Isotopes and Nuclear Symbols Example Problem

How to Find the Number of Protons and Neutrons in an Isotope Atom

All isotopes of an element have the same number of protons, but different numbers of neutrons.
How to Find the Number of Protons and Neutrons in an Isotope Atom. ALFRED PASIEKA / Getty Images

This worked problem demonstrates how to determine the number of protons and neutrons are in the nucleus of an isotope.

Finding Protons and Neutrons in an Isotope Problem

One of the harmful species from nuclear fallout is the radioactive isotope of strontium, 9038Sr (assume the super and subscripts line up). How many protons and neutrons are there in the nucleus of strontium-90?


The nuclear symbol indicates the composition of the nucleus.

The atomic number (number of protons) is a subscript at the lower left of the symbol of the element. The mass number (sum of the protons and neutrons) is a superscript to the upper left of the element symbol. For example, the nuclear symbols of the element hydrogen are:

11H, 21H, 31H

Pretend that the superscipts and subscripts line up on top of each other - they should do so in your homework problems, even though they don't in my computer example ;-)

The number of protons is given in the nuclear symbol as the atomic number, or the lower left subscript, 38.

Obtain the number of neutrons by subtracting the number of protons from the mass number, or the upper left superscript:

number of neutrons = 90 - 38
number of neutrons = 52


9038Sr has 38 protons and 52 neutrons