The periodic table is a tabular arrangement of the chemical elements, ordered by their atomic number (number of protons), electron configurations, and recurring chemical properties. This ordering shows periodic trends, such as elements with similar behavior in the same column. The rows of the table are called periods; the columns are called groups. Six groups have names as well as numbers: for example, group 17 elements are the halogens; and group 18, the noble gases. The periodic table can be used to derive relationships between the properties of the elements, and predict the properties of new elements yet to be discovered or synthesized. The periodic table provides a useful framework for analyzing chemical behaviour, and is widely used in chemistry and other sciences. The Russian chemist Dmitri Mendeleev published the first widely recognized periodic table in 1869. He developed his table to illustrate periodic trends in the properties of the then-known elements. Mendeleev also predicted some properties of then-unknown elements that would be expected to fill gaps in this table. Most of his predictions were proved correct when the elements in question were subsequently discovered. Mendeleev's periodic table has since been expanded and refined with the discovery or synthesis of further new elements and the development of new theoretical models to explain chemical behaviour. All elements from atomic numbers 1 (hydrogen) to 118 have been discovered or synthesized, with the most recent additions being confirmed by the International Union of Pure and Applied Chemistry (IUPAC) on December 30, 2015 and officially named on November 28, 2016: they complete the first seven rows of the periodic table. The first 94 elements exist naturally, although some are found only in trace amounts and were synthesized in laboratories before being found in nature. Elements with atomic numbers from 95 to 118 have only been synthesized in laboratories or nuclear reactors. Synthesis of elements having higher atomic numbers is being pursued. Numerous synthetic radionuclides of naturally occurring elements have also been produced in laboratories.
technically speaking, all elements are radioactive, it's just the elements that aren't described as radioactive take a MUCH longer time to decay. The basic characteristics that an element needs to have to be considered a radioactive element are listed below. DISINTEGRATES The nucleus of a radioactive element is unstable. The nucleus will break down over time, reducing the amount of the element remaining. This disintegration occurs naturally and does not need an outside stimulus to occur. All man-made elements are radioactive and break down. The speed at which an element breaks down is called "half-life," or how long it takes for half of the atoms present to disintegrate. This measure can determine how relatively stable or unstable the element is. For example, the half life of uranium is over 4 billion years, while the half life of francium is just over 20 minutes. DIFFERENT ELEMENTS As the element disintegrates, the subatomic particles of the nucleus form different elements. These particles are not lost to the environment. For example, uranium disintegrates in a number of steps, becoming different elements along the way. These include thorium, protactinium, radium, radon, polonium, bismuth and lead. The last step in the series, lead, is a stable element that does not disintegrate. These created elements are called daughters of the parent element. RADIATION EMISSION Radiation is the energy released from the atom as the element disintegrates from one element to another. There are many types of radiation, including light and microwaves. When radioactive elements release their energy, the radiation is called ionizing radiation, which includes charged particles. These charged particles are the harmful radiation that is dangerous to living organisms. However, not all radiation emitted from the elements is harmful to humans and are classed as alpha and beta ray radiation. DETECTION A number of tools are used to detect the presence of radioactive materials and elements. A Geiger counter is a well-known device used to measure radiation levels. The device works by creating electrical charges when it encounters radiation emitted from radioactive materials. The more radioactive material, the higher the reading on the device.
Radioactive decay- Is the spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter from the nucleus. Remember that a radioisotope has unstable nuclei that does not have enough binding energy to hold the nucleus together. Radioisotopes would like to be stable isotopes so they are constantly changing to try and stabilize. In the process, they will release energy and matter from their nucleus and often transform into a new element. This process, called transmutation, is the change of one element into another as a result of changes within the nucleus. The radioactive decay and transmutation process will continue until a new element is formed that has a stable nucleus and is not radioactive. Transmutation can occur naturally or by artificial means.