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

Which of the following statements is always correct?

Dalton's atomic theory successfully explained which of the following laws?
$(i)$ Law of conservation of mass
$(ii)$ Law of constant composition
$(iii)$ Law of radioactivity
$(iv)$ Law of multiple proportion

$(a)$ What are canal rays? Who discovered them? What is the charge and mass of canal rays?
$(b)$ How are the canal rays different from electrons in terms of charge and mass?

$(a)$ Describe the main features of Bohr's model of an atom. Draw a neat and labelled diagram of energy levels.
$(b)$ Which of the following pairs are isotopes and which are isobars?
$(i)$ $_{26}^{58} A, _{28}^{58} B$
$(ii)$ $_{35}^{79} X, _{35}^{80} Y$
$(c)$ Elements $A$ and $B$ have atomic numbers $18$ and $16$ respectively. Which of these two would be more reactive and why?

An atom of an element has full $K$ and $L$ shells and has $5$ electrons in its $M$ shell. What is its atomic number?