The isotope of uranium used as a fuel in nuclear reactors and carbon used in radiocarbon dating are

Write five applications of isotopes in various fields.

$(a)$ 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.

$(a)$ 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.

The first model of an atom was given by

State whether the following statement is true or false:
The relative number and kinds of atoms are varying in a given compound.