Give reasons:
$(a)$ Mass number of an atom excludes the mass of an electron.
$(b)$ The nucleus of an atom is charged.
$(c)$ The alpha particle scattering experiment was possible by using gold foil only and not by foil of any other metal.

For the following statements,write $T$ for True and $F$ for False.
$(a)$ $J$.$J$. Thomson proposed that the nucleus of an atom contains only nucleons.
$(b)$ $A$ neutron is formed by an electron and a proton combining together. Therefore,it is neutral.
$(c)$ The mass of an electron is about $\frac{1}{2000}$ times that of a proton.
$(d)$ An isotope of iodine is used for making tincture iodine,which is used as a medicine.

Answer the following in one line or one word:
$(a)$ Who discovered the neutron?
$(b)$ On the basis of Thomson's model of an atom,explain how the atom is neutral as a whole.
$(c)$ What is the maximum number of electrons that can be accommodated in the outermost shell of an atom?
$(d)$ What term is assigned to the atoms of different elements having the same atomic mass but different atomic numbers?
$(e)$ How many neutrons are present in a hydrogen atom?

$(a)$ 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.

$(a) (i)$ Name the scientist who proposed this model of the atom.
$(ii)$ Write the three postulates of this model.
$(iii)$ How many maximum electrons can be accommodated in the $M$ orbit?
$(b)$ What are canal rays? Give the characteristics of canal rays.

Show diagrammatically the electron distributions in a sodium atom and a sodium ion and also give their atomic number.