[Music] hi and welcome back to free science lessons by the end of this video you should be able to describe early attempts to arrange the elements into a periodic table you should then be able to explain by Demitri mela's periodic table successfully predicted the properties of elements and finally you should be able to describe how the elements are arranged in the modern periodic table here is the modern periodic table and you are given a copy of this in your exam the periodic table is arranged into columns which we call groups so we have group one and group two we then have this big block which we call the transition elements and then we have groups three four 5 6 7 and finally group zero so why is it called the periodic table well the word periodic means occurring at regular intervals and in the periodic table we can see that elements with similar properties occur at regular intervals so for example we can see that group one consists of highly reactive metals such as lithium sodium and potassium on the other side of the table group seven consist of highly reactive nonmetals such as Florine chlorine and bromine all the elements in a group have similar chemical properties in other words they react in a similar way and that's because all the elements in a group have the same number of electrons in their outer energy level now a long time ago scientists began to see if they could put the elements into some sort of order the scientist Johan Dober noticed that elements with similar chemical properties often occurred in threes and he called these Triads a good example is lithium sodium and potassium which are three metals that all react rapidly with water another example is chlorine bromine and iodine which are all reactive nonmetal Metals so from this scientists began to wonder whether the elements could be arranged into some sort of logical order around 40 years later the scientist John newand noticed something interesting he arranged the elements in order of increasing atomic weight and he saw that every eighth element reacts in a similar way if we count lithium as element one then element 8 is sodium if we now count sodium as element one then element eight is potassium we can do the same with brillium magnesium and calcium and again these three elements also react in a similar way to each other nuland called this his law of octaves now nulan system did have problems by always sticking to the exact order of atomic weight sometimes elements were grouped together when they had totally different properties so that meant that nand's law of o octaves was not really taken seriously by other scientists okay I'm showing you here the scientist Dimitri mandev Mev developed the first modern periodic table and you've got to be able to explain how he did this mandala started by arranging all the elements in order of increasing atomic weight however he then did two things that no one had done before firstly if he needed to mandev would switch the order of specific elements so they fitted the patterns of other elements in the same group secondly Mel have realized that some elements had not been discovered so he simply left gaps in his periodic table where he thought an element was missing Mel was so confident that his table was correct that he predicted the properties of the Undiscovered elements based on other elements in the same groups several years later the missing elements were discovered and their properties marched mandala's predictions and and because of that other scientists now accepted that mela's table was correct now the modern periodic table is almost exactly the same as mener layers apart from a couple of important differences firstly in the modern periodic table the elements are arranged in order of atomic number in other words the number of protons when we order by atomic number every element is in the correct group when Mel have developed his table protons had not been discovered so he ordered the elements back atomic weight the problem with ordering by atomic weight is that elements can appear in the wrong order due to the presence of Isotopes a good example are torium and iodine which we can see here ordering by atomic weight puts these elements in the wrong groups but ordering by atomic number puts them in the correct groups and remember that Mev addressed this problem by switching the order of elements when he needed to secondly the modern periodic table has group zero the noble gases these elements had not been fully discovered when mandala published his table you'll find plenty of questions on this topic in our vision workbook which you can get by clicking on the link [Music] above