Identify the $Mg^{2+}$ ion from the figures,where $n$ and $p$ represent the number of neutrons and protons,respectively.

$(a)$ An atom is considered electrically neutral. Explain.
$(b)$ State the valency of Helium.
$(c)$ If an atom has an atomic mass of $4$ and $2$ protons in its nucleus,how many neutrons does it have?

Give reasons for the following:
$(a)$ Isotopes of an element are chemically similar.
$(b)$ An atom is electrically neutral.
$(c)$ Noble gases show least reactivity.
$(d)$ Nucleus of an atom is heavy and positively charged.
$(e)$ Ions are more stable than atoms.

$(a)$ Calculate the number of electrons,protons,and neutrons in an atom of an element with atomic number $20$ and mass number $40$. Write the electronic configuration and draw the structure of the atom.
$(b)$ An atom has complete $K$ and $L$ shells. Is this an atom of a metal,non-metal,or noble gas? Justify.

$(a)$ 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?

Which of the following species have $18$ electrons? $Ca^{2+}, K^{+}, Na, Cl^{-}, Ar$. Also,state the reason why the chemical properties of all isotopes of an element are similar.