An element $X$ has a mass number $4$ and atomic number $2$. What is the valency of this element?

In the Gold foil experiment of Geiger and Marsden,which paved the way for Rutherford's model of an atom,$\sim 1.00\%$ of the $\alpha$-particles were found to deflect at angles $> 50^o$. If one mole of $\alpha$-particles were bombarded on the gold foil,compute the number of $\alpha$-particles that would deflect at angles less than $50^o$.

The presence of protons and neutrons at the center of the atom was suggested by:

$(a)$ Both helium and beryllium have two electrons in their valence shells. Helium is a noble gas whereas beryllium is a metal. Justify.
$(b)$ Hydrogen exists in three isotopic forms. Why are the isotopes of hydrogen chemically alike?

$A, B$ and $C$ are three metals with atomic numbers $4, 11$ and $13$ respectively. Arrange the metals in the increasing order of their valency.

The first model of an atom was given by