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, ionisation enthalpy and electron gain enthalpy) will let you conclude that the periodicity in properties basically depends on the electronic configuration of an element.
Table of Contents
The trends of fundamental properties of an element.
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.
Ionisation Enthalpy:
While moving from left to right in a period, the 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, ionisation energy generally increases. However, when we see the trend of ionisation 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 ionisation energy.
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.
Chemical reactivity
The 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.