This slight difference is more pronounced in hydrogen than in heavier elements. Isotopes can be either stable or unstable. Unstable isotopes are the ones which undergo radioactive decay and in the process, change into other elements.
Some unstable isotopes have very short half lives less than a nanosecond , while others have extremely long ones trillions of years. Stable isotopes do not decay over time and tend to be the most commonly found isotopes of any given element.
An example to illustrate the point would be different isotopes of carbon. Common isotopes of carbon include carbon, carbon, and carbon, often written as 12 C, 13 C, and 14 C.
These types of atoms have 6, 7, and 8 neutrons respectively. In most types of nuclear power production, a specific isotope of uranium , U, is used. This is the only naturally occurring fissile nucleus found on Earth although people have made other fissile isotopes of plutonium. In nature, uranium exists as a mixture of U, U, and U. The Energy education team has adapted the following simulation from the University of Colorado. This simulation shows how neutrons and protons sit in energy levels and make up the nucleus.
The number of neutrons and protons maintain particular ratios for the nucleus to be stable. Take care not to confuse mass numbers and relative atomic masses:. For example, the relative atomic mass of chlorine is This is because chlorine contains two different isotopes, chlorine and chlorine Isotopes Atoms of the same element must have the same number of protons , but they can have different numbers of neutrons.
Isotopes of an element have: the same atomic number different mass numbers Three isotopes of hydrogen All hydrogen atoms contain one proton and one electron , but they can contain different numbers of neutrons.
Relative atomic mass curriculum-key-fact. 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.
The complete nuclear symbol for helium-4 is drawn below:. The following nuclear symbols are for a nickel nucleus with 31 neutrons and a uranium nucleus with neutrons. In the nickel nucleus represented above, the atomic number 28 indicates the nucleus contains 28 protons, and therefore, it must contain 31 neutrons in order to have a mass number of The uranium nucleus has 92 protons as do all uranium nuclei and this particular uranium nucleus has neutrons.
Another way of representing isotopes is by adding a hyphen and the mass number to the chemical name or symbol. Thus the two nuclei would be Nickel or Ni and Uranium or U, where 59 and are the mass numbers of the two atoms, respectively. Note that the mass numbers not the number of neutrons are given to the side of the name.
For all atoms with no charge, the number of electrons is equal to the number of protons. To find the number of neutrons, subtract the number of protons from the mass number. Learning Objectives Explain what isotopes are and how an isotope affect an element's atomic mass.
Determine the number of protons, electrons, and neutrons of an element with a given mass number. An Example: Hydrogen Isotopes Hydrogen is an example of an element that has isotopes. What is the atomic number and the mass number of an isotope of lithium containing 4 neutrons? Solution A lithium atom contains 3 protons in its nucleus irrespective of the number of neutrons or electrons.
Stability of Isotopes Atoms need a certain ratio of neutrons to protons to have a stable nucleus. The complete nuclear symbol for helium-4 is drawn below: The following nuclear symbols are for a nickel nucleus with 31 neutrons and a uranium nucleus with neutrons. Summary The number of protons is always the same in atoms of the same element. The number of neutrons can be different, even in atoms of the same element. Atoms of the same element, containing the same number of protons, but different numbers of neutrons, are known as isotopes.
Isotopes of any given element all contain the same number of protons, so they have the same atomic number for example, the atomic number of helium is always 2.
Isotopes of a given element contain different numbers of neutrons, therefore, different isotopes have different mass numbers.
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