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Because this outermost shell is the most important shell, it is given the name “valence shell,” and the electrons in that shell are called “valence electrons.” The word “valence” means “importance.” The valence electrons are the most important electrons in each atom.

Observation 3: successive ionization energies of the atom

There is another more direct way for us to observe the number of valence electrons in each atom and to show that they are all in about the same shell. We can attempt to remove each electron in the valence shell, one after the other, increasing the charge on the ion to higher and higher values. We have discussed the first ionization energy, which is the energy to remove one electron. The second ionization energy is the energy needed to remove an electron from the positive ion to form an ion with a +2 charge. There are also third and fourth ionization energies and beyond:

First ionization energy IE 1 A (g) → A + (g) + e - (g)

Second ionization energy IE 2 A + (g) → A 2+ (g) + e - (g)

Third ionization energy IE 3 A 2+ (g) → A 3+ (g) + e - (g)

Fourth ionization energy IE 4 A 3+ (g) → A 4+ (g) + e - (g)

The experimental data observed when doing these “successive” ionizations is shown in [link] :

Successive ionization energies of the atoms (kj/mol)
Na Mg Al Si P S Cl Ar
IE 1 496 738 578 787 1012 1000 1251 1520
IE 2 4562 1451 1817 1577 1903 2251 2297 2665
IE 3 6912 7733 2745 3231 2912 3361 3822 3931
IE 4 9543 10540 11575 4356 4956 4564 5158 5770
IE 5 13353 13630 14830 16091 6273 7013 6542 7238
IE 6 16610 17995 18376 19784 22233 8495 9458 8781
IE 7 20114 21703 23293 23783 25397 27106 11020 11995

Let’s analyze this data, looking for evidence of the valence shell and the number of valence electrons. First, for every atom listed, IE 2 is always greater than IE 1 , IE 3 is greater than IE 2 , and so forth. This makes sense when we remember that the negatively charged electrons in an atom repel one another. Once an electron has been removed from an atom, the remaining electrons will have lower energy and be harder to remove.

Looking more closely, though, the increases in the ionization energies are not very constant. In Na, IE 2 is greater than IE 1 by a factor of almost nine, but IE 3 is greater than IE 2 by a fraction, and the same is true with the higher ionization energies. This means that the first electron in Na is fairly easy to remove, but the second one is much harder. This means that the first electron removed from Na is in the valence shell, far from the nucleus, but the second electron removed is closer in and more strongly attracted. The third electron removed is harder still to remove, but not much. This means that the third electron removed is in the same shell as the second one. Therefore, Na has only one valence electron.

Now look at Mg. The first electron doesn’t require much energy to remove, although it is more than in Na. The second electron is harder again, but the real change is when we try to remove the third electron. Suddenly we see an increase of a factor of 5 in the ionization energy. This means that Mg has two relatively easily removed electrons, and therefore Mg has two valence electrons. Looking at the data in [link] , we can simply count the number of valence electrons in each atom. (It is hard to do this for Cl and Ar, because it is hard to remove this many electrons from a single atom.)

The shell model of the atom tells us how the electrons are arranged in each atom, but it does not tell us why. We don’t know why the electrons can’t all be added to a single shell, because we don’t know why a shell seems to “fill up.” It is clear from the data that the number of valence electrons in each noble gas is the number needed to fill the valence shell. But we don’t have a reason why this is true. These questions will require further observations and reasoning.

Review and discussion questions

  1. Provide the experimental evidence that reveals the electrons in an atom are grouped into a valence shell and inner shell electrons.
  2. State and explain the evidence that reveals the outer shell of each inert gas is full.
  3. Why does the ionization energy for each successive ionization increase for every atom? Why is the increase from IE 4 to IE 5 in Si much larger than any of the other increases for Si?

By John S. Hutchinson, Rice University, 2011

Questions & Answers

where we get a research paper on Nano chemistry....?
Maira Reply
nanopartical of organic/inorganic / physical chemistry , pdf / thesis / review
what are the products of Nano chemistry?
Maira Reply
There are lots of products of nano chemistry... Like nano coatings.....carbon fiber.. And lots of others..
Even nanotechnology is pretty much all about chemistry... Its the chemistry on quantum or atomic level
no nanotechnology is also a part of physics and maths it requires angle formulas and some pressure regarding concepts
Preparation and Applications of Nanomaterial for Drug Delivery
Hafiz Reply
Application of nanotechnology in medicine
what is variations in raman spectra for nanomaterials
Jyoti Reply
ya I also want to know the raman spectra
I only see partial conversation and what's the question here!
Crow Reply
what about nanotechnology for water purification
RAW Reply
please someone correct me if I'm wrong but I think one can use nanoparticles, specially silver nanoparticles for water treatment.
yes that's correct
I think
Nasa has use it in the 60's, copper as water purification in the moon travel.
nanocopper obvius
what is the stm
Brian Reply
is there industrial application of fullrenes. What is the method to prepare fullrene on large scale.?
industrial application...? mmm I think on the medical side as drug carrier, but you should go deeper on your research, I may be wrong
How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
how nano science is used for hydrophobicity
Do u think that Graphene and Fullrene fiber can be used to make Air Plane body structure the lightest and strongest. Rafiq
what is differents between GO and RGO?
what is simplest way to understand the applications of nano robots used to detect the cancer affected cell of human body.? How this robot is carried to required site of body cell.? what will be the carrier material and how can be detected that correct delivery of drug is done Rafiq
analytical skills graphene is prepared to kill any type viruses .
Any one who tell me about Preparation and application of Nanomaterial for drug Delivery
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.
how did you get the value of 2000N.What calculations are needed to arrive at it
Smarajit Reply
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Source:  OpenStax, Concept development studies in chemistry 2013. OpenStax CNX. Oct 07, 2013 Download for free at http://legacy.cnx.org/content/col11579/1.1
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