Read the given passage and answer the questions that follow:
When an electron is added to a neutral gaseous atom (X) to convert it into a negative ion, the enthalpy change accompanying the process is defined as the Electron Gain Enthalpy (∆egH). Electron gain enthalpy provides a measure of the ease with which an atom adds an electron to form anion as represented by equation X(g) + e – → X – (g) . Depending on the element, the process of adding an electron to the atom can be either endothermic or exothermic....
1. The first electron gain enthalpy values are positive for the noble gases. Why?
2. (i)The first electron gain enthalpy values for the 3rd period elements in the P block is more than the respective
(ii) period elements, why?
3. Second electron gain enthalpy is generally negative, why?
4. What is the trend in electron affinity along the period and group?
5. Electron gain enthalpy of F is lower than the Cl while the I.E value is higher. Explain
Answers
Answer:
The noble gases present in the group 18 of periodic table, have electronic configuration ns2np6. ... Thus, in case of noble gases, where it resists the addition of electrons, extra energy is required in order to force the electron to bind to the stable atom. Therefore, making the electron gain enthalpies positive.
Electron gain enthalpy value of the 3rd peirod p-block elements of the mordern periodic table are generally more negative than the 2nd period element of the same group. <br> Reason: Due to smaller atomic size of the 2nd peirod element, its electron density is high which eases the addition of electron.
Why is the second electron gain enthalpy negative (i.e. energy is absorbed)? Second is to be added to negatively charged ions which will repel the incoming electron. The energy required to overcome repulsion is more than energy released on adding electron.
Electron affinity generally increases across a period in the periodic table and sometimes decreases down a group. These trends are not necessarily universal. The chemical rationale for changes in electron affinity across the periodic table is the increased effective nuclear charge across a period and up a group.
It is due to small size of fluorine atom. As, there are strong inter electron repulsion in the relatively small 2P orbits of fluorine and thus, the incoming electron does not experience much attraction. Therefore chlorine has the electron gain enthalpy than the fluorine.