left to right across a period and consequently it will be easier to add an electron to a smaller atom since the added electron on an average would be closer to the positively charged nucleus. We should also expect electron gain enthalpy to become less negative as we go down a group because the size of the atom increases and the added electron would be farther from the nucleus. This is generally the case (Table . ).
However, electron gain enthalpy of O or F is less negative than that of the succeeding element. This is because when an electron is added to O or F, the added electron goes to the smaller n = quantum level and suffers significant repulsion from the other electrons present in this level. For the n = quantum level (S or Cl), the added electron occupies a larger region of space and the electron- electron repulsion is much less. Problem .
Which of the following will have the most negative electron gain enthalpy and which the least negative? P, S, Cl, F. Explain your answer. Electron gain enthalpy generally becomes more negative across a period as we move from left to right.
Within a group, electron gain enthalpy becomes less negative down a group. However, adding an electron to the p- orbital leads to greater repulsion than adding an electron to the larger p- orbital. Hence the element with most negative electron gain enthalpy is chlorine; the one with the least negative electron gain enthalpy is phosphorus. (e) Electronegativity A qualitative measure of the ability of an atom in a chemical compound to attract shared electrons to itself is called electronegativity .
Unlike ionization enthalpy and electron gain enthalpy, it is not a measureable quantity. However, a number of numerical scales of electronegativity of elements viz., Pauling scale, Mulliken-Jaffe scale, Allred-Rochow scale have been developed. The one which is the most widely used is the Pauling scale. Linus Pauling, an American scientist, in assigned arbitrarily a value of .
to fluorine, the element considered to have the greatest * In