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A = N + Z , size 12{A=N+Z} {}

where A size 12{A} {} is also called the mass number    . This name for A size 12{A} {} is logical; the mass of an atom is nearly equal to the mass of its nucleus, since electrons have so little mass. The mass of the nucleus turns out to be nearly equal to the sum of the masses of the protons and neutrons in it, which is proportional to A size 12{A} {} . In this context, it is particularly convenient to express masses in units of u. Both protons and neutrons have masses close to 1 u, and so the mass of an atom is close to A size 12{A} {} u. For example, in an oxygen nucleus with eight protons and eight neutrons, A = 16 size 12{A="16"} {} , and its mass is 16 u. As noticed, the unified atomic mass unit is defined so that a neutral carbon atom (actually a 12 C size 12{"" lSup { size 8{"12"} } C} {} atom) has a mass of exactly 12 u size 12{u} {} . Carbon was chosen as the standard, partly because of its importance in organic chemistry.

Masses of the proton, neutron, and electron
Particle Symbol kg u MeV c 2
Proton p 1 . 67262 × 10 27 size 12{1 "." "67262" times "10" rSup { size 8{ - "27"} } } {} 1.007276 938.27
Neutron n 1 . 67493 × 10 27 size 12{1 "." "67493"´"10" rSup { size 8{-"27"} } } {} 1.008665 939.57
Electron e 9 . 1094 × 10 31 size 12{9 "." "1094"´"10" rSup { size 8{-"31"} } } {} 0.00054858 0.511

Let us look at a few examples of nuclides expressed in the Z A X N notation. The nucleus of the simplest atom, hydrogen, is a single proton, or 1 1 H (the zero for no neutrons is often omitted). To check this symbol, refer to the periodic table—you see that the atomic number Z of hydrogen is 1. Since you are given that there are no neutrons, the mass number A is also 1. Suppose you are told that the helium nucleus or α particle has two protons and two neutrons. You can then see that it is written 2 4 He 2 . There is a scarce form of hydrogen found in nature called deuterium; its nucleus has one proton and one neutron and, hence, twice the mass of common hydrogen. The symbol for deuterium is, thus, 1 2 H 1 (sometimes D is used, as for deuterated water D 2 O ). An even rarer—and radioactive—form of hydrogen is called tritium, since it has a single proton and two neutrons, and it is written 1 3 H 2 . These three varieties of hydrogen have nearly identical chemistries, but the nuclei differ greatly in mass, stability, and other characteristics. Nuclei (such as those of hydrogen) having the same Z size 12{Z} {} and different N size 12{N} {} s are defined to be isotopes    of the same element.

There is some redundancy in the symbols A size 12{A} {} , X size 12{X} {} , Z size 12{Z} {} , and N size 12{N} {} . If the element X size 12{X} {} is known, then Z size 12{Z} {} can be found in a periodic table and is always the same for a given element. If both A size 12{A} {} and X size 12{X} {} are known, then N size 12{N} {} can also be determined (first find Z size 12{Z} {} ; then, N = A Z size 12{N=A-Z} {} ). Thus the simpler notation for nuclides is

A X , size 12{"" lSup { size 8{A} } X} {}

which is sufficient and is most commonly used. For example, in this simpler notation, the three isotopes of hydrogen are 1 H, 2 H, and 3 H, while the α particle is 4 He . We read this backward, saying helium-4 for 4 He , or uranium-238 for 238 U . So for 238 U , should we need to know, we can determine that Z = 92 for uranium from the periodic table, and, thus, N = 238 92 = 146 size 12{N="238"-"92"="146"} {} .

A variety of experiments indicate that a nucleus behaves something like a tightly packed ball of nucleons, as illustrated in [link] . These nucleons have large kinetic energies and, thus, move rapidly in very close contact. Nucleons can be separated by a large force, such as in a collision with another nucleus, but resist strongly being pushed closer together. The most compelling evidence that nucleons are closely packed in a nucleus is that the radius of a nucleus    , r size 12{r} {} , is found to be given approximately by

Questions & Answers

what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
so some one know about replacing silicon atom with phosphorous in semiconductors device?
s. Reply
Yeah, it is a pain to say the least. You basically have to heat the substarte up to around 1000 degrees celcius then pass phosphene gas over top of it, which is explosive and toxic by the way, under very low pressure.
Do you know which machine is used to that process?
how to fabricate graphene ink ?
for screen printed electrodes ?
What is lattice structure?
s. Reply
of graphene you mean?
or in general
in general
Graphene has a hexagonal structure
On having this app for quite a bit time, Haven't realised there's a chat room in it.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Concepts of physics with linear momentum. OpenStax CNX. Aug 11, 2016 Download for free at http://legacy.cnx.org/content/col11960/1.9
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