Themes > Science > Chemistry > General Chemistry > Elements > Periodic Table of the Elements > What is the Periodic Law and how was it formulated?
Mendeleev, a Russian Chemist, was one of the first to be partially sucessful
in arranging the known elements in the 1870's into a chart that would allow the
prediction of properties. He arranged the elements known in those days according
to increasing atomic masses. The first Periodic Law proposed by him stated:
"The properties of the elements are a periodic function of their atomic masses"
When Mendeleev arranged the known elements using this principle as a guide, he found that certain elements grouped themselves into vertical groupings sometimes called families. If one measured a property of each of the elements in the group, Mendeleev noted that the value for that property would either be high or low depending upon the group under observation. For example, measuring the atomic radius of the atoms of elements which is the distance from the outermost valence region to the nucleus of an atom, one would find that in the case of the first group of elements on the left side of the chart all the elements seemed to have characteristically high values where those elements grouped on the right side of the chart seemed to have characteristically small radii values. All of the elements in group 1 had the highest radius value of any element in its respective period. A period is a horizontal row of elements in the table. Likewise all the elements in group 17 on the far right of the chart had the lowest radius value of any element in its respective period. This is a manifestation of the periodicity exhibited by the elements between their atomic weights and their property measurements. One could conceivably show this periodicity using other properties such as boiling points, melting points, etc.
There were some inconsistencies in the arrangement of the elements according to his law, however it wasn't until the early 1900's (1914) that a Prof Moseley, a British Physicist, was able to determine the atomic numbers of all the known elements using an experimental technique. Moseley then proceeded to rearrange the elements according to increasing atomic numbers. Moseley's arrangement seemed to clear up the contradictions and inconsistencies of Mendeleev's arrangement, but Moseley based his arrangement on atomic numbers and not atomic masses.
Moseley's periodic law is now considered the current Periodic Law. It resulted in a slight alteration of Mendeleev's arrangement, but the slight difference was enough to correct the inconsistencies that existed in Mendeleev's arrangement.
The elements are arranged in vertical columns known as Groups. The elements in each group have consistently high or low values for certain properties. The horizontal rows of elements are referred to as "periods"
Group 1 is also called the alkali metal group. These are strong metals that are unusually soft and very reactive toward Oxygen forming Oxides and water forming hydroxides of the metal. These elements are so reactive toward Oxygen and water vapor that they are stored under an inert liquid to protect them from Oxygen and water vapor.
Group 2 is called the alkaline earth metals. These metals are not as soft as Group 1 metals. They also react more mildly with Oxygen to produce oxides of the metals and only react with water at temperatures where the water is steam.
Groups 3-12 are referred to as the transition metal groups. These metals are not as predictable because of the shielding effect of the inner electrons. As for the "shielding effect" this refers to the inner electrons found in the transition state elements and the inner transition (rare earth) elements. These electrons had a tendency to block the electrical effect of the positive nucleus upon the outer valence electrons of those atoms. This shielding effect helps to partially explain the erratic placement of the electrons in the d and f orbitals relative to the s and p orbitals.
Groups 1-2 and 13-18 are referred to as the representative elements
Group 17 is referred to as the halogen group
Group 18 is referred to as the Noble gas group previously known as the inert gas group.
There are two special series of elements that occur right after the transition metal element Actinium (Actinides) and Lanthanum(Lathanides). These special inner transition state metals were first rearranged by Dr. Glen Seaborg of Univ. of Calif. at Berkeley in the 1950's. It caused quite a stir until it was pointed out and demonstrated that this arrangement seemed to predict the properties of several newly synthesized man-made elements. I would call Dr. Seaborg's work the third milestone in our quest to make order out of the behavior of elemental substances.
The metals which tend to have their atoms losing electrons during a chemical change are roughly found to the left Group 14
Non-metals which tend to have their atoms gaining electrons during chemical change are roughly found in Group16-17 with some elements in the lower parts of Groups 15.
Metalloids which tend to have their atoms sometimes losing and sometimes gaining electrons during chemical change are generally found in Groups 14-16
The Noble gases really belong to their own category since their atoms tend neither to lose or gain electrons. There are only a handful of compounds involving the Noble Gases (mostly involving Xenon).
Concerning the so called "representative elements" (Groups 1,2,13,14,15,16,17,18) the following general trends are observed which have their basis in the periodicity exhibited by the elements:
As you proceed downward in a group two factors are at work. The nuclear charge is increasing which should if it were the only factor make the electrons more tightly bound to the atom. However as you proceed downward the valence shell is moving farther away from the positive nucleus. This factor overweighs the first because of the shielding effect of inner electrons as you proceed downward. Even though the nuclear charge is increasing as you move downward there are more electrons between the valence outer electrons and the nucleus. These inner electrons will shield the valence electrons from the attractive influence of the positive nucleus. Therefore the distance factor is the dominate factor and the energy requirement for removing a valence electron decreases.
As you proceed to the left in a period or as you proceed down within a group:
The ionization potential (energy required to remove an electron from an atom) decreases. Ionization Potential is the energy required to remove an electron from an atom. Basically the ionization exhibits periodicity by decreasing as you proceed to the left along a horizontal row called a period. As you proceed downward in a column of elements called a group the ionization potential will also decrease. The first trend is because as you proceed to the left in a period the nuclear charge decreases making it easier to remove electrons so the energy requirement will decrease.
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