An isotope of an element is defined by the number of protons and neutrons in its nucleus. In this case, you have an iridium isotope with 77 protons and 115 neutrons.
The number of protons identifies the element, and since iridium has 77 protons, this confirms that the element is indeed iridium (Ir). To find the mass number of the isotope, you add the number of protons and neutrons together:
Mass number \( = \) Number of protons \( + \) Number of neutrons
Mass number \( = 77 + 115 = 192 \)
Therefore, the isotope of iridium you are referring to is Iridium-192 (often written as \( \text{Ir}^{192} \)).