The stability of the nucleus means that the nucleus will not spontaneously change its number of protons, neutrons and its basic properties. According to the stability of the nucleus, it can be divided into two types: stable nuclei and unstable (or radioactive) nuclei.
1. Atomic nuclei with a number of protons equal to and greater than 84 are unstable. That is, the elements after the atomic number 84 are all radioactive elements.
2. A nucleus with less than 84 protons has an even number of protons and neutrons, and its nucleus is stable.
3. Atomic nuclei with a number of protons or neutrons equal to 2, 8, 20, 28, 50, 82, 126 are particularly stable. These numbers are called magic numbers. Both the number of protons and the number of neutrons are magic numbers, called double magic number nuclei.
4. The ratio of the number of neutrons to the number of protons is n/p. When Z<20, n/p=1, and the nucleus is stable. As the atomic number increases, the value of n/p increases, and the larger the ratio, the worse the stability.
Nuclear decay
Unstable atomic nuclei will spontaneously transform into another nucleus while emitting radiation. This change is called radioactive decay. There are three kinds of rays emitted by the nucleus during the decay process: α rays, β rays and γ rays.
Alpha rays are alpha particle streams, which are positively charged helium nuclei. Beta rays are a stream of high-speed moving electrons.
There are two types of β decay: β+ and β-. When β decays, in addition to positrons or negative electrons, neutrinos or antineutrinos are also emitted. Beta-decay is the conversion of neutrons in the nucleus to protons (remaining in the nucleus) and releases an electron and an antineutrino associated with the electron. Beta+ decay is the fact that there are fewer neutrons in the nucleus, and protons are converted into neutrons (remaining in the nucleus), and a positron and a neutrino are released at the same time.
Gamma rays are a stream of photons. It is usually emitted when a new nucleus is formed after alpha decay or beta decay. This is because the radioactive mother nucleus becomes an excited daughter nucleus after the aforementioned decay, and when the daughter nucleus transitions to a normal state, it generally radiates gamma photons.
The total number of charges and total mass of particles before decay is equal to the total number of charges and total mass of all particles after decay
Law of radioactive decay
There are N nuclei in the sample at time t, and dN decays within dt time.
t=0, N=N0, yes
The above formula is called the law of radioactive decay.
The physical meaning is: at time t, the ratio of the number of nuclei that decay per unit time to the total number of nuclei at that time. The larger, the faster the decay.
It is customary to use half-life to characterize the rate of decay of radioactive elements. The definition of half-life is: the time required for the nucleus to decay to N=N0/2. Expressed by T.
Sometimes the average lifetime τ is also used to indicate the speed of decay. The average lifetime refers to the average value of the time that each nucleus exists before it decays.
Radioactivity (also called radioactivity) refers to the number of nuclear decays of a radioactive source per unit time.
In the International System of Units, the unit of activity is becquerel (Bq). 1Bq represents the activity of a radioactive source that undergoes nuclear decay once per second. The commonly used unit is Curie (Ci).