Short Answer

Electron affinity is influenced by several factors that determine how easily an atom can accept an electron.

• Atomic size, nuclear charge, shielding effect, and electron configuration are the main factors.
• Smaller atoms with high nuclear charge, like fluorine, have high electron affinity, while larger atoms or noble gases have low or positive electron affinity.

Detailed Explanation :

Definition of Electron Affinity

Electron affinity (EA) is the energy change when a gaseous atom gains an electron to form an anion. It measures how strongly an atom attracts electrons. The value can be negative (energy released) or positive (energy absorbed) depending on the element.

The ability of an atom to gain an electron is affected by several factors which determine the magnitude of electron affinity.

Factors Affecting Electron Affinity

1. Atomic Size

• Smaller atoms have electrons closer to the nucleus, so added electrons experience stronger nuclear attraction.
• Larger atoms have outer electrons farther from the nucleus, so attraction for the incoming electron is weaker.
• Example: Fluorine (small atomic radius) has higher EA than chlorine in the same group.

2. Nuclear Charge

• More protons in the nucleus → stronger positive charge → stronger pull on the added electron.
• Higher nuclear charge increases electron affinity because the incoming electron is held more tightly.
• Example: Across a period, EA increases due to increasing nuclear charge from left to right.

3. Electron Shielding

• Inner shell electrons shield outer electrons from the nucleus, reducing the effective nuclear attraction.
• Greater shielding → weaker pull on the added electron → lower electron affinity.
• Example: Down a group, additional electron shells increase shielding → EA decreases.

4. Electron Configuration and Stability

• Atoms with half-filled or fully filled orbitals are more stable → resist gaining electrons → lower EA.
• Atoms with unfilled orbitals gain electrons more easily → higher EA.
• Example: Nitrogen (half-filled 2p³) has lower EA than expected, while oxygen (2p⁴) has higher EA.

5. Sublevel of Incoming Electron

• Electrons added to s-orbitals (closer to nucleus) are strongly attracted → higher EA.
• Electrons added to p-orbitals experience less nuclear pull → lower EA.
• Example: Halogens gain electrons in p-orbitals, releasing significant energy but slightly less than s-orbitals.

6. Periodic Position of Elements

1. Across a Period (Left to Right):
   • EA generally increases.
   • Reason: Nuclear charge increases, atomic radius decreases → electrons pulled closer.
2. Down a Group (Top to Bottom):
   • EA generally decreases.
   • Reason: Increased distance from nucleus and greater shielding reduce attraction.

Examples of Factor Effects

• Halogens: High EA due to small size, high nuclear charge, and unfilled p-orbitals.
• Alkali Metals: Low EA because large atomic size and single valence electron → weak attraction.
• Noble Gases: Positive or very low EA due to fully filled stable orbitals, resisting electron gain.

Significance of Factors

1. Predicting Reactivity: High EA → reactive non-metals; low EA → inert metals.
2. Formation of Anions: Determines stability and energy released in ionic compounds.
3. Periodic Trends: Explains variation in EA across periods and groups.
4. Chemical Bonding: Helps understand ionic and covalent bond formation.
5. Industrial Applications: Used in chemical synthesis, halogen reactions, and material design.

Conclusion

Electron affinity is affected by atomic size, nuclear charge, electron shielding, electron configuration, and orbital type. These factors control how strongly an atom attracts an extra electron, influencing reactivity, anion formation, and periodic trends. Understanding these factors is essential for predicting chemical behavior and bonding patterns of elements in the periodic table.