Radioactive decay (also known as nuclear decay, radioactivity or nuclear radiation) is the process by which an unstable atomic nucleus loses energy (in terms of mass in its rest frame) by emitting radiation, such as an alpha particle, beta particle with neutrino or only a neutrino in the case of electron capture, or a gamma ray or electron in the case of internal conversion. A material containing such unstable nuclei is considered radioactive. Certain highly excited short-lived nuclear states can decay through neutron emission, or more rarely, proton emission.
Radioactive decay is a stochastic (i.e. random) process at the level of single atoms. According to quantum theory, it is impossible to predict when a particular atom will decay,[1][2][3] regardless of how long the atom has existed. However, for a collection of atoms, the collection's expected decay rate is characterized in terms of their measured decay constants or half-lives. This is the basis of radiometric dating. The half-lives of radioactive atoms have no known upper limit, spanning a time range of over 55 orders of magnitude, from nearly instantaneous to far longer than the age of the universe.
A radioactive nucleus with zero spin can have no defined orientation, and hence emits the total momentum of its decay products isotropically (all directions and without bias). If there are multiple particles produced during a single decay, as in beta decay, their relativeangular distribution, or spin directions may not be isotropic. Decay products from a nucleus with spin may be distributed non-isotropically with respect to that spin direction, either because of an external influence such as an electromagnetic field, or because the nucleus was produced in a dynamic process that constrained the direction of its spin. Such a parent process could be a previous decay, or a nuclear reaction.[4][5][6][note 1]
The decaying nucleus is called the parent radionuclide (or parent radioisotope[note 2]), and the process produces at least one daughter nuclide. Except for gamma decay or internal conversion from a nuclear excited state, the decay is a nuclear transmutation resulting in a daughter containing a different number of protons or neutrons (or both). When the number of protons changes, an atom of a different chemical element is created.
The first decay processes to be discovered were alpha decay, beta decay, and gamma decay. Alpha decay occurs when the nucleus ejects an alpha particle (helium nucleus). This is the most common process of emitting nucleons, but highly excited nuclei can eject single nucleons, or in the case of cluster decay, specific light nuclei of other elements. Beta decay occurs in two ways: (i) beta-minus decay, when the nucleus emits an electron and an antineutrino in a process that changes a neutron to a proton, or (ii) beta-plus decay, when the nucleus emits a positron and a neutrino in a process that changes a proton to a neutron. Highly excited neutron-rich nuclei, formed as the product of other types of decay, occasionally lose energy by way of neutron emission, resulting in a change from one isotope to another of the same element. The nucleus may capture an orbiting electron, causing a proton to convert into a neutron in a process called electron capture. All of these processes result in a well-defined nuclear transmutation.
Atoms are the smallest units of matter that still retain the fundamental chemical properties of an element. Much of the study of chemistry, however, involves looking at what happens when atoms combine with other atoms to form compounds. A compound is a distinct group of atoms held together by chemical bonds. Just as the structure of the atom is held together by the electrostatic attraction between the positively charged nucleus and the negatively charged electrons surrounding it, the stability within chemical bonds is also due to electrostatic attractions. To illustrate further, consider the two major types of chemical bonds: covalent bonds and ionic bonds. In covalent bonds, two atoms share pairs of electrons, while in ionic bonds, electrons are fully transferred between two atoms so that ions are formed. Let’s consider both types of bonds in detail.
<em><u>Refractory periods are a short phase in time following an action potential where another action potential cannot be generated. </u></em>
<em><u>It is the period immediately following the transmission of an impulse in nerve or muscle, in which a neuron or muscle cell regains its ability to transmit another impulse. </u></em>
There are two types of refractory period, that is the absolute refractory period and the relative refractory period. Absolute refractory period is the first part of a refractory period during which, the neuron will not fire again no matter how great the stimulation and this only lasts for a short time.
Relative refractory period occurs when a stronger than usual stimulus is required to trigger the action potential before the neuron returns to resting state.
