Electron Affinity & Electronegativity
Definition
Electron affinity is the amount of energy released or absorbed when an isolated gaseous atom gains an electron to form a negative ion.
For example:
Electronegativity is the relative tendency of an atom in a chemical bond to attract the bonded electron pair toward itself. It is a dimensionless property and is expressed on scales such as the Pauling scale.
Main Content
1. Electron Affinity
- Electron affinity is a measure of the energy change that occurs when an atom in the gaseous state accepts an electron. If energy is released, the electron affinity is considered favorable and usually expressed as a negative value in thermodynamic convention.
- It is a property of isolated atoms, unlike electronegativity, which is observed in bonded atoms. Elements with a high tendency to gain electrons, such as halogens, generally have high electron affinity. For example, chlorine has a very high electron affinity because it readily accepts an electron to attain a stable noble gas configuration.
Important explanation:
Electron affinity is influenced by atomic size, nuclear charge, shielding effect, and electronic configuration. Smaller atoms with higher effective nuclear charge attract an added electron more strongly, so they usually show greater electron affinity. However, exceptions exist due to extra stability of half-filled and fully filled orbitals. For instance, nitrogen has a low electron affinity because its half-filled 2p subshell is relatively stable.
2. Electronegativity
- Electronegativity is the ability of an atom in a molecule to pull the shared electron pair toward itself. It determines bond polarity and helps explain whether a bond is more ionic or more covalent in nature.
- It is not a directly measurable quantity for isolated atoms; instead, it is assigned using comparative scales. The most common scale is the Pauling scale, where fluorine is given the highest electronegativity value of 4.0.
Important explanation:
Electronegativity depends on atomic radius, nuclear charge, and shielding. As atomic size decreases and effective nuclear charge increases, electronegativity increases. Hence, electronegativity increases from left to right across a period and decreases from top to bottom down a group. Fluorine is the most electronegative element, while cesium and francium are among the least electronegative. Electronegativity is crucial in predicting dipole moment, bond strength, and chemical reactivity.
3. Comparison and Periodic Trends
- Electron affinity and electronegativity are related because both describe the attraction of electrons, but they differ in definition, measurement, and context. Electron affinity is an atomic property related to the gain of an electron, while electronegativity is a bonding property related to the attraction of shared electrons.
- Both properties generally show similar periodic trends: they increase across a period and decrease down a group. However, electron affinity has more irregularities than electronegativity because it depends strongly on electronic configuration and subshell stability. For example, noble gases usually have very low or positive electron affinity because adding an electron would require entry into a new shell, while electronegativity values for noble gases are often not assigned or are very low in typical chemistry contexts.
Important explanation:
These properties help explain why nonmetals tend to gain or attract electrons, while metals tend to lose them. High electron affinity and high electronegativity are characteristic of nonmetals such as fluorine, oxygen, and chlorine. Low values are typical of alkali metals such as sodium and potassium. The periodic behavior of these properties is an important part of Unit VII: Periodic properties.
Working / Process
1. Determine atomic structure and position in the periodic table
- Identify the element’s atomic number, period, and group.
- Use these to estimate whether the atom is likely to have a small radius, high effective nuclear charge, or strong shielding.
- These factors help predict both electron affinity and electronegativity.
2. Analyze electron gain or bond attraction
- For electron affinity, consider how much energy is released when an atom gains an electron in the gaseous state.
- For electronegativity, examine how strongly an atom attracts bonding electrons within a molecule.
- Compare the element with others in the same period or group to infer relative trends.
3. Apply trends and exceptions
- Across a period, both properties usually increase due to increasing nuclear charge and decreasing atomic size.
- Down a group, both usually decrease due to increased atomic size and shielding.
- Check for exceptions caused by stable electronic configurations, such as half-filled or fully filled subshells, which can lower electron affinity.
Advantages / Applications
- Helps predict the type of bond formed between atoms, such as ionic, polar covalent, or nonpolar covalent.
- Explains chemical reactivity trends of elements, especially the high reactivity of halogens and the low reactivity of noble gases.
- Useful in understanding molecular polarity, dipole moment, acid strength, and electron transfer in redox reactions.
Summary
- Electron affinity measures the energy change when an atom gains an electron.
- Electronegativity measures how strongly an atom attracts shared electrons in a bond.
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Both properties generally increase across a period and decrease down a group, with important exceptions.
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Important terms to remember: electron affinity, electronegativity, effective nuclear charge, shielding effect, atomic radius, Pauling scale, bond polarity, periodic trend