Notes On Group 17: Atomic Properties - CBSE Class 12 Chemistry
Trends of some of the atomic properties of group seventeen elements: Atomic properties include atomic and ionic radii, ionisation enthalpy, electron gain enthalpy and electro-negativity. Trend of atomic and ionic radii: As we move down the group, the atomic radii and ionic radii increase due to the addition of a new principal energy level in each successive element. These elements have the least atomic radii when compared to other elements in the corresponding periods. This is because of maximum effective nuclear charge. Ionisation enthalpy: These elements show very high values of ionisation enthalpy. As a consequence, the atoms of these elements have little tendency to lose electrons and form positive ions. As we move down the group, the value of ionisation energy decreases. This is due to the gradual increase in the atomic size, which reduces the force of attraction between the valence electrons and the nucleus. The ionisation enthalpy of fluorine is appreciably higher than any other halogen, which is attributed to its small size. Electron gain enthalpy: Halogens have the maximum negative electron gain enthalpy in the respective periods. The electron gain enthalpy becomes less negative on descending the group. Fluorine has less negative electron gain enthalpy than chlorine. I.e. chlorine has the maximum negative electron gain enthalpy among all the elements. It is because of the small size and compact 2p sub-shell of the fluorine atom. Owing to the small size of the fluorine atom, the incoming electron experiences a greater amount of repulsion from the electrons that are already present. The electron-electron repulsions between the incoming electron and the electrons already present .outweigh the attraction between the added electron and the nucleus Electro-negativity: The halogens have very high electro-negativity values. You can see from the values of electro-negativity in the table that the electro-negativity decreases gradually on moving down the group from fluorine to iodine due to the corresponding increase in the atomic radii. Fluorine is the most electronegative element in the periodic table.

Summary

Trends of some of the atomic properties of group seventeen elements: Atomic properties include atomic and ionic radii, ionisation enthalpy, electron gain enthalpy and electro-negativity. Trend of atomic and ionic radii: As we move down the group, the atomic radii and ionic radii increase due to the addition of a new principal energy level in each successive element. These elements have the least atomic radii when compared to other elements in the corresponding periods. This is because of maximum effective nuclear charge. Ionisation enthalpy: These elements show very high values of ionisation enthalpy. As a consequence, the atoms of these elements have little tendency to lose electrons and form positive ions. As we move down the group, the value of ionisation energy decreases. This is due to the gradual increase in the atomic size, which reduces the force of attraction between the valence electrons and the nucleus. The ionisation enthalpy of fluorine is appreciably higher than any other halogen, which is attributed to its small size. Electron gain enthalpy: Halogens have the maximum negative electron gain enthalpy in the respective periods. The electron gain enthalpy becomes less negative on descending the group. Fluorine has less negative electron gain enthalpy than chlorine. I.e. chlorine has the maximum negative electron gain enthalpy among all the elements. It is because of the small size and compact 2p sub-shell of the fluorine atom. Owing to the small size of the fluorine atom, the incoming electron experiences a greater amount of repulsion from the electrons that are already present. The electron-electron repulsions between the incoming electron and the electrons already present .outweigh the attraction between the added electron and the nucleus Electro-negativity: The halogens have very high electro-negativity values. You can see from the values of electro-negativity in the table that the electro-negativity decreases gradually on moving down the group from fluorine to iodine due to the corresponding increase in the atomic radii. Fluorine is the most electronegative element in the periodic table.

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