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:
|SHELL NUMBER (n)||ABSOLUTE MAX CAPACITY||VALENCE CAPACITY|
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.
|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.
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.
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.
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.
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.
The next set of pictures will take us through the lanthanide series
of inner transtion elements.