[Alext180]

Could someone explain electron affinity? I don't understand the definition "the change in energy that occurs when a neutral atom gains or loses an electron"
Why would a neutral atom want to gain or lose an electron?
Why is its trend similar to atomic radius? What does it have to with atomic radius?

Thank you

[Ben Bob2]

The definition:
   
Electron affinity is explained by “octet stability.” If you understand the rule of octet stability, think about this: how will the energy of an atom change if it gains an electron and reaches the electron configuration of a noble gas?
    
This paragraph answers your second question better than I can:

Electron affinity generally decreases down a group of elements because each atom is larger than the atom above it (this is the atomic radius trend, which will be discussed later in this text). This means that an added electron is further away from the atom's nucleus compared to its position in the smaller atom. With a larger distance between the negatively-charged electron and the positively-charged nucleus, the force of attraction is relatively weaker. Therefore, electron affinity decreases.

Source: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Trends_of_Elemental_Properties/Periodic_Trends

[Alext180]

The definition:
   
Electron affinity is explained by “octet stability.” If you understand the rule of octet stability, think about this: how will the energy of an atom change if it gains an electron and reaches the electron configuration of a noble gas?
    
This paragraph answers your second question better than I can:

Electron affinity generally decreases down a group of elements because each atom is larger than the atom above it (this is the atomic radius trend, which will be discussed later in this text). This means that an added electron is further away from the atom's nucleus compared to its position in the smaller atom. With a larger distance between the negatively-charged electron and the positively-charged nucleus, the force of attraction is relatively weaker. Therefore, electron affinity decreases.

Source: http://chemwiki.ucdavis.edu/Inorganic_Chemistry/Descriptive_Chemistry/Periodic_Trends_of_Elemental_Properties/Periodic_Trends

I see, so electron affinity deals with both the energy an atom has after it becomes stable, AND its ability to gain the electrons to get there? Then, how is it different from electronegativity? According to the website, electronegativity is the ability to attract, while electron affinity is the ability to accept. How is that not the same thing?

Thanks for the help

[mjc123]

Electron affinity is not "the change in energy that occurs when a neutral atom gains or loses an electron". It is the change in energy when an atom or ion gains an electron. I say "or ion" because you can have first, second etc. electron affinities. For example for oxygen the first EA is the energy of the process
O(g) + e  O^-(g)
and the second EA is the energy change of the process
O^-(g) + e  O^2-(g)
Ionisation energy is the energy change when an atom or ion loses an electron.
The difference between EA and electronegativity is that EA is a well-defined, measurable quantity, while electronegativity is a vaguer concept. EA is a property of free atoms in the gas phase, electronegativity is "the tendency of an atom in a molecule to attract electron density to itself". That is harder to quantify, and there are several different ways of estimating electronegativity - see e.g. the Wikipedia article on "electronegativity".