Chemistry

The Modern Form of the Periodic Table



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The study of Chemistry involves the study of the more than 100 chemical elements found on Earth in various quantities. Some elements exist in abundance, some only in traces; some exist as the elements themselves (e.g. gold and other less reactive elements) while many others exist only in compounds form (e.g. iron, which exists as its oxides and other compounds) as they are more reactive and react readily with the chemicals found in the atmosphere.

To make sense out of the huge maze of knowledge for the reactions of each and every element extant on Earth, to find the path out of the labyrinth of the myriad chemistry of the elements, chemists have devised ways to systematize the study of these elements, based on the fact that, in essence, all chemical reactions involve the exchange of electrons between the reacting atoms of the various elements. So for a simple example, in the formation of sodium chloride, NaCl, sodium atom loses one electron to an atom of chlorine, Cl, thus forming the ionic compound sodium chloride, or common salt. In the case of reactions between non-ionic compounds e.g. the aromatic and other substances in which the bonding is mostly covalent, what takes place is the sharing of one or more of the lone pairs of electrons in the outermost shells of the atoms involved.

THE MODERN FORM OF THE PERIODIC TABLE

I do not propose to go into detail about the history of the periodic table: how various scientists developed theories and generalizations about the properties of discovered elements - scientists like Mendeleev, Dobereiner, Lothar Meyer etc. each of whom made some contributions towards realizing the modern, informative form of the periodic table.

Under the present, modern system (IUPAC ordained and sanctioned), elements are grouped in strict adherence to their atomic number or the number of electrons in each elemental atom in its neutral state. So, hydrogen, the element with atomic number 1, is placed first and the element meitnerium, Mt, with atomic number 109, is placed last. Note that many of the latter elements are not 'natural' in the sense that they have been synthesized in the laboratories. The over 100 elements are placed in vertical Groups and horizontal Periods so that, with the exception of few elements, every element is identified with 'co-ordinates' viz its Group Number and Period Number.

For illustration, take the element magnesium, Mg, which is placed in Group II and in Period 3. The vertical Groups are numbered Groups I, II, etc until Group O whereas the horizontal groups are from Period 1 to Period 7. Magnesium has atomic number 12 with electronic configuration 1s22s22p63s2 meaning its atom has 2 electrons in its outermost shell and that is why it is placed in Group II. Elements with similar chemical properties are grouped together vertically - thus we have the s-block elements in Group I and II, the more reactive metals Li, Na, K, Rb, Cs, Fr and Be, Mg, Ca, Sr, Ba, Ra. The transition metals or the f-block and d-block elements are placed in the middle of the Periodic Table, flanked by Groups I, II on the left and Groups III, IV, V, VI, VII, VIII or the p-block non-metals and lastly the inert gases in Group O. This is the UK system, probably a little different from that of the US form of the Periodic Table.

The most important and general observation to be made about the Periodic Table is that for elements placed in the same group (vertical column), the reactivity of the elements decreases as we go down the group; so Li is more reactive than Na, which is more reactive than K, and so on down the group. In its reaction with water, H2O, for instance, Li acts explosively whereas Cs and Fr much less violently. But the common denominator among elements in the same group is that they share similar chemical properties - because they have similar electronic configurations in their atoms. So, atoms of elements of the same group, say Group III, all have 3 electrons in their outermost shell.

A colour scheme is often used to distinguish the various s-block, d-block, p-block and f-block elements; non-metals and weak metals ( elements which are not very prominent in their supposedly metallic properties ) are thus coloured differently for elements in Groups III to VIII. Of course the noble or inert gases of He through Rn are coloured the same to indicate they belong to the generally highly inactive chemical elements characterized by the 'filled' outer-shell electronic configuration, resulting in a highly stable state not easily susceptible to reaction.

Across a particular period ( the horizontal row of elements, moving from left to right ), because of the fact that the outermost shell is progressively filled (e.g. for Period 2, Li is 2.1, Be is 2.2, B is 2.3, C is 2.4, N is 2.5, O is 2.6 , F is 2.7 and Ne is 2.8) there is a decrease in atomic radius because the increasing positive charge of the nucleus (more protons ) helps pull the electrons closer. In addition, moving from left to right, there is generally a rise in what is known as 1st ionization energy, the energy required to remove 1 electron from each atom in 1 mole of gaseous atoms to produce 1 mole of gaseous ions with a single positive charge. This is because, moving from left to right across the same period, there is an increase in nuclear charge and the resulting decrease in atomic radius causes the electrons to be held more strongly, making it more difficult or more energy-expending to remove the outermost electron.

The elements also exhibit periodicity (i.e a gradual gradation or progression) in chemical reactivity among its compounds. It may be noted that, generally as we move across a period from left to right, the chlorides of the relevant elements change from more ionic (metallic) chlorides to more covalent, non-ionic and molecular non-metal chlorides, as exemplified by elements of say, Period 3. Thus sodium chloride, NaCl is an ionic chloride, whereas phosphorus chloride, PCl3 or PCl6 are non-metallic, covalent and 'molecular' in structure as compared to the giant, lattice structure of ionic NaCl.

While I hope that I have succeeded in a small way to helping you readers understand the Periodic Table, it is not possible to write all about the intricacies of the Periodic Table, discussing in great detail all the information that can be obtained about the chemical elements within the short confines of this article. I therefore urge that interested readers go read up more if they are intent on finding out more about the Periodic Table.

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