$A$ helium atom has $2$ electrons in its valence shell,but its valency is not $2$. Explain.

$(a)$ 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?

State whether the following statement is true or false:
Atoms combine in the ratio of small whole numbers to form compounds.

$(a)$ 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?

$(a)$ What are isobars?
$(b)$ Atomic number of an element $Y$ is $17$.
$(i)$ Write its electronic configuration.
$(ii)$ What is the number of valence electrons in $Y$?
$(iii)$ How many electrons are needed to complete the octet of $Y$?
$(iv)$ Is it a metal or non-metal?
$(c)$ The valency of $Na$ is $1$ and not $7$. Give reason.

Find out the valency of the atoms represented by the Fig. $(a)$ and $(b)$.