In a sample of ethyl ethanoate $(CH_{3}COOC_{2}H_{5})$,the two oxygen atoms have the same number of electrons but different number of neutrons. Which of the following is the correct reason for it?

List applications of any three isotopes in various fields.

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?

State whether the following statement is true or false:
Atomic radius is measured in nanometres. $1 \, m = 10^9 \, nm$.

$(a)$ Why did Rutherford choose gold and not any other lighter element (such as aluminium) for his experiment? Explain your answer giving two reasons.
$(b)$ Write the name and symbol of the particle chosen by Rutherford for bombardment against the gold foil in his experiment.

Write the charge and mass of an electron.