The periodic table. An atom that is not electrically balanced, that is, an atom with a different number of electrons and protons, is called an ion, and its charge depends on the lack or excess in the number of electrons in comparison with the number of protons. They all have one proton, but deuterium also has a neutron, and tritium has two neutrons. One element that is well known, due to its isotopes and use in nuclear processes, is uranium, marked U, which has six main isotopes.
They all have 92 protons, of course, but differ in the number of neutrons. But some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in their numbers of neutrons are called isotopes. Many isotopes occur naturally. Usually one or two isotopes of an element are the most stable and common.
Different isotopes of an element generally have the same physical and chemical properties because they have the same numbers of protons and electrons. Hydrogen is an example of an element that has isotopes.
Most hydrogen atoms have just one proton, one electron, and lack a neutron. These atoms are just called hydrogen. Some hydrogen atoms have one neutron as well. These atoms are the isotope named deuterium. Other hydrogen atoms have two neutrons. These atoms are the isotope named tritium. For most elements other than hydrogen, isotopes are named for their mass number. These atoms are the isotope called carbon A lithium atom contains 3 protons in its nucleus irrespective of the number of neutrons or electrons.
Notice that because the lithium atom always has 3 protons, the atomic number for lithium is always 3. The mass number, however, is 6 in the isotope with 3 neutrons, and 7 in the isotope with 4 neutrons.
In nature, only certain isotopes exist. For instance, lithium exists as an isotope with 3 neutrons, and as an isotope with 4 neutrons, but it doesn't exist as an isotope with 2 neutrons or as an isotope with 5 neutrons. Atoms need a certain ratio of neutrons to protons to have a stable nucleus. Having too many or too few neutrons relative to protons results in an unstable, or radioactive, nucleus that will sooner or later break down to a more stable form. Isotopes of the same element will have the same atomic number but different mass numbers.
Atomic number, chemical symbol, and mass number : Carbon has an atomic number of six, and two stable isotopes with mass numbers of twelve and thirteen, respectively.
Its average atomic mass is Scientists determine the atomic mass by calculating the mean of the mass numbers for its naturally-occurring isotopes. Often, the resulting number contains a decimal. For example, the atomic mass of chlorine Cl is Given an atomic number Z and mass number A , you can find the number of protons, neutrons, and electrons in a neutral atom.
Isotopes are various forms of an element that have the same number of protons, but a different number of neutrons. Isotopes are various forms of an element that have the same number of protons but a different number of neutrons.
Some elements, such as carbon, potassium, and uranium, have multiple naturally-occurring isotopes. Isotopes are defined first by their element and then by the sum of the protons and neutrons present. While the mass of individual isotopes is different, their physical and chemical properties remain mostly unchanged.
Isotopes do differ in their stability. Carbon 12 C is the most abundant of the carbon isotopes, accounting for Carbon 14 C is unstable and only occurs in trace amounts. Neutrons, protons, and positrons can also be emitted and electrons can be captured to attain a more stable atomic configuration lower level of potential energy through a process called radioactive decay.
The new atoms created may be in a high energy state and emit gamma rays which lowers the energy but alone does not change the atom into another isotope. These atoms are called radioactive isotopes or radioisotopes. Carbon is normally present in the atmosphere in the form of gaseous compounds like carbon dioxide and methane. Carbon 14 C is a naturally-occurring radioisotope that is created from atmospheric 14 N nitrogen by the addition of a neutron and the loss of a proton, which is caused by cosmic rays.
This is a continuous process so more 14 C is always being created in the atmosphere. Once produced, the 14 C often combines with the oxygen in the atmosphere to form carbon dioxide. Carbon dioxide produced in this way diffuses in the atmosphere, is dissolved in the ocean, and is incorporated by plants via photosynthesis. Animals eat the plants and, ultimately, the radiocarbon is distributed throughout the biosphere.
In living organisms, the relative amount of 14 C in their body is approximately equal to the concentration of 14 C in the atmosphere. When an organism dies, it is no longer ingesting 14 C, so the ratio between 14 C and 12 C will decline as 14 C gradually decays back to 14 N. This slow process, which is called beta decay, releases energy through the emission of electrons from the nucleus or positrons.
After approximately 5, years, half of the starting concentration of 14 C will have been converted back to 14 N. This is referred to as its half-life, or the time it takes for half of the original concentration of an isotope to decay back to its more stable form.
Because the half-life of 14 C is long, it is used to date formerly-living objects such as old bones or wood. Comparing the ratio of the 14 C concentration found in an object to the amount of 14 C in the atmosphere, the amount of the isotope that has not yet decayed can be determined. On the basis of this amount, the age of the material can be accurately calculated, as long as the material is believed to be less than 50, years old.
This technique is called radiocarbon dating, or carbon dating for short.
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