Isotopes of the same element contain an identical number of protons and have very similar physical and chemical properties (similarities); Isotopes of the same element have a different number of neutrons and different mass numbers and they do differ in their stability (differences).
Isotopes are various forms of an element that have the same number of protons but a different number of neutrons.
Together, the number of protons and the number of neutrons determine an element’s mass number: mass number = protons + neutrons.
Some elements, such as carbon, potassium, and uranium, have multiple naturally-occurring isotopes. Isotopes are defined first by their element and then by the sum of the protons and neutrons present.
Some isotopes are stable, but others can emit, or kick out, subatomic particles to reach a more stable, lower-energy, configuration. Such isotopes are called radioisotopes, and the process in which they release particles and energy is known as decay.
Learn more about isotopes from sources below:
www.khanacademy.org
www.boundless.com
www.britannica.com