Electronic Structure of the Atom


Chemical Bonding

Structure of the Atom

In general chemistry, we focus on three subatomic particles that make up all atoms.  These are the electron, proton, and neutron.  We will not find it necessary to sort out the "particle zoo" that is the realm of particle physics.  We will find that the chemistry we look at can be explained by the 3 particles mentioned above, and that most of the time, we focus our attention on the electron.

In the atom, the protons and neutrons are found in the center to form what we call the nucleus, while the electrons are found moving around in the space surrounding the nucleus.  Each proton is said to carry 1 unit of positive charge, while, neutrons, as the name suggests, are electrically neutral.  The electrons each carry one unit of negative charge.  Thus, the nucleus is positively charged, and the negative charge is distributed around the nucleus to fill what we might call the volume of the atom.

The element to which an atom belongs is characterized by the number of protons in its nucleus.  This is called the atomic number.  Two elements can belong to the same element and yet differ in the number of neutrons.  These are called isotopes.  Figure 1 shows the the arrangement of the particles in hydrogen atoms.

The above drawings are called "Bohr Model Drawings" because they suggest that the electron orbits around the nucleus of the atom much like the planets of our solar system orbit around the sun, as the Danish physist Niels Bohr proposed.  We now know that the motion of the electrons around the nucleus is not so predictable.  We can not precisely locate the electrons, but we must instead speak of the probability of finding an electron within a particular region of space.  Nevertheless, there is still much we can explain proceeding from nothing more than an "old" Bohr model, so I have decided to develop as much of the theory as possible using this model (because it is simpler) and then develop the quantum mechanical description as it is needed.  The "Lewis Diagrams" -- which are still frequently used in general chemistry, and which we shall learn how to draw -- are essentially just abbreviated Bohr Model drawings.

Hydrogen atoms are the simplest atoms, having only a single proton in the nucleus.  Other elements, of course, have more than one proton, and generally consist of more than one nuclide -- that is, they exist as isotopes.  For simplicity, however, I will only draw one of the element's nuclides.  Our principle focus here is going to be on the arrangement of the electrons, not the number of isotopes, or the number of neutorns in those isotopes.

In the Bohr model, the electrons are pictured as orbiting around the nucleus like planets around the sun.  But unlike our solar system, where each planet has its own orbit, in the Bohr model of the atom, more than one electron can share the same "orbit".  These "orbits" are usually called "shells".  The first shell can hold a maximum of 2 electrons, so we can draw an electrically neutral helium atom (atomic number 2) like this:

When we get to the element lithium (atomic number 3) we need a second shell to hold the third electron, since the first shell can only hold 2 electrons.  Shown below is a Bohr model drawing of a lithium atom with a mass number of 7.

At this point, you're probably wondering how to know when to open up another shell to contain additional electrons.  There are two shell capacities we need to be concerned with.  One is the absolute maximum capacity, and the other is the valence shell capacity.  The absolute maximum capacity of a shell is, of course, the largest number of electrons that can ever fit in that shell at the same time.  The valence shell capacity is normally 8, but is 2 for the first shell.  The valence shell is the highest (that is, most distant from the nucleus) shell that contains electrons.  In figures 1 and 2, only the first shell was used, so it was the valence shell.  All the other shells were empty, so were not drawn.  Likewise, in Figure 3, shells 3 and up are not drawn, because they are empty.  The valence shell in the lithium atom is the second shell.  We number the shells from the inside out, so the shell closest to the nucleus is shell number 1, the next one is shell number 2 and so on.  We usually use the variable n to represent the shell number, and speak of, for example, the n=1 shell (read "n equals one shell").  With this as background, I present the following table of electron capacities:
1 2 2
2 8 8
3 18 8
4 32 8
n 2n2 8

Notice that the higher the shell, the larger its absolute maximum electron capacity.  However, when a shell is a valence shell, it normally holds no more than 8 electrons, regardless of the capacity it would otherwise have.  This is referred to as the octet rule.  A notable exception to the octet rule is the first shell.  Its absolute maximum electron capacity is only 2 electrons, which applies also when it is a valence shell.  (We might call the situation for the first shell, the duet rule.)  But the octet rule has an even broader application than merely pointing out that valence shells are normally full when they contain 8 electrons.  As we will see, atoms are most stable when they have full valence shells, and the resulting tendency to obtain such arrangements is the driving force behind chemical reactions, and explains why elements combine in the proportions they do.

The table above gives the absolute maximum electron capacities for the first 4 shells.  The last entry in the table is a generic one, which shows how the other entries in the table were calculated.  The absolute maximum electron capacity is twice the square of the shell number.  Try taking the shell number, squaring it, and multiplying by 2, and you will indeed reproduce the entries shown as absolute maximum capacities for shells 1 through 4.

It is primarily the number of electrons in an atom's valence shell that determines the atom's chemical properties.  The number of electrons the atom already has (in its valence shell) determines how it will obtain a full shell.  Thus, two different atoms (that is, belonging to different elements) will behave similarly if they have the same number of valence electrons.

The periodic table is a listing of the elements based on similarity of chemical properties.  The Russian chemist Dmitri Mendeleev was the first to publish what has evolved into our modern periodic table.  Mendeleev listed the known elements in order of increasing atomic weight.  Rather than listing them in a continuous row, he broke the row into columns so that elements with similar properties fell in the same column.  His assumption at the time was that when listed in order of increasing atomic weight, elements with similar chemical properties would occur at regular intervals.  This was referred to as the periodic law.  In order to make this work, however, Mendeleev found he sometimes had to leave gaps in his table.  He had to do this to make elements with similar properties fall in the same column.  Consider, for example, the following excerpt of the table as it might have appearred in his day.
1 H 1
2 Li 7 Be 9 B 11 C 12 N 14 O 16 F 19
3 Na 23 Mg 24 Al 27 Si 28 P 31 S 32 Cl 35
4 K 39 Ca 40 ?   ?? Ti 48 V 51 Cr 52 Mn 55
5 Cu 64 Zn 65 ?   ?? ?   ?? As 75 Se 79 Br 80
6 Rb 85 Sr 87 Y 89 Zr 91 Nb 93 Mo 96 ?   ??
7 Ag 108 Cd 112 In 115 Sn 119 Sb 122 Te 128 I 127

Atomic weights of the elements are shown along with the symbols, to illustrate that the elements have been placed in the table in order of increasing atomic weight.  Notice the gaps (highlighted in yellow) that sometimes had to be left in the table.  Consider row 5, for example.  Even though no elements with atomic weights between zinc (Zn) and arsenic (As) were known at the time, Mendeleev did not place arsenic immediately after zinc.  This would have placed arsenic in the same column as boron and aluminum, but arsenic is not chemically similar to these elements.  Based on its chemical properties, arsenic belongs in the same column as nitrogen and phosphorous, so Mendeleev left two gaps in the table, and placed arsenic where its chemical properties indicated it should go.  He reasoned that the gaps belonged to elements that had not been discovered yet.  Based on the regular way in which the properties change as one goes through the table, he was able to predict the properties of the missing elements.  When the missing elements were discovered, it was found that his predictions were generally quite accurate, which lead to widespread acceptance of the table.

Another problem with Mendeleev's table -- more disturbing than the missing elements (yellow cells) -- was that sometimes elements had to be deliberately placed out of order to make the elements fall in the columns that were consistent with their chemical properties (red cells).  Notice in row 7, that if listing the elements strictly on the basis of atomic weight, we would be forced to put I in column VI and Te in column VII.  But this would put the elements in columns that do not match their observed chemical properties.  So Mendeleev temporarily violated his own periodic law, and placed the elements in the order that placed them in the proper columns, with regard to their chemical properties.  At the time, Mendeleev assumed that the atomic weights of tellurium and iodine had been incorrectly measured (they are close, after all) and he believed that a "correct" measurement would reveal that the atomic of tellurium was less than that of iodine.  But that was not the case.  We know today that the elements must be listed in order of increasing atomic number, not increasing atomic weight.  When this is done, discrepancies like that seen with tellurium and iodine disappear, and all elements naturally fall into their rightful place in the periodic table.

The arrangement of elements in the periodic table is based on the chemical properties of the elements.  The chemical properties, in turn, are determined mainly by the number of electrons in the valence shell.  One reason we can be so confident in our modern understanding of the electronic structure of the atom is that it explains the periodic table so well.  The next several pictures show Bohr model drawings of atoms, and an outline of the modern periodic table, with the element's position highlighted.

In the above series of pictures, we see that as electrons are added to the atom's valence shell, the element's position moves to the right in the periodic table.  When an atom's shell becomes full, the atom's positon has reached the end of the row.  Not counting the transition elements (the 10 shorter columns in the middle of the table) there are 8 columns.  The elements in the first 2 columns and the last 6 columns are called main group elements, or representative elements.  These are elements in which electrons are being added to the valence shell.  With the exception of the first shell, valence shells can hold 8 electrons, and are considered full when they have this number.  This explains why the main group elements are 8 columns wide.

When we begin adding electrons to the third shell, we encounter, for the first time, a shell for which the absolute maximum electron capacity is larger than the valence shell capacity.  While the n=3 shell is the valence shell, it can hold only 8 electrons, but when a pair of electrons is present in the n=4 shell, the n=3 shell gains the ability to hold an additional 10 electrons, reaching its true limit of 18 electrons total.  The third row in the periodic table fills out in the same manner as the second row.

aluminum and the periodic table

silicon and the periodic table

phosphorous and the periodc table

sulfur and the periodic table

chlorine and the periodic table

argon and the periodic table

In the above series of pictures, electrons were added one by one to the third shell, as we moved across the third row of the periodic table.  When we reached 8 electrons in this shell, the shell was "full" because as a valence shell, that's all it holds.  The next electron we place in the atom will have to go into the fourth shell, so once the next electron is added, the fourth shell -- not the third shell -- will be the valence shell.  For this reason, you might think that immediately after the first electron enters the fourth shell, the third shell would continue filling.  However, it turns out that electrons tend to be dealt with in pairs, so we will actually need TWO electrons in the fourth shell before the third shell begins filling again.

potassium and the periodic table

calcium and the periodic table

Now that the fourth shell has a pair of electrons, the third shell "recognizes" its true capacity of 18 electrons.  Since it only has 8 electrons at this point, it begins filling again.  It can take on 10 more electrons to reach its true capacity.  Notice that we are about to enter a section of 10 shorter columns in the periodic table.  This part of the table did not exist in rows 1, 2, and 3.  Notice that we had a huge gap between columns 2 and 3 in those rows.  This gap is filled in rows 4 and below with the transition elements.  These are elements in which electrons are being added to the shell immediately behind the valence shell, rather than to the valence shell itself.  Thus, for the transition elements in row 4, electrons are being added to shell 3, and for the transition elements in row 5, electrons are being added to shell 4, and so on.

You might be wondering what happens to the numbering of columns at this point.  We had previously referred to the colum that contains the element boron (B) as column 3.  Now that we have to begin considering the transition elements, does boron's column become column 13?  While we sometimes number all the columns sequentially in this manner, it is often more useful  to consider them separately and distinguish them by letters.  In our general chemistry courses here at Palo Alto College, we most often designate the 8 columns we considered from early on with the letter A, and the group of 10 that we have just now begun to consider with the letter B.  Unfortunately, the use of the letters A and B is not universally recognized.  However, we can eliminate confusion by referring to the elements in the "A-groups" as main group elements, also called representative elements.

scandium and the periodic table

titanium and the periodic table

vanadium and the periodic table

So far, the electronic structure of atoms has followed a pattern without exception.  It would be nice if this were the case with all the remaining elements, but unfortunately, there are a number of elements that break the pattern, and chromium is the first of several exceptions.  In the transition elements so far considered, each new element in sequence adds an additional electron to the third shell, and the fourth shell (the valence shell for the series of elements we are now considering) is left alone.  In chromium, we can think of the situation this way: when we add a fourth electron above the octet in the third shell, one of the valence electrons drops from the fourth shell to the third shell.  We end up with 5 electrons above an octet (rather than 4) in the third shell, and only 1 valence electron.  The predicted and experimental electronic structures are shown in Figures 6(f) and 6(g) respectively.

the predicted electronic structure of chromium

the experimentally determined electronic structure of chromium

For the remainder of the transition elements in the fourth row, all except copper follow the expected pattern.  Copper deviates in the same manner as chromium -- that is, a fourth shell (valence shell) electron shifts to the third shell.  The electronic structures are shown in the following figures.  For copper, only the actual electronic structure is shown, but it is noted in the figure that the structure deviates from that predicted by location in the periodic table.

manganese and the periodic table

iron and the periodic table

cobalt and the periodic table

nickel and the periodic table

copper and the periodic table

zinc and the periodic table

With the element zinc (Figure 6m) the third shell has been filled to capacity.  As we continue to add electrons to the atom, the fourth shell (valence shell) begins filling again.  It currently has 2 electrons, and will fill to 8 in the manner expected.  Upon receiving its 8th electron, it will be temporarily "full" (because it is a valence shell) and the 5th shell will start to fill.  The next series of figures shows the filling of the fourth shell to 8 electrons.

gallium and the periodic table

germanium and the periodic table

arsenic and the periodic table

selenium and the periodic table

bromine and the periodic table

krypton and the periodic table

The fourth shell, since it is the valence shell, is now "full" with 8 electrons.  Its true capacity is 32 electrons, so it can hold 24 more electrons than it now holds.  However, the fifth shell must take on a pair of electrons before the fourth shell "realizes" that it can hold more electrons.  After the fifth shell takes on a pair of electrons, the fourth shell will take on 10 more electrons as we proceed across the block of transition elements, which we have seen, is 10 elements wide.  But that takes care of only 10 electrons of the 24 additional electrons we said this shell could hold.  There are still 14 more electrons that the fourth shell can hold.  It will acquire those electrons as we go across the block of inner-transition elements.  Notice the two long rows of elements below the main body of the periodic table.  If you count the number of columns, you will find that there are 14 -- accounting for the remaining electron capacity of the fourth shell.  Earlier, we saw that in the transition elements, electrons are being added to the shell immediately behind the valence shell, rather than to the valence shell itself.  The inner-transition elements are those elements in which electrons are being added to a shell that is TWO shells behind the valence shell.  Thus, the final 14 electrons will not be added to the fourth shell until the sixth shell takes on a pair of electrons.  We will see this as we proceed through the rest of the periodic table.

rubidium and the periodic table

strontium and the periodic table

The fifth shell now has a pair of electrons, and the fourth shell begins filling again.  This occurs as we go across the transition elements in the fifth row of the table (remember, in the transition elements, electrons are being added to the shell behind the valence shell).  Six of the 10 elements in this row have electron configurations that would not be predicted from their locations in the periodic table.  As before, the discrepancies result from valence shell electrons shifting to the shell immediately behind the valence shell.  In this case, it means fifth shell electrons shifting to the fourth shell.  In what follows, I present the actual electron configurations as indicated by experimental evidence and note in the caption that the configuration is not the expected one.

yttrium and the periodic table

zirconium and the periodic table

Niobium and the periodic table

Molybdenum and the periodic table

Technetium and the periodic table

Ruthenium and the periodic table

Rhodium and the periodic table

Palladium and the periodic table

Silver and the periodic table

Cadmium and the periodic table

The fourth shell now contains 18 electrons.  It can hold 32 electrons, which leaves room for 14 more, but it will not acquire those until the sixth shell takes on a pair of electrons.  The 14 remaining electrons are "inner-transition electrons", which means they are added to a shell two shells behind the valence shell.  Before the sixth shell can begin taking electrons, we must fill the fifth shell up to an octet, which makes it temporarily full (since it is currently a valence shell).  The next set of pictures shows the Bohr model drawings and periodic table locations for indium (In) through xenon (Xe).

Indium and the Periodic Table

Tin and the periodic table

Antimony and the periodic table

Tellurium and the periodic table

Iodine and the periodic table

Xenon and the periodic table

The fifth shell (because it is still a valence shell) is now temporarily full with 8 electrons.  The next 2 electrons (because they tend to come in pairs) will enter the sixth shell.  After that, the general feature is that the fourth shell will fill, taking the remaining 14 electrons needed to fill it to capacity, followed by filling the fifth shell from its current 8 electrons to 18 electrons.  However, there will be a few exceptions to this general pattern.  As we consider the first row of inner-transition elements (the lanthanide series, so called because it follows the element lanthanum), we will see that the exceptions to the pattern arise from a shift of fourth shell electrons to the fifth shell.  In other words, the fifth shell does not always "wait" for the fourth shell to fill before taking on additional electrons.  There will be some configurations in which an electron has been removed from the fourth shell and added to the fifth shell.  These exceptions (like the pattern exceptions we have seen previously) arise because the electron energy levels in question are extremely close together.  As a result, subtle effects can make a difference in which energy level is higher.  The unifying principle in all of this is that electrons will always fill the lower energy states before the higher ones.  The apparent random disruption of what would otherwise be a very nice predictable patters is simply nature's way of ensuring that the set of electrons has the minimum possible energy.

The next set of pictures will take us through the lanthanide series of inner transtion elements.