Elements with valency $1$ are

Give one point of similarity and one point of difference between isotopes of ${ }_{6}^{14} C$ and ${ }_{6}^{12} C$.

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.

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?

Which of the following in the figure do not represent Bohr's model of an atom correctly?

Write the names of three elementary particles which constitute an atom. Which of these particles determine the mass of an atom? Justify your answer with an example.