History of Classification

Matter exists in many different forms such as solid, liquid, gas, etc., in our surroundings. The foremost effort to classify the elements started in 1789 by Antoine Lavoisier, who classified elements into metals, non-metals, and gases based on properties of the elements. As the number of elements discovered was increasing day-to-day, there was a need to classify the elements for an easy understanding.

Dobereiner Triads Classification

Initially, only 30-35 elements were known, when German physicist Johann Wolfgang Dobereiner in 1817 started to classify the elements after it has been proved that the properties of elements are related to their atomic masses. Based on the similar properties of elements, three elements could be grouped each of which he named as triads. In triads, the atomic weight of the second element is found to be appropriately equal to the average of the atomic weights of the first and third elements.

Limitations:

The limitation of this classification was that not all elements could be arranged in triads.

Newland’s Octaves

As there was an advancement in discovering many elements, in 1865, British chemist John Newlands arranged the known 56 elements based on the ascending order of the atomic weights. While arranging, the repetition of the properties of elements for every eight elements was noticed by him. Hence, it is named after him as Newland’s Octaves.

Limitations:

Few limitations of Newland’s law were that new elements discovered were not able to arrange based on the octave rule and there were no gaps for new elements. Among the arranged elements the octave rule was followed only till calcium. To arrange all the existing elements, two elements (Co and Ni; Ba and V, etc.,) were placed in the same position with different properties.

Mendeleev’s Periodic Classification

As the number of elements discovered was increasing to 63, Dmitri Mendeleev, a Russian chemist, in 1869 started classifying the elements based on the atomic masses which is the fundamental property of elements. The elements were arranged based on

the atomic weights in ascending order and could analyze that, at regular intervals, the physical and chemical properties of elements were repeating.

Thus, Mendeleev's periodic law is stated as “At regular intervals, the various Physico-chemical properties were the periodic functions of the atomic masses of the elements”.

Based on these facts, Mendeleev arranged the elements in a tabular column in which the vertical columns present were named ‘groups’, whereas the horizontal rows present were named periods.

Merits and Demerits

Few merits of the periodic table arranged by Mendeleev’s are

• Few elements atomic weights were changed thus to provide those elements a suitable and appropriate place in the periodic table. Beryllium, whose mass was previously determined as 14.09 was changed to 9.4 and placed before boron.
• Based on the properties of the adjacent elements, vacant space has been provided for the elements which are to be discovered in the future. Few elements like eka-boron, eka-aluminum, and eka-silicon with atomic weights 44, 68, and 72, respectively were named and assigned. Later on, among the newly discovered elements scandium (Sc) (eka-boron), gallium (Ga) (eka-aluminum), and germanium (Ge) (eka-silicon) properties were well coincided by Mendeleev predictions.
• When gases such as helium, neon, and argon were classified as noble gases, without disturbing the created periodic table, Mendeleev created a new group named as ‘zero groups’ and placed these gases in these groups.
• The periodic properties such as atomic radius, ionization energy, etc., of elements in a particular group shows regular gradation i.e., either decreases or increases from alkali metals (left) to noble gases (right). Similarly, regular gradation can be noticed from the first period (top) to the seventh period (bottom) of the periodic table.

Demerits in Mendeleev’s Arrangement of the Periodic Table

• The dilemma in the position of a hydrogen atom was the main drawback of this method of distribution of elements. The hydrogen’s chemico-physical properties were similar to both IA (alkali) metals like Li, Na, K, etc., and the VII group i.e., BF, CI, Br, and I. based on its properties it was difficult to decide a correct place for hydrogen either in the VII group with halogens or with alkali metals.
• Few elements such as Co and Ni have the same whole atomic mass and were arranged in decreasing atomic mass which created ambiguity regarding the sequence of elements in the periodic table.
• Isotopes were discovered after Mendeleev’s periodic table was framed. Thus, there was no vacant space for the isotopes to be inserted in the arranged periodic table.
• An increase in the atomic masses was found to be not constant as the new elements were incorporated into the vacant places.

Modern Periodic Table

As new elements could not follow Mendeleev’s increase in the atomic mass pattern, Moseley redefined it in 1913 and explained that the elements should be sorted according to increasing order of the atomic number and not atomic masses. Based on this criterion, Mendeleev’s periodic law was modified as Modern periodic law. This law illustrates that “At regular intervals, the elements chemical and physical properties were the periodic functions of the atomic numbers”. Thus, in designing the Modern periodic table, Neil Bohr arranged all elements in ascending order of the atomic numbers of the elements.

While designing the Modern periodic table there were 118 known elements that are arranged based on their atomic numbers. There are seven periods named from I to VII. The first period is the shortest period with two elements, the second and third periods are short period in which 8 elements are present in each, 18 elements each are present in the fourth and fifth periods which are considered as long periods with 18 elements and the sixth and seventh period is the longest period with 32 elements each. The seventh period was said to be incomplete but there was an addition of four new elements recently. There are 18 groups (1–18) which are subdivided as A and B. Group I to VII is subdivided into A and B and group VIII do not have any subdivision. Group I which is present extreme left in the periodic table contains alkali metals and group 18 present at the extreme right in the periodic table contains noble gases i.e., zero group elements. There are two series, namely lanthanides (58–71) and actinides (90–103), which are placed completely below in the periodic table.

Modern periodic table elements are classified into four groups as, s, p, d, and f- block elements based on the orbital in which the last electron or the valence electron enters. Groups 1 and 2 along with hydrogen come under s-block elements, p-block elements comprise groups 13 (IIIA) to 18 (zero groups), groups 3 to 12 come under d-block elements and two series i.e., lanthanides and actinides come under f-block elements.

Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for

• B.Sc. in Chemistry
• M.Sc. in Chemistry