did Rutherford choose gold and not any other lighter element (such as aluminium) for his experiment? Explain your answer giving two reasons.
$(b)$ Write the name and symbol of the particle chosen by Rutherford for bombardment against the gold foil in his experiment.