For the most part the nuclei of the elements that make up the matter around us are stable, but some spontaneously change into other elements in one of a variety of ways. When these unstable atomic nuclei spontaneously breakdown, they are said to have decayed. Collectively these processes are called natural radioactivity. The major particles released during these changes are the alpha particle, beta particle and the positron. In addition to the previously mentioned particles, energy is released in the form of neutrinos, and gamma radiation.
All atoms having a nucleus with the same number of protons form a unique chemical element.
If the number of protons in an atom changes, a new element is formed. But, when atoms of an element have different numbers of neutrons they form what is referred to as an isotope of that element. Most of the elements occur naturally as more than one isotope. The number of protons in an atom is called its atomic number, while the sum of the protons and neutrons is called its mass number. As you will see later on, it is the ratio of the number of neutrons to the number of protons that allows scientists to determine how stable an atom is.