whenever electron enters the atom, energy is released. In other words the first electron affinity has a negative value because energy is released. why is energy released?
Perhaps the easiest way to grasp this is to look at the opposite first. Take sodium, electron structure 2.8.1. To remove the outer orbit electron [1st ionization energy] you must pull it off .. ie put in energy, positive value.
So with chlorine, ES 2.8.7 you are doing the exact opposite. Cl atoms attract an extra electron to achieve the ES of the nearest rare gas 2.8.8 and energy is released. 1st EA is negative value.
Have a look at
http://www.chemguide.co.uk/atoms/properties/eas.html
http://www.chemguide.co.uk/atoms/properties/ies.html
where these terms are well explained
Perhaps the easiest way to grasp this is to look at the opposite first. Take sodium, electron structure 2.8.1. To remove the outer orbit electron [1st ionization energy] you must pull it off .. ie put in energy, positive value.
So with chlorine, ES 2.8.7 you are doing the exact opposite. Cl atoms attract an extra electron to achieve the ES of the nearest rare gas 2.8.8 and energy is released. 1st EA is negative value.
Have a look at
http://www.chemguide.co.uk/atoms/properties/eas.html
http://www.chemguide.co.uk/atoms/properties/ies.html
where these terms are well explained
References :
retired chemistry examiner