Whenever we wish to take up a subject and study it, we have to go through a systematic arrangement of the syllabus that will help us proceed so that we are not left more confused than before about the subject. In chemistry, elements are very important as these combine to give rise to compounds which we find everywhere around us. We also use elements in their pure form every day and hence it becomes important to study the elements that are like alphabets in the world of chemistry. To conduct a systematic study, we need to categorize the components and then tackle them so that we go through similar topics at the same time and do not jump all over the board in an effort to soak up knowledge. In order to develop a proper understanding of elements, chemists felt the need to categorize and group them so that a common trend among the multitudes of elements can be recognized and possibly exploited for our needs. The periodic table is the result of that effort which started with Antoine Lavoisier in 1789 and was brought to fruition by Dmitri Mendeleev in 1869.
Before Mendeleev presented his table, there had been many attempts to group the elements together so that a link can be established between them and reasons can be provided for their similar behavior. At the time of Mendeleev, the existence of neutrons and protons was not proven, nor was it speculated. Mendeleev decided to base his periodic table on increasing atomic weights and tried to group the elements based on their chemical properties when he felt dissatisfied with the characterization that atomic weight provided. This was a special and keen observation on his part that led to the acceptance of the table along with one more factor: he left blanks in his table because he was confident that some elements were missing and could be found in the future to fill the gap. This prediction was proven to be correct when gallium and germanium occupied the spaces that Mendeleev left in his table.
Modern periodic law
The modern periodic table is based on the modern periodic law which states that the periodic trends in the properties of elements are a consequence of their atomic numbers and not atomic weights. Thus, the elements in the modern periodic table are arranged according to their increasing atomic numbers. Unlike atomic weights, atomic numbers are integers and are consecutive in nature. An atomic number is the number of protons that are present in the nucleus of the atom. In an uncharged atom, number of protons is equal to the number of electrons and so atomic numbers represent the number of electrons for a neutral atom. As the chemical properties of elements is determined largely by the arrangement of electrons, the arrangement of elements based on their atomic numbers allows for a better prediction of chemical properties.
A periodic table is called so because it aims at showing a periodic trend that occurs when you move across the row or down the column in the table. In all, there are 18 columns and 7 rows in the periodic table. The modern periodic table shows a similarity among the elements in various ways. The elements can be observed in a column or row, which is also called period, and it can be seen that there are some similar properties or some sort of trend. The elements can also be observed with the four blocks in mind: s-block, p-block- d-block and f-block. These blocks are named so because of the subshells s, p, d, f in which the valence electron of the group resides. The importance and relevance of the periodic table can be seen in the fact that there is a clear distinction between what are metals, non-metals and metalloids. The left-hand side bulk of the periodic table is composed of metals: alkali metals, alkaline earth metals and transition metals. Non-metals occur in smaller numbers and occupy the right-hand side of the periodic table. In between the metals and non-metals the metalloids occupy the periodic table and show properties that are similar to metals and non-metals.
Problems with the periodic table
Learning is a continuous process and whenever we come across something new we try to understand it to the best of our ability. The periodic table is a shot at understanding the nature of elements, finding patterns that will give rise to similar properties and predicting behavior of the elements. But the modern periodic table has its shortcomings as well. Most of its shortcomings are being sought to be eradicated by modern scientists with the help of improved models of the periodic table. The most popular among them is the spiral model which places hydrogen at the centre and then the transition elements and the f-block elements are present on individual peninsulas.
Hydrogen and helium
Hydrogen and helium are not placed according to their valence electron configuration. Hydrogen should belong to the 1st group and helium should belong to the 2nd group according to their electronic distribution. But neither hydrogen nor helium exhibit the properties of these groups. Hydrogen shows the properties of both the alkali metals and halogens and sometimes the author of the periodic table shows hydrogen to be separately placed without a group. Helium does not show the properties of alkaline earth metals. But placing it with rest of the noble gases poses a problem as well because of its complete duplet instead of a complete octet that other noble gases have.
The transition metals have various definitions but IUPAC provides them the strict definition of being those metals which have partially filled d sub-shell. Since the elements of group 12 have a completely filled d sub-shell, as per the strict definition, group 12 members are not considered transition metals. However, many scientists continue to include them among transition metals based on the fact that they are able to exhibit properties similar to transition elements.