The following nuclear symbols are for a nickel nucleus with 31 neutrons and a uranium nucleus with 146 neutrons. Nuclear symbol for helium-4: The element symbol is He, the mass number to the top left is 4, and the atomic number to the bottom left is 2 The complete nuclear symbol for helium-4 is drawn below: To write a nuclear symbol, the mass number is placed at the upper left (superscript) of the chemical symbol and the atomic number is placed at the lower left (subscript) of the symbol. These two ways include writing a nuclear symbol or by giving the name of the element with the mass number written. It is important to note that the mass number is not given on the periodic table. There are two main ways in which scientists frequently show the mass number of an atom they are interested in. Luckily, aside from having different masses, most other properties of different isotopes are similar. For most of our purposes in chemistry, we will do the same thing and deal with the average mass of the atoms. As a result, when he performed his measurements, he was actually observing the averaged properties of all the different isotopes in the sample. In a chunk of lithium, \(93\%\) will always be lithium with 4 neutrons, while the remaining \(7\%\) will always be lithium with 3 neutrons.ĭalton always experimented with large chunks of an element-chunks that contained all of the naturally occurring isotopes of that element. Moreover, it always contains the two in the same relative amounts (or "relative abundance"). In other words, a piece of lithium always contains both types of naturally occurring lithium (the type with 3 neutrons and the type with 4 neutrons). But if atoms of a given element can have different numbers of neutrons, then they can have different masses as well! How did Dalton miss this? It turns out that elements found in nature exist as constant uniform mixtures of their naturally occurring isotopes.
According to Dalton, atoms of a given element are identical. This whole discussion of isotopes brings us back to Dalton's Atomic Theory. In contrast, the carbon isotopes called carbon-12 and carbon-13 are stable. The isotope of carbon known as carbon-14 is an example of a radioisotope. This is why radioactive isotopes are dangerous and why working with them requires special suits for protection. When they decay, they release particles that may be harmful. Many isotopes have radioactive nuclei, and these isotopes are referred to as radioisotopes. This process is called radioactive decay. 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. 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.Ītoms need a certain ratio of neutrons to protons to have a stable nucleus. The mass number, however, is 6 in the isotope with 3 neutrons, and 7 in the isotope with 4 neutrons. Notice that because the lithium atom always has 3 protons, the atomic number for lithium is always 3.
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