Why are some isotopes radioactive?

Some isotopes are radioactive because their nuclei are unstable and decay to achieve stability, releasing radiation in the process.

Radioactivity is a property of certain isotopes that arises from the instability of their atomic nuclei. The nucleus of an atom is made up of protons and neutrons. The number of protons determines the atomic number and thus the element, while the number of neutrons can vary, creating different isotopes of the same element.

In some isotopes, the balance of protons and neutrons is such that the nucleus is unstable. This instability is due to the forces within the nucleus. The protons, being positively charged, repel each other, but are held together by the strong nuclear force, which acts between both protons and neutrons. If the balance of protons and neutrons isn't just right, the nucleus can become unstable.

This instability leads to radioactive decay, a process in which the nucleus changes to become more stable. There are three main types of radioactive decay: alpha decay, beta decay, and gamma decay. In alpha decay, the nucleus emits an alpha particle (two protons and two neutrons), effectively becoming a different element. In beta decay, a neutron changes into a proton and an electron, with the electron being emitted as a beta particle. Gamma decay involves the emission of high-energy photons, known as gamma rays, and doesn't change the number of protons or neutrons in the nucleus.

The rate at which a radioactive isotope decays is measured in terms of its half-life, the time it takes for half of the atoms in a sample to decay. This can range from fractions of a second to billions of years, depending on the isotope. The process of radioactive decay is random, but the half-life of an isotope is a constant, characteristic property.

In summary, some isotopes are radioactive because their nuclei are unstable. This instability leads to radioactive decay, in which the nucleus changes to become more stable, releasing radiation in the process.