State the atoms represented as shown below: (a) State the relationship between the two atoms. (b) What is the difference between them? (c) Give two examples...

The diagram shows the same chemical symbol X written twice, each time with a subscript on the lower left and a superscript on the upper left:

• Left atom: \( {}^{q}_{p}\text{X} \)
• Right atom: \( {}^{r}_{p}\text{X} \)

In the standard nuclide notation \( {}^{A}_{Z}\text{X} \), the lower figure is the atomic number \(Z\) (number of protons) and the upper figure is the mass number \(A\) (protons plus neutrons). Reading the two atoms off the diagram:

Both atoms carry the same lower figure \(p\), so both have the same number of protons and are therefore atoms of the same element X. They differ only in the upper figure (\(q\) against \(r\)).

(a) Relationship between the two atoms

They are isotopes of the same element. Isotopes are atoms of the same element that have the same atomic number but different mass numbers.

(b) The difference between them

They have different mass numbers (\(q\) for the left atom and \(r\) for the right atom). Since the mass number is protons plus neutrons and both have the same number of protons \(p\), the difference lies wholly in the number of neutrons.

Number of neutrons is given by \( N = A - Z \):

Their number of protons (\(p\)) and, in the neutral state, their number of electrons are the same, so they have identical chemical properties and differ only in relative atomic mass and physical properties that depend on mass.

(c) Two examples of elements that exhibit this phenomenon (isotopy)

1. Chlorine: \( {}^{35}_{17}\text{Cl} \) and \( {}^{37}_{17}\text{Cl} \)
2. Carbon: \( {}^{12}_{6}\text{C} \) and \( {}^{14}_{6}\text{C} \)

(Hydrogen, \( {}^{1}_{1}\text{H} \), \( {}^{2}_{1}\text{H} \) and \( {}^{3}_{1}\text{H} \), and oxygen, \( {}^{16}_{8}\text{O} \) and \( {}^{18}_{8}\text{O} \), are also acceptable examples.)