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Learning Objectives

define what isotopes are and also how one isotope affect an element"s atomic mass. Determine the number of protons, electrons, and neutrons that an aspect with a provided mass number.

You are watching: How do isotopes of a given element differ?

All atoms of the same facet have the same variety of protons, however some might have various numbers of neutrons. Because that example, every carbon atoms have actually six protons, and most have actually six neutrons together well. Yet some carbon atoms have seven or eight neutrons instead of the normal six. Atoms of the same aspect that different in your numbers of neutrons are called isotopes. Many isotopes occur naturally. Normally one or two isotopes the an aspect are the most stable and common. Various isotopes of an element generally have actually the exact same physical and historicsweetsballroom.comical properties because they have actually the very same numbers of protons and electrons.

An Example: Hydrogen Isotopes

Hydrogen is an example of an facet that has isotopes. 3 isotopes the hydrogen space modeled in number \(\PageIndex1\). Many hydrogen atoms have just one proton, one electron, and also lack a neutron. This atoms room just called hydrogen. Part hydrogen atoms have actually one neutron as well. This atoms are the isotope named deuterium. Other hydrogen atoms have two neutrons. This atoms space the isotope named tritium.

Figure \(\PageIndex1\): The three most stable isotope of hydrogen: protium (A = 1), deuterium (A = 2), and also tritium (A = 3). (CC SA-BY 3.0; Balajijagadesh).

For most elements other than hydrogen, isotopes are named for their mass number. For example, carbon atoms v the usual 6 neutrons have actually a mass number of 12 (6 protons + 6 neutrons = 12), so they are referred to as carbon-12. Carbon atoms with 7 neutrons have actually an atomic mass that 13 (6 proton + 7 neutrons = 13). This atoms are the isotope referred to as carbon-13.

Stability that Isotopes

Atoms require a specific ratio of neutron to protons to have actually a steady nucleus. Having too many or too couple of neutrons loved one to protons results in one unstable, or radioactive, nucleus that will sooner or later breakdown to a an ext stable form. This procedure is called radioactive decay. Countless isotopes have actually radioactive nuclei, and also these isotope are described as radioisotopes. Once they decay, they release particles that may be harmful. This is why radioactive isotopes space dangerous and why working with them requires special suits for protection. The isotope the carbon well-known as carbon-14 is an instance of a radioisotope. In contrast, the carbon isotopes called carbon-12 and also carbon-13 are stable.

This whole conversation of isotopes brings us ago to Dalton"s atomic Theory. Follow to Dalton, atoms of a given aspect are identical. But if atom of a given facet can have different numbers the neutrons, then they have the right to have different masses together well! how did Dalton miss out on this? It turns out that facets found in nature exist as constant uniform mixtures of their naturally emerging isotopes. In various other words, a piece of lithium constantly contains both species of naturally emerging lithium (the form with 3 neutrons and the type with 4 neutrons). Moreover, it constantly contains the 2 in the exact same relative quantities (or "relative abundance"). In a chunk of lithium, \(93\%\) will constantly be lithium with 4 neutrons, if the continuing to be \(7\%\) will always be lithium through 3 neutrons.

Dalton always experimented with big chunks of one element—chunks that contained all of the naturally arising isotopes of that element. Together a result, as soon as he carry out his measurements, he was in reality observing the average properties of all the different isotopes in the sample. For many of our objectives in historicsweetsballroom.comistry, we will perform the exact same thing and resolve the median mass the the atoms. Luckily, as well as having various masses, most other properties of different isotopes space similar.

There space two main ways in which scientists commonly show the mass number of an atom they room interested in. The is important to keep in mind that the fixed number is not provided on the periodic table. These two ways incorporate writing a nuclear symbol or by providing the surname of the aspect with the massive number written.

To compose a nuclear symbol, the massive number is put at the upper left (superscript) the the historicsweetsballroom.comical symbol and the atomic number is put at the lower left (subscript) that the symbol. The finish nuclear symbol because that helium-4 is drawn below:


The following nuclear symbols space for a nickel nucleus v 31 neutrons and also a uranium nucleus v 146 neutrons.


\< \ce ^238_92U\>

In the nickel nucleus stood for above, the atomic number 28 shows that the nucleus contains 28 protons, and therefore, it need to contain 31 neutrons in order to have a mass variety of 59. The uranium nucleus has 92 protons, together all uranium nuclei do; and also this particular uranium nucleus has 146 neutrons.

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Another way of representing isotopes is by including a hyphen and also the mass number to the historicsweetsballroom.comical surname or symbol. For this reason the two nuclei would certainly be Nickel-59 or Ni-59 and also Uranium-238 or U-238, whereby 59 and 238 are the mass numbers of the two atoms, respectively. Keep in mind that the mass numbers (not the variety of neutrons) are provided to the side of the name.


Exercise \(\PageIndex3\)

How plenty of protons, electrons, and also neutrons are in each atom?

\(^60_27\ceCo\) Na-24 \(^45_20\ceCa\) Sr-90 Answer a: 27 protons, 27 electrons, 33 neutrons Answer b: 11 protons, 11 electrons, 13 neutron Answer c: 20 protons, 20 electrons, 25 neutron Answer d: 38 protons, 38 electrons, 52 neutrons