Science and Technology for UPSC Examinations [Chemistry] Part 8- Structure of Periodic Table

Structure of the Periodic Table:

The periodic table has 7 horizontal rows which are called periods and 18 vertical columns which are called groups. Please note that there is an extended version of the periodic table which contains 8th period. No element of the eighth period has yet been synthesized.

Understanding Periods

We can see in the above table that elements have been arranged in increasing sequence of their atomic numbers. We take example of period 4 to understand the common features of the periods

In the above 4^th period, everybody among recognizes that the first element is K that is Potassium. Second is Ca that is Calcium. Among others which everybody knows are Fe that is Iron, Co that is Cobalt, Ni that is Nickel, Cu that is Copper, Zn that is Zinc, Ga that is Gallium. The last two are Br Bromine and Kr Krypton.We should know that Potassium is a highly reactive element and it oxidizes rapidly in the air. Calcium is little less reactive. Then we find some elements which we call metals such as Iron, Copper and Zinc. The penultimate element is Bromine that is a halogen gas and last element Krypton is a Noble gas.

• A metal is some element which can lose electrons easily. The most reactive metals are alkali metals. In the above row K is an alkali metal and its metallic properties i.e. properties to lose electrons is highest. Since they are highly reactive they are never found in nature in elemental form. Rather mostly they are found as oxides , carbonates etc.
• After Alkali metals, we have alkaline earth metals such as Calcium in the above row. When the oxides of these alkaline earth metals are dissolved in water, they give basic alkaline solution and this is the basic reason why they are called alkaline earth metals. The metallic property is little less than the alkali metals.
• We find that the metallic property of the elements decreases when we move from left to right in a period in the periodic table and this is a common property of all the periods.
  
• The chemical reactiveness of alkali metals is highest. It decreases , but in the last we find that halogens are also very reactive. And Finally the Nobel gases are very less reactive.
• In a period of a periodic table, the chemical reactiveness first decreases and then increases till halogens. However, this is not applicable to last group of Nobel gases.
• In a period the valency of the elements increases from 1 to 4 and later diminishes and becomes zero.

What is Valency?

Valency or Valence more precisely refers to measure of the number of chemical bonds formed by the atoms of the given elements. For example, carbon forms four bonds, oxygen two and hydrogen one.

But this definition is ambiguous. Phosphorus makes 3 bonds sometimes and 5 at other times. So the IUPAC which is the supreme body of pure & applied chemistry tried to give an unambiguous definition of valency. Now the valency is defined as:

The maximum number of univalent atoms (originally they were hydrogen or chlorine atoms) that may combine with an atom of the element under consideration, or with a fragment, or for which an atom of this element can be substituted is known as Valency.

Lets take an example.

We know that salt is Sodium Chloride. It is represented as NaCl. Na refers to Sodium and Cl refers to chlorine. When NaCl is dissolved in water it gets dissociated into Na+ and Cl- ions. In general sense it means that there is one atom of Na and one atom of Cl that combine to give one molecule of NaCl. The sign + denotes the valency. Valency of Na is 1. Valency of Ca in the above row is 2 as it makes either CaCl2 or CaO or Ca(OH)2. The Ca ion is denoted as Ca++. In case of ferrous the valency is 3. Fe+++ or Fe3+.

In the periodic table when we move from left to right the number of valence electrons increases from group 1 to group 8.

Fluorine has highest valency that is 6

Other common properties of a period:

• In a periodic table the values of the electron affinity also increases when we move from left to right.
• The values of the electron negativity also increases from left to right.
• The values for the ionization potential also increased from left to right.
• Size of the atomic radius decreases in a period from left to right.
• The basic or alkaline character of the oxides of the elements decreases when we move from left to right.

Understanding Groups

There are 18 columns in the periodic table and they are called groups. The elements of a group have similar configuration in the outermost electron shells of their atmos. This outermost shell is called valence shell. The atoms in which outermost shell is complete, tend to be inert chemically, but if they are with one or more extra electrons they become highly reactive because they can be removed very easily from the shell and make positive ion (due to loss of electrons).

The first group of alkali metals has 1 extra electron and its valency is 1. Second group of alkaline earth metals has two extra electrons so has a valency 2. Each group is called a family. The 18 families in the periodic table are as follows:

• Group 1 : The alkali metals or lithium family
• Group 2: The alkaline earth metals or beryllium family
• Group 3: The scandium family
• Group 4 : The titanium family
• Group 5 : The vanadium family
• Group 6: The chromium family
• Group 7 : The manganese family
• Group 8 : The iron family
• Group 9 : The cobalt family
• Group 10 : The nickel family
• Group 11 : The coinage metals (this is unofficial name) or copper family
• Group 12 : The zinc family
• Group 13 : The boron family
• Group 14 : The carbon family
• Group 15 : The nitrogen family or pnictogens
• Group 16 : The oxygen family or chalcogens
• Group 17 : The Fluorine family or halogens
• Group 18 : The helium family/neon family (In this group six are Nobel gases).

Common Properties in a Group

• The metallic property in a group increases from top to bottom.
• The chemical reactiveness of the metallic elements increases from top to bottom , while decreases in the case of nonmetallic elements.
• Valency is same in all groups (there are some exceptions)
• Values of the electron affinity decreases from top to bottom in a group.
• Values of ionization potential decreases from top to bottom.
• The size of the atomic radius increases from top to bottom.