What is electron gain enthalpy $\left( \Delta_{eg} H \right)$? When does its value become positive and negative?

(N/A) 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 $\left( \Delta_{eg} H \right)$.'
Electron gain enthalpy provides a measure of the ease with which an atom adds an electron to form an anion,as represented by the equation:
$X_{(g)} + e^{-} \rightarrow X_{(g)}^{-}; \Delta_{eg} H$
Depending on the element,the process of adding an electron to the atom can be either endothermic or exothermic.
For many elements,energy is released when an electron is added to the atom,and the electron gain enthalpy is negative.
For example,group $17$ elements (the halogens) have very high negative electron gain enthalpies because they can attain stable noble gas electronic configurations by picking up an electron.
On the other hand,noble gases have large positive electron gain enthalpies because the electron has to enter the next higher principal quantum level,leading to a very unstable electronic configuration.
It may be noted that electron gain enthalpies have large negative values toward the upper right of the periodic table,preceding the noble gases.