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Learning objectives

By the end of this section, you will be able to:

  • Define and discuss the nucleus in an atom.
  • Define atomic number.
  • Define and discuss isotopes.
  • Calculate the density of the nucleus.
  • Explain nuclear force.

The information presented in this section supports the following AP® learning objectives and science practices:

  • 3.G.3.1 The student is able to identify the strong force as the force that is responsible for holding the nucleus together. (S.P. 7.2)

What is inside the nucleus? Why are some nuclei stable while others decay? (See [link] .) Why are there different types of decay ( α size 12{α} {} , β size 12{β} {} and γ size 12{γ} {} )? Why are nuclear decay energies so large? Pursuing natural questions like these has led to far more fundamental discoveries than you might imagine.

The first image shows a lump of coal. The second image shows a pair of hands holding a metal uranium disk. Third image shows a cylindrical glass tube containing slivery-brown cesium.
Why is most of the carbon in this coal stable (a), while the uranium in the disk (b) slowly decays over billions of years? Why is cesium in this ampule (c) even less stable than the uranium, decaying in far less than 1/1,000,000 the time? What is the reason uranium and cesium undergo different types of decay ( α size 12{α} {} and β size 12{β} {} , respectively)? (credits: (a) Bresson Thomas, Wikimedia Commons; (b) U.S. Department of Energy; (c) Tomihahndorf, Wikimedia Commons)

We have already identified protons    as the particles that carry positive charge in the nuclei. However, there are actually two types of particles in the nuclei—the proton and the neutron , referred to collectively as nucleons    , the constituents of nuclei. As its name implies, the neutron    is a neutral particle ( q = 0 size 12{q=0} {} ) that has nearly the same mass and intrinsic spin as the proton. [link] compares the masses of protons, neutrons, and electrons. Note how close the proton and neutron masses are, but the neutron is slightly more massive once you look past the third digit. Both nucleons are much more massive than an electron. In fact, m p = 1836 m e size 12{m rSub { size 8{p} } ="1836" m rSub { size 8{e} } } {} (as noted in Medical Applications of Nuclear Physics and m n = 1839 m e size 12{m rSub { size 8{n} } ="1839" m rSub { size 8{e} } } {} .

[link] also gives masses in terms of mass units that are more convenient than kilograms on the atomic and nuclear scale. The first of these is the unified atomic mass    unit (u), defined as

1 u = 1 . 6605 × 10 27 kg. size 12{"1 u"=1 "." "6605"´"10" rSup { size 8{-"27"} } " kg"} {}

This unit is defined so that a neutral carbon 12 C atom has a mass of exactly 12 u. Masses are also expressed in units of MeV/ c 2 . These units are very convenient when considering the conversion of mass into energy (and vice versa), as is so prominent in nuclear processes. Using E = mc 2 size 12{E= ital "mc" rSup { size 8{2} } } {} and units of m size 12{m} {} in MeV/ c 2 size 12{"MeV/"c rSup { size 8{2} } } {} , we find that c 2 size 12{c rSup { size 8{2} } } {} cancels and E size 12{E} {} comes out conveniently in MeV. For example, if the rest mass of a proton is converted entirely into energy, then

E = mc 2 = ( 938.27 MeV/ c 2 ) c 2 = 938.27 MeV. size 12{E= ital "mc" rSup { size 8{2} } = \( "938" "." "27" "MeV/"c rSup { size 8{2} } \) c rSup { size 8{2} } ="938" "." "27"" MeV"} {}

It is useful to note that 1 u of mass converted to energy produces 931.5 MeV, or

1 u = 931.5 MeV/ c 2 . size 12{"1 u"="931" "." 5" MeV/"c rSup { size 8{2} } } {}

All properties of a nucleus are determined by the number of protons and neutrons it has. A specific combination of protons and neutrons is called a nuclide    and is a unique nucleus. The following notation is used to represent a particular nuclide:

Z A X N , size 12{"" lSub { size 8{Z} } lSup { size 8{A} } X rSub { size 8{N} } } {}

where the symbols A size 12{A} {} , X size 12{X} {} , Z size 12{Z} {} , and N size 12{N} {} are defined as follows: The number of protons in a nucleus is the atomic number     Z size 12{Z} {} , as defined in Medical Applications of Nuclear Physics . X is the symbol for the element , such as Ca for calcium. However, once Z size 12{Z} {} is known, the element is known; hence, Z size 12{Z} {} and X are redundant. For example, Z = 20 size 12{Z="20"} {} is always calcium, and calcium always has Z = 20 size 12{Z="20"} {} . N size 12{N} {} is the number of neutrons in a nucleus. In the notation for a nuclide, the subscript N size 12{N} {} is usually omitted. The symbol A size 12{A} {} is defined as the number of nucleons or the total number of protons and neutrons ,

Questions & Answers

In what ways does physics affect the society both positively or negatively
Princewill Reply
how can I read physics...am finding it difficult to understand...pls help
rerry Reply
try to read several books on phy don't just rely one. some authors explain better than other.
Ju
And don't forget to check out YouTube videos on the subject. Videos offer a different visual way to learn easier.
Ju
hope that helps
Ju
I have a exam on 12 february
David Reply
what is velocity
Jiti
the speed of something in a given direction.
Ju
what is a magnitude in physics
Jiti Reply
Propose a force standard different from the example of a stretched spring discussed in the text. Your standard must be capable of producing the same force repeatedly.
Giovani Reply
What is meant by dielectric charge?
It's Reply
what happens to the size of charge if the dielectric is changed?
Brhanu Reply
omega= omega not +alpha t derivation
Provakar Reply
u have to derivate it respected to time ...and as w is the angular velocity uu will relace it with "thita × time""
Abrar
do to be peaceful with any body
Brhanu Reply
the angle subtended at the center of sphere of radius r in steradian is equal to 4 pi how?
Saeed Reply
if for diatonic gas Cv =5R/2 then gamma is equal to 7/5 how?
Saeed
define variable velocity
Ali Reply
displacement in easy way.
Mubashir Reply
binding energy per nucleon
Poonam Reply
why God created humanity
Manuel Reply
Because HE needs someone to dominate the earth (Gen. 1:26)
Olorunfemi
why god made humenity
Ali
and he to multiply
Owofemi
stuff happens
Ju
God plays dice
Ju
Is the object in a conductor or an insulator? Justify your answer. whats the answer to this question? pls need help figure is given above
Jun Reply
ok we can say body is electrically neutral ...conductor this quality is given to most metalls who have free electron in orbital d ...but human doesn't have ...so we re made from insulator or dielectric material ... furthermore, the menirals in our body like k, Fe , cu , zn
Abrar
when we face electric shock these elements work as a conductor that's why we got this shock
Abrar

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Source:  OpenStax, College physics for ap® courses. OpenStax CNX. Nov 04, 2016 Download for free at https://legacy.cnx.org/content/col11844/1.14
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