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

Which of the $Na^+$ and $He$ has completely filled $K$ and $L$ shells? Give a reason to support your answer.

Which of the following statements about Rutherford's model of the atom are correct?
$(i)$ Considered the nucleus as positively charged.
$(ii)$ Established that the $\alpha$-particles are four times as heavy as a hydrogen atom.
$(iii)$ Can be compared to the solar system.
$(iv)$ Was in agreement with Thomson's model.

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?

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.

The presence of electrons outside the nucleus and the nucleus at the centre of the atom was suggested by: