Atomic number, atomic mass, and isotopes (article) | Khan Academy
Each nucleus can be characterized by two numbers: A, the atomic mass and the repulsive electrostatic force between protons has interesting. Alkali metals are white, highly reactive substances cut easily by a knife. All six are found in Group I of the periodic table, which lists elements in. Radioactivity is the release of energy and matter that results from changes in the nucleus Elements with atomic numbers of 83 and less, have isotopes (stable.
During radioactive decay, principles of conservation apply. Some of these we've looked at already, but the last is a new one: There are three common types of radioactive decay, alpha, beta, and gamma.Radioactivity: Expect the unexpected - Steve Weatherall
The difference between them is the particle emitted by the nucleus during the decay process. Alpha decay In alpha decay, the nucleus emits an alpha particle; an alpha particle is essentially a helium nucleus, so it's a group of two protons and two neutrons. A helium nucleus is very stable.
An example of an alpha decay involves uranium The process of transforming one element to another is known as transmutation. Alpha particles do not travel far in air before being absorbed; this makes them very safe for use in smoke detectors, a common household item.
Beta decay A beta particle is often an electron, but can also be a positron, a positively-charged particle that is the anti-matter equivalent of the electron.
If an electron is involved, the number of neutrons in the nucleus decreases by one and the number of protons increases by one. An example of such a process is: In terms of safety, beta particles are much more penetrating than alpha particles, but much less than gamma particles. Gamma decay The third class of radioactive decay is gamma decay, in which the nucleus changes from a higher-level energy state to a lower level.
Similar to the energy levels for electrons in the atom, the nucleus has energy levels. The concepts of shells, and more stable nuclei having filled shells, apply to the nucleus as well. When an electron changes levels, the energy involved is usually a few eV, so a visible or ultraviolet photon is emitted.
Radioactivity, Atomic Structure, Atomic Number and Atomic Mass - Pass My Exams
In the nucleus, energy differences between levels are much larger, typically a few hundred keV, so the photon emitted is a gamma ray. Gamma rays are very penetrating; they can be most efficiently absorbed by a relatively thick layer of high-density material such as lead. A list of known nuclei and their properties can be found in the on-line chart of the nuclides. Radioactivity Making a precise prediction of when an individual nucleus will decay is not possible; however, radioactive decay is governed by statistics, so it is very easy to predict the decay pattern of a large number of radioactive nuclei.
The rate at which nuclei decay is proportional to N, the number of nuclei there are: Whenever the rate at which something occurs is proportional to the number of objects, the number of objects will follow an exponential decay. In other words, the equation telling you how many objects there are at a particular time looks like this: The decay constant is closely related to the half-life, which is the time it takes for half of the material to decay.
If you want to calculate how many neutrons an atom has, you can simply subtract the number of protons, or atomic number, from the mass number.
The atomic mass of a single atom is simply its total mass and is typically expressed in atomic mass units or amu. By definition, an atom of carbon with six neutrons, carbon, has an atomic mass of 12 amu.
In general, though, an atom's atomic mass will be very close to its mass number, but will have some deviation in the decimal places. The relative atomic mass is an average of the atomic masses of all the different isotopes in a sample, with each isotope's contribution to the average determined by how big a fraction of the sample it makes up.
Relationship Between the Atomic Number and the Chemical Reactivity of Alkali Metals | Sciencing
The relative atomic masses given in periodic table entries—like the one for hydrogen, below—are calculated for all the naturally occurring isotopes of each element, weighted by the abundance of those isotopes on earth.
Extraterrestrial objects, like asteroids or meteors, might have very different isotope abundances. Isotopes and radioactive decay As mentioned above, isotopes are different forms of an element that have the same number of protons but different numbers of neutrons. Many elements—such as carbon, potassium, and uranium—have multiple naturally occurring isotopes.
A neutral atom of Carbon contains six protons, six neutrons, and six electrons; therefore, it has a mass number of 12 six protons plus six neutrons.
Neutral carbon contains six protons, eight neutrons, and six electrons; its mass number is 14 six protons plus eight neutrons. These two alternate forms of carbon are isotopes. Some isotopes are stable, but others can emit, or kick out, subatomic particles to reach a more stable, lower-energy, configuration.
Beta decay Beta decay itself comes in two kinds: Gamma decay After a nucleus undergoes alpha or beta decay, it is often left in an excited state with excess energy.
Just as an electron can move to a lower energy state by emitting a photon somewhere in the ultraviolet to infrared range, an atomic nucleus loses energy by emitting a gamma ray.
Gamma radiation is the most penetrating of the three, and will travel through several centimetres of lead. Beta particles will be absorbed by a few millimetres of aluminium, while alpha particles will be stopped in their tracks be a few centimetres of air, or a sheet of paper — although this type of radiation does the most damage to materials it hits.
Half-lives and probability Radioactive decay is determined by quantum mechanics — which is inherently probabilistic. The half-life of a radioactive isotope is the time after which, on average, half of the original material will have decayed.
After two half-lives, half of that will have decayed again and a quarter of the original material will remain, and so on. Uranium and plutonium are only weakly radioactive but have very long half-lives — in the case of uranium, around four billion years, roughly the same as the current age of the Earth, or the estimated remaining lifetime of the Sun. So half of the uranium around now will still be here when the Sun dies.
Other radioisotopes of iodine are even shorter-lived. Caesium, however, sticks around for longer. It has a half-life of around 30 years, and, because of this and because it decays via the more hazardous beta process, is thought to be the greatest health risk if leaked into the environment. In the background There is a natural level of radiation all around us, which comes from several sources. Some gamma radiation comes from space as cosmic rays.