Alpha decay

Atoms are stable when the number of neutrons in the nucleus is roughly equal to the number of protons. When the difference is too large, the atom becomes unstable.
The heavy nuclei of neutron-rich isotopes with an atomic number higher than that of iron (N = 26) are assembled at very high temperatures (>10⁹ K) during the explosion of massive stars (supernovas).
However, all the nuclei of matter seek a parsimonious energy stability. So to return to a stable state, they must transmute by expelling energy in the form of mass or radiation. This leads to the spontaneous disintegration of the nucleus. This is called radioactivity.
Radioactivity is therefore a natural random phenomenon which occurs in the nucleus, deep within the atom and which gives rise to a new, more stable nucleus.
In 1908, Ernest Rutherford (1871-1937) identified alpha particles as helium-4 nuclei composed of 2 protons and 2 neutrons. Thus, the stable nucleus resulting from radioactive decay has two neutrons and two protons less than the unstable nucleus from which it originated.
The emission of an alpha particle mainly concerns very large nuclei such as radium-226 (88 protons and 138 neutrons), thorium-232 (90 protons and 142 neutrons), uranium-238 (92 protons and 136 neutrons), etc.
Such nuclei are unstable because the Coulomb repulsion between protons, which increases as the square of the number of protons, is stronger than the nuclear attraction between nucleons. This is when the nucleus expels a bunch of four nucleons.

For the matter, it is the fastest and most economical way to regain stability.
Radium has no stable isotopes. The main isotope of radium discovered by Pierre and Marie Curie on December 21, 1898 by extraction of pitchblende (uranium ore), is radium-226, whose half-life or half-life is 1,600 years (time required for half of the nuclei initially present to give rise to other nuclei).
Radium has amazing natural properties (radioluminescence, spontaneous heat production, radioactive source,...). In addition, it makes the atmosphere conductive of electricity and remotely discharges capacitors, which made it possible to design the Geiger-Müller counter. The Geiger counter, developed in 1928, is used to measure a large number of ionizing radiations (alpha, beta, gamma radiation and X-rays).
Radium was used until 1950, for its radioluminescence properties, in particular in paints intended for watchmaking. From the 1920s, occupational diseases were identified among workers, who refined their brushes of paint enriched with radium, by bringing them to their mouths. These diseases led to an initial epidemiological investigation and the gradual closing of the factories.
Due to their large mass and charge, alpha particles are not very penetrating. In general they can be stopped by the skin. However, if they are ingested, they become dangerous. Sufficiently high doses can cause cancer.

nota: Ionization is the action of removing or adding electromagnetic charges to an atom.