What information do you get from the figure about the atomic number,mass number,and valency of atoms $X$,$Y$,and $Z$? Give your answer in a tabular form.

Name any two noble gases.

$(a)$ 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.

The nucleus of a hydrogen atom is known as:

If an ion $M^{3+}$ contains $10$ electrons and $14$ neutrons, what are the atomic number and mass number of the element $M$?

$(a)$ Which popular experiment is shown in the figure?
$(b)$ List three observations of this experiment.
$(c)$ State conclusions drawn from each observation of this experiment.
$(d)$ State the model of atom suggested on the basis of the above experiment.