The element that may exist naturally as a single isotope based on the given information is sodium (Na) with a relative atomic mass of 23.0.
Isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. When an element exists naturally as a single isotope, it means that all atoms of that element in nature have the same atomic mass.
In the case of sodium (Na), the given relative atomic mass of 23.0 indicates that it exists as a single isotope. This is because the value is not a decimal or a range, which would suggest the presence of multiple isotopes with different abundances. Therefore, sodium (Na) likely exists as a single isotope, specifically sodium-23 (23Na).
On the other hand, magnesium (Mg) has a given relative atomic mass of 24.3. This suggests that magnesium exists with multiple isotopes, as the value is not a whole number. The presence of a decimal value implies a mixture of isotopes with different atomic masses and abundances.
In summary, based on the provided information, sodium (Na) may exist naturally as a single isotope (sodium-23), while magnesium (Mg) is likely to have multiple isotopes.