دوران جديد طبقه بندی عناصر از کارهای ژوليوس لوتار ماير(1869) و بويژه ديميتري مندليف(1869) ريشه گرفت. مندليف قانون تناوبی را مطرح کرد که می  گفت: وقتی عناصر را به ترتيب وزن اتمی شان مطالعه کنيم، شباهت هايی در خواص آنها بصورت تناوبی مشاهده ميشود. درجدول مندليف عناصر به گونه ای تنظيم شده بودند که عناصر شبيه به هم در ستون های عمودی، موسوم به گروه ظاهر می شدند. برای قرار دادن عناصر مشابه زير يکديگر، مندليف مجبور شد در جدول خود جاهای خالی برای عناصر کشف نشده پيش بينی کند. اسکانديوم، گاليوم و ژرمانيوم که خواص هر يک از آنها به خواص پيش بينی شده توسط مندليف شباهت فراوانی داشت، اعتبار سيستم تناوبی را نشان داد. وجود گازهای نجيب(Rn,Xe,Kr,Ar,Ne,He)توسط مندليف پيش بينی نشد. اما پس از کشف آنها در سالهای 1892 تا 1898 اين عناصر به خوبی در جدول تناوبی قرار گرفتند. طرح جدول تناوبی چنان بود که سه عنصر(I, Ni, K) خارج از ترتيب تعيين شده با افزايش وزن اتمی، قرار ميگرفتند. مثلآ يد بر مبنای وزن اتمی بايد عنصر شماره 52 باشد؛ اما به طور اختياری بعنوان عنصر 53 معرفی شد تا در گروهی از عناصر(Br, Cl, F) که با آنها شباهت شيميايی دارد، قرار گرد. مطالعه بعدی طبقه بندی تناوبی، بسياری از شيميدان ها را متوجه اين امر ساخت که خاصيت بنيادی ديگری، علاوه بر وزن اتمی، ميتواند عامل خصلت تناوبی مشاهده شده باشد. پيشنهاد شد که اين خاصيت بنيادی، از جهتی به عدد اتمی که در آن زمان فقط يک شماره ساده بدست آمده از سيستم تناوبی بود بستگی دارد.

Early in the nineteenth century, chemists became interested in the chemical and physical similarities between elements. In 1817 and 1829, Johann W. Dobereiner published articles in which he examined the properties of sets of elements that he called triads (Ca , Sr , Ba , Li , Na , K , Cl , Br , I , and S , Se , Te ). The elements of each set have similar properties, and the atomic weight of the second element of a set is approximately equal to the average of the atomic weight of the other two elements of the set.

   In following years, many chemists attempted of classify the elements into groups of the basis of similarities in properties. In the years 1863-66, John A. R. Newlands proposed and developed his” law of octaves”. Newlands ststed that when the elements are listed by increasing atomic weight, the eighth element is similar to the first, the ninth to the second, and so forth. He compared this relationship to octaves of muscial notes. Unfortunately, the actual relationship is not so simple as Newlands supposed. At the time he presented it, his work seemed forced and was not taken seriously by other chemists. Much later, however, Newlands was awardedthe Davy Medal by the royal Society for this work.

   The modern periodic classification of the elements stems from the works of Julius Lothar Meyer (1869) and, in particular, Dmitri Mendeleev (1869). Mendeleev proposed a periodic law: when the elements are studird in order of increasing atomic weight, similarities in properties recur periodically. Mendeleev,s table listed the elements in such a way that similar elements appeared in vertical columns, called group.

   In order to make similar elements appear under one another, Mendeleev had to leave blanks for undiscovered elements in this table. On the basis of his system, he predicted the properties of three of the missing elements. The subsequent discovery of scandium, gallium, and germanium, each of which was found to have properties much like those predicted by Mendeleev, demonstrated the validity of the periodic system. The existence of the nobel gases (He, Ne, Ar, Kr, Xe, and Rn) was unforeseen by Mendeleev. Nevertheless, after their discovery in the years 1892-98, these elements readily fitted into the periodic table.

   The plan of the periodic table required that three elements (K, Ni, and I) be placed out of the order determined by increasing atomic weight. Iodine, for example, should be element number ۵۲ on the basis of atomic weight. Instead, iodine was arbitrarily made element number 53 so that it would fall in a group of the table with chemically similar elements (F, Cl, and Br). Subsequent study of the periodic classification convinced many that some fundamental property other than atomic weight is the cause of the observed periodicity. It was proposed that this fundamental property is in some way related to atomic number, which at that time was only a serial number derived from the periodic system.