Which of the following species is electrically neutral and why?
$K^{+}, Cl^{-}, S^{2-}, P^{3-}, K$

The average atomic mass of a sample of an element $y$ is $35.5 \,u$. What are the percentages of isotopes $_{17}^{37}y$ and $_{17}^{35}y$ in the sample?

An atom of an element $X$ may be written as ${ }_{4}^{9} X$.
$(a)$ What does the figure $9$ indicate?
$(b)$ How many neutrons does this element $X$ contain?

$(a)$ An ion $M^{2+}$ contains $10$ electrons and $12$ neutrons. What is the atomic number and mass number of the element $M$?
$(b)$ Is it possible in an atom to have $12$ protons and $13$ electrons? Explain.
$(c)$ Why is helium gas inert?

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.

The ion of an element has $3$ positive charges. The mass number of the atom is $27$ and the number of neutrons is $14$. What is the number of electrons in the ion?