In the exercise, we are shown an isotope of Osmium, Os. Osmium has an atomic number equal to 76.
An isotope is a variation of a chemical element, which has the same atomic number but varies the number of neutrons, i.e. those particles that have a neutral charge.
There are two important terms that we must know, which are atomic number and atomic mass.
Atomic number: It is the number of protons present in the nucleus of an atom.
Atomic mass: It is the sum of the protons and neutrons present in the nucleus.
So, in an isotope, the atomic mass will vary but the atomic number will remain the same because if the number of protons is changed it will mean that we have a different element.
Now let's see the representation of an isotope in the following figure:
In the figure we see how an isotope is represented, as an example, we have carbon 11. The upper number gives us the atomic mass, i.e. the sum of neutrons and protons and the lower number tells us the atomic number, i.e. the number of protons. The number of electrons is determined by the number of protons since they are in the same quantity but with opposite charges, that is negative charge.
In this example we have as upper number 35, this is the atomic mass (neutrons + protons). And as lower number 17, this is the atomic number and equal to the number of protons in the nucleus.
If we subtract these two values, we will have the number of neutrons in the nucleus.
The number of electrons is equal to the number of protons.
So, if we want to calculate the number of neutrons, we have to substrate the atomic mass minus atomic number, as follows:
Neutrons = Atomic mass - Atomic number
Neutrons = 191 - 76 = 115
The answer will be: There are 115 neutrons