Which of the following statements is always correct?

An ion $Y^{3-}$ contains $18$ electrons and $16$ neutrons. Calculate the atomic number and mass number of the element $Y$. Name the element $Y$.

Is it possible for the atom of an element to have one electron,one proton,and no neutron? If so,name the element.

Give a brief account of the development of the presently accepted model of the atom with the contributions made by different scientists. Write two features of this atomic model.

$(a)$ 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.

Elements with valency $1$ are