Which of the following element has a mass of 23? 

Na 

The correct answer is option 1. 23. 

The atomic mass of an element is the average mass of the atoms of that element, taking into account the relative abundances of its isotopes in nature. Each isotope contributes to the overall atomic mass based on its abundance and mass.

In the case of sodium (\( \text{Na} \)), the atomic mass is approximately 23 atomic mass units (amu). This means that, on average, each sodium atom has a mass of approximately 23 atomic mass units.

Sodium has only one stable isotope, which is sodium-23 (\(^{23}\text{Na}\)). This isotope makes up 100% of naturally occurring sodium. Since there are no other isotopes of sodium to consider, the atomic mass of sodium is simply equal to the mass of its only stable isotope, which is approximately 23 amu.

Therefore, sodium (\( \text{Na} \)) is the element with an atomic mass of 23.