Historically, the discovery of triads using atomic weights rather than atomic numbers was the first hint of any quantitative patterns among the atomic weights of the elements. The atomic number of the middle element is the average of the elements above and below it. Working with triadsĪ triad is the name given to a group of three elements falling vertically in any group of the periodic table. Janet's original left-step table leads to the loss of a perfect atomic number triad, in moving helium out of the noble gas group, whereas this new table retains the helium triad. There is an additional bonus in presenting the periodic table in this manner. This new version relocates hydrogen to the top of the halogens, while retaining the overall form of the left-step table (figure 4). More recently it has been suggested that the left-step table could be modified to avoid the heresy of placing helium among the alkaline earths. Some authors avoid the hydrogen-helium problem altogether by removing them from the main body and by allowing them to float above the rest of the table. It has even been suggested that hydrogen should be grouped at the head of the carbon group since, like carbon, its atom has a half-filled outer shell.įigure 4 - A periodic table with hydrogen in group 17 As a result it can be placed in group 1, as it usually is, or with the halogens in group 17 ( fig 4). It can both lose as well as gain an electron. Hydrogen doesn't fit eitherĮven the first element, hydrogen, has been causing trouble for some time. This question of where to place helium is one sign of trouble in the periodic table. While helium is the most unreactive of all the elements in the periodic reactive. From a chemical point of view, the placement of helium among these metals amounts to complete heresy. This move can be justified on the basis of the outer-electron structure of helium, which possesses two such electrons, as do the members of the alkaline earth group. There is an annoying drawback in that the element helium is removed from its usual place among the noble gases and moved to join the alkaline earths like beryllium, magnesium and calcium. 2 It also displays the elements in one continuous sequence without any gaps or interruptions. Recently this form has enjoyed a revival because it also seems to display the order of orbital filling more clearly than the conventional form. In the conventional modern periodic table, known as the medium-long form, these elements fall into groups 1 and 18 respectively - the alkali metals and the noble gases (figure 2). 1 Over 1000 of them have been published in articles or on websites.Ĭuriously the first two elements, hydrogen and helium, present something of an anomaly and there has always been some disagreement as to exactly which groups they should be assigned to. Given these basic facts it is a little surprising to find so many different forms of the periodic table. For instance, all these metals react with water, although the reactivity increases as one descends the group from lithium to potassium and beyond to caesium and rubidium. It is the outer-shell electrons which govern their chemical properties and that's why they behave similarly. The elements in group 1, for example, all have one outer-shell electron although they differ in their inner shell structures. The modern explanation why the elements fall into vertical columns, showing similar properties, has been provided by quantum mechanics and describes the shell structure of electrons orbiting the nucleus of each kind of atom. Take a look at all our articles and resources about the periodic table of elements or visit our interactive periodic table, featuring history, alchemy, podcasts, videos, and data trends.
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