Periodic Table of Elements

Dobereiner’s Triads

A German chemist Dobereiner observed relationship between atomic masses of several groups of three elements called triads. In these groups, the main or middle component had an atomic mass average of the other two components.

One triad group example is that of calcium (40), strontium (88) and barium (137). The atomic mass of strontium is the average of the atomic masses of calcium and barium. Just a couple of components could be set up in this way. This category did not get wide acceptance.

Newlands Octaves

After a successful decision of appropriate atomic masses of components by Cannizzaro in 1860, efforts were again started to organize components. In 1864 British chemistNewlands put forward his observations in the form of ‘law of octaves’.

He kept in mind that there was a repetition in chemical properties of every 8th component if they were set up by their increasing atomic masses. He compared it with musical notes. His work could not get much recognition as no space was left for undiscovered elements. The noble gases were also not known at that time.

Mendeleev’s Periodic Table

In 1871, a Russian chemist, Dmitri Mendeleev, offered a better and detailed plan for the classification of components. He provided the first regular table of elements in which elements of similar chemical properties were set up in 8 vertical columns called Groups.

The horizontal rows of the table were called Periods. Mendeleev set up the known elements (just 63) in order of increasing atomic masses, in horizontal rows called periods. So that components with similar properties remained in the exact same vertical columns. This arrangement of elements was called Periodic Table.

He put forward the results of his work in the form of the periodic law, which is stated as:

“properties of the elements are periodic functions of their atomic masses”.

Demerits of Mendeleev’s Periodic Table

Although, Mendeleev periodic table was the very first attempt to organize the elements, yet it has a couple of demerits in it. His failure to discuss the position of isotopes and wrong order of the atomic masses of some elements recommended that atomic mass of an element can not serve as the basis for the arrangement of components.

Improvements in Mendeleev’s Periodic Table

In order to make the periodic table more useful and precise, a few improvements were made in Mendeleev s periodic table. In1913 H. Moseley discovered a new property of the components i.e. an atomic number. He observed that atomic number instead of atomic mass must identify the position of the component in the periodic table and accordingly the periodic law was amended as:

” properties of the elements are periodic function of their atomic numbers”.

An atomic number of an element equals the number of electrons in a neutral atom. So atomic number provides the basis of electronic configuration as well.

The Modern Periodic Table:

In the modern periodic table, all the elements are arranged in increasing order of their atomic numbers. Following are the essential features of the periodic table.

1. Group and Periods

Elements with similar properties are placed in vertical columns called Groups. There are 8 groups, which are normally numbered by Roman characters I to VIII. Each group is divided into two subgroups, designated as A and B subgroups. The subgroups, including the representative or normal elements, are identified as A subgroups, whereas B subgroup consists of less typical elements, called transition elements and are organized in the centre of the periodic table of elements. The horizontal rows of the table of elements are called Periods. The essential features of the periods are as follows:

• a) There are 7 periods in the table of elements numbered by Arabic characters 1 to 7.
• b) The period 1 includes only two elements, hydrogen and helium.
• c) The period 2 and 3 include 8 elements each and are called short periods. All the elements in these periods are representative elements and belong to A subgroup. In these periods, every 8th component resembles in properties with the first component. As lithium and beryllium in the second period look like in the majority of their properties with sodium and magnesium of the 3rd period, respectively. Similarly, boron and aluminium both show oxidation state of +3, fluorine in 2nd period have close similarities with chlorine of the 3rd period.
• d) The periods 4 and 5 are called long periods. Each long period includes eighteen elements. Out of these, eight are representative elements coming from A subgroup comparable to 2nd and third periods. Whereas the other 10 elements, positioned in the centre of the table come from B subgroups and are called transition elements. In these periods, the repetition of properties amongst the elements happens after 18 elements. As after ₁₉K (having atomic number 19) the next element with similar properties is ₃₇
• e) The 6^th period is also a long period, which contains thirty-two elements. In this duration there are 8 representative elements, 10 transition elements and a new set of fourteen elements called Lanthanides as they begin after ₅₇ Lanthanides have remarkably similar properties and are normally shown individually at the bottom of the periodic table.
• f) The 7^th period is incomplete so far. It includes only two typical elements ₈₇Fr and ₈₈Ra, ten transition elements and fourteen inner transition elements. The inner transition elements of this period are called Actinides, as they follow ₈₉ The actinides are also shown at the bottom of the periodic table of elements under the Lanthanides. Due to their scarcity, the inner transition elements are also called rare earth elements.

2. Some More Families in the Periodic Table:

While studying about periods you have noticed that certain rows of elements with similar properties have actually appointed typical names such as transition elements, Lanthanides, Actinides or Rate Earth elements.

Similarly, due to their peculiar attributes, some common elements coming from sub-groups A, have actually also been assigned family names. For example, elements of the group IA are called Alkali Metals, because of their property to form strong alkalies with water.

Likewise, due to their existence in Earth’s crust and alkaline character, the elements of group IIA are referred to as Alkaline Earth Metals. Another essential family in the table of elements is Halogen family.

The name “Halogens” is provided to the components of group VIIA, due to their salt-forming properties. As the gases of group VIIIA ‘are least reactive they are called “Noble Gases”. These family names work for a fast acknowledgement of an element in the periodic table.

3. Blocks in the Periodic Table

Elements in the periodic table can also be classified into 4 blocks. This classification is based upon the valence orbital of the elements involved in chemical bonding. According to this category, elements of IA and IIA subgroups are called s-block elements because their valence electrons are readily available in s orbital. The elements of IIIA to VlllA subgroups (other than He) are called p-block components as their valence electrons exist in the p orbital.

Similarly, in transition elements, electrons in d-orbital are responsible for their valency thus they are called d-block elements. For Lanthanides and Actinides valence electrons exist in f- orbital hence these elements are called f-block elements. This category is quite useful in understanding the chemistry of elements and predicting their properties especially the principle of valency or oxidation state.

4. Metals, Non-metals and Metalloids

Another basis for classifying the elements in the periodic table is their metal character. Generally, the components on the left-hand side, in the centre and at the bottom of the periodic table are metals, while the non-metals remain in the upper right corner of the table.

Some elements, specifically lower members of groups, IIIA, IVA and VA have properties of both metals as well as non-metals. These elements are called semi-metals or metalloids. In the periodic table elements of groups IVA to VIIIA, at the top right-hand corner above the stepped line, are non-metals. The elements simply under the “steps’ such as Si, As, and Te are the metalloids. All the remaining elements, other than hydrogen, are metals.

MCQs with Answers: Periodic Table of Elements

 

Frequently Asked Questions (FAQs): Periodic Table of Elements