1. The nuclei of isotopes contain different numbers of .

protons

neutrons

electrons

Neutrons

Step-by-step explanation:

Isotopes are atoms of the same element, but whose nuclei have a different number of neutrons, but same number of protons and electrons. Protons and neutrons are found inside the nucleus while electorns are outside and around the it.

Since the amount of neutrons is different, isotopes have a different mass number compared with the element (remember mass number is the sum of protons and neutrons).  However, they have the same atomic number, because they have same amount of protons and electrons, and that also indicates they are electrically neutral.

There can be many different isotopes of the same element because they can have different number of neutrons. So this means most chemical elements have more than one isotope.

Some elements have natural but unstable isotopes, for example uranium, whose isotopes can decay into other more stable isotopes. Through this process, they emit radiation. They can be used for estimating the age of natural samples, such as rocks and organic matter because the average rate of decay of a given isotope is known, relative to those that have already decayed.