Topic 5

Electron Affinity

                            Electron affinity is a measure of an atom's ability to attract an additional electron. It's defined as the energy change (either released or absorbed) when an electron is added to a neutral gaseous atom to form a negative ion (anion).

Key points:

·         Energy Release or Absorption:

                        Electron affinity can be either positive or negative:

o    Positive: 

                        If energy is absorbed when an electron is added, the electron affinity is positive, indicating that the atom is less likely to accept an electron.

o    Negative: 

                        If energy is released when an electron is added, the electron affinity is negative, indicating that the atom has a strong affinity for electrons.

·         Trend Across a Period:


o    Increases: 

                    Generally, electron affinity increases from left to right across a period. As atomic size decreases, the nucleus has a stronger attraction for electrons, making it more likely to accept one.

·         Trend Down a Group:


o    Decreases: 

                    Electron affinity generally decreases from top to bottom within a group. As atomic size increases, the shielding effect of inner electrons becomes stronger, reducing the attraction between the nucleus and the incoming electron.

Examples:

·         Halogens (Group 17): Halogens typically have high negative electron affinities, indicating a strong tendency to gain electrons and form anions.

·         Noble Gases (Group 18): Noble gases have relatively low electron affinities, as their filled outer electron shells make them relatively stable and less likely to accept additional electrons.

Factors affecting electron affinity:


·         Atomic size: 

                    Smaller atoms generally have higher electron affinities due to stronger nuclear attraction.

·         Shielding effect: 

                    The presence of inner electrons can shield the nucleus from the incoming electron, reducing the attraction.

·         Electron configuration: 

                    Atoms with half-filled or completely filled subshells may have slightly higher electron affinities due to the stability associated with these configurations.

In summary: 

            Electron affinity is a measure of an atom's ability to attract an electron. It generally increases across a period and decreases down a group, reflecting trends in atomic size and shielding effects. Understanding electron affinity is important for understanding the formation of ions and chemical bonding.