Trends of chemical reactivity in Periodic Table

The systematic arrangement of elements in a periodic table discloses certain periodic trends in the properties of elements. For example, atomic radii and ionic radii decrease from left to right, moving in a period. Understanding the trends in fundamental properties of elements (atomic and ionic radii, ionization enthalpy and electron gain enthalpy) will let you conclude that the periodicity in properties basically depends on the electronic configuration of an element. After studying the concepts in detail, we can discover a relationship between chemical properties and fundamental properties of elements.

Let us first understand the trends of fundamental properties of an element.

1. Atomic Radii and Ionic radii:
   

   The atomic radii and ionic radii of elements decrease while moving from left to right in a period. They increase on moving from top to bottom in a group as the number of shells increases with the increase in atomic number.

2. Ionization Enthalpy:
   

   While moving from left to right in a period, atomic radius decreases. So if the size of an atom decreases, the attractive force between the nucleus and the outermost electrons increases. Due to this, across a period in the periodic table, ionization energy generally increases. However, when we see the trend of ionization enthalpy in the groups, it decreases from top to bottom in a group. This is because the number of shells increases down the group, due to which the outermost electrons will be far away from the nucleus and consequently, the effective nuclear charge is less. Secondly, the shielding effect also increases down the group with the increasing number of shells, which in turn results in the decreasing ionization energy.

3. Electron gain enthalpy:
   

   Electron gain enthalpy becomes more negative as we move from left to right in a period.

   • Negative: When energy is released while accepting an electron.
   • Positive: When energy is supplied to an atom while adding an electron.

Concluding the trends of properties in the periodic table from above, we can say that the elements at the two extremities of the periodic table are highly reactive (note: noble gases have completely filled shells; hence they are least reactive) and the elements at the center are the lowest reactive element. The extreme left elements are the alkalis which easily lose electrons to form cations. On the other side are halogens – the elements on the extreme right which easily gain electrons to form an anion. We can relate it to the metallic and non-metallic characteristics of elements. Metallic property decreases while non-metallic characteristics increase while moving from left to right in a period.  In a group, metallic properties increases and non-metallic property decreases down the group.

3. Chemical reactivity:
   

   Chemical reactivity of an element can be clearly understood by studying the reaction of the element with oxygen and halogens. Elements combine with oxygen to form oxides. Elements on the extreme left of the periodic table react with oxygen to form basic oxides (Na₂O), while elements on the extreme right form acidic oxides on reaction with oxygen (Cl₂O₇). Oxides of the elements in the centre of the periodic table are amphoteric (Al₂O­₃). Amphoteric oxides are those which behave as acids as well as bases.

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