Classification of Elements - Part VII

Characteristics of groups
In the first group, called group I A, we have Hydrogen (H), Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Cesium (Cs) and Fransium (Fr). They have all only one electron in their outermost shells. H is an exception in this group and has to be dealt separately.  So other than H all the elements show metallic properties and have valence +1.  They give up an electron easily. They are highly metallic. The group is known as the alkali metals.

The second group (group II A) has Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra). They have two electrons in their last shell and their valence is +2 as they give up two electrons to form compounds. The elements in group II A are not as metallic as the alkali metals. They form oxides easily and are known as alkali earth metals.

As we proceed to group III and further, we will notice that the number of valence electrons increases by one in each subsequent group. The number of valence electrons in the group VII A, the halogen group, is uniformly 7. This group  consists of elements like Fluorine (F), Chlorine (Cl), Bromine (Br), Iodine (I), Astatine (At). These have their outermost electron orbits with electrons one less than that required for complete filling. These elements are gaseous in nature and have valence -1, they borrow electrons to stabilize their electronic configuration.

In group VIII A we have Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe) and Radon (Rn). The electronic configuration will show that these atoms all have completely filled shells. These elements are therefore chemically non-interacting and inert. They are therefore gaseous in nature. They are Noble gases or inert gases.

We can observe here that all the group member elements have the same valence electrons and display same valencies. Also the atomic radii increases in a group.

On going down a particular group, the properties of elements get enhanced. For example, the figure below shows group I A, the alkali metal elements.

As the atomic radii increases in the alkali metal elements, the last electron is farther away from the attractive forces of the nuclear charge. So it is relatively easy for the element to give up its last electron. And hence show more metallicity.  To say it in terms of electro positive character of elements, we can say that the electro positive character increases as we go down the group I A.

Now if we see the behaviour of elements in the group VII A or the halogen elements, we see that the electro negative character reduces as we go down the group. This means that fluorine (F) is more reactive than chlorine (Cl). The reason for this is that the orbit where the extra electron is captured is closer to the nucleus in F than in Cl. Thus the extra electrons get attracted into the F-atom in a stronger manner than that in Cl.

Summary of  the characteristics of elements in a group  

• The atomic numbers are not consecutive.

• The number of valence electrons in the elements is same in a group.

• The elements of the same group  have the same valencies. The atomic radii increase while going from top to bottom in a group.

• Metallic character increases while going from top to bottom in a group for metallic groups. For non-metallic groups, the non-metallic nature decreases while going from top to bottom

• Chemical reactivity increases while going from top to bottom in a group for metallic groups. For non-metallic groups, the chemical reactivity decreases while going from top to bottom.

Advantages of the Modern periodic table (long form of periodic table) 

1. The table is based on atomic numbers of elements. This is a more fundamental quantity than the atomic mass concept used earlier. (The confusion regarding atomic mass was that scientists did not know if they should consider atomic mass or molecular mass of elements such as H or O).

2. The table shows why elements in the same group display same properties. The table also shows how and why the properties of elements differ in the same period.

3.The periodicity is related to electron configuration. This is displayed very clearly by the periodic table. All chemical and physical properties are a manifestation of the electron configuration of the elements. Hence the periodic table is very systematic and follows the fundamental electronic structure of the elements to classify them.

The periodic table was modified in the middle of the 20^th century. The current periodic table in use is shown below.

Periodic Table in current use The last major changes to the periodic table was done in the middle of the 20th Century. Glenn Seaborg is given the credit for it. Starting with his discovery of plutonium in 1940, he discovered all the transuranic elements from 94 to 102. He reconfigured the periodic table by placing the actinide series below the lanthanide series. In 1951, Seaborg was awarded the Noble prize in chemistry for his work. Element 106 has been named seaborgium (Sg) in his honor. Of the periodic table, he has said that "the Periodic Table is arguably the most important concept in chemistry, both in principle and in practice. It is the everyday support for students, it suggests new avenues of research to professionals, and it provides a succinct organization of the whole of chemistry. Glenn Seaborg It is a remarkable demonstration of the fact that the chemical elements are not a random cluster of entities but instead display trends and lie together in families. An awareness of the periodic table is essential to anyone who wishes to disentangle the world and see how it is built up from the fundamental building blocks of the chemistry, the chemical elements."    Click here for an Enlarged View of the Periodic Table

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