Electron affinity describes the energy change when a neutral atom in the gas phase gains an electron to form a negative ion, called an anion. It matters because it helps explain why some elements readily form negative ions while others do not. High electron affinity is especially important for nonmetals such as halogens, which strongly attract extra electrons.
The idea connects atomic structure, periodic trends, and chemical bonding.
Understanding Chemistry: Electron Affinity
An incoming electron feels two competing effects. It is attracted to the positively charged nucleus, which can lower the energy of the atom. At the same time, it is repelled by electrons already present.
The balance between these effects depends on the atom's size and its electron arrangement. A small atom can hold an added electron close to its nucleus, so the attraction is strong.
A larger atom places the new electron farther away, where nuclear attraction is weaker and shielding by inner electrons is greater. Shielding means inner electrons partly block the pull of the nucleus.
Electron arrangements explain important exceptions to broad periodic patterns. Filled and half filled sublevels can be unusually stable. An added electron may have to enter an orbital that already contains one electron.
This creates extra electron repulsion. For this reason, oxygen releases less energy when gaining an electron than sulfur, even though oxygen is above sulfur in the same group. Nitrogen is another useful case.
Its outer p sublevel is half filled, so forcing one more electron into it disrupts a relatively stable arrangement. Trends are useful predictions, not rules that replace careful evidence.
The energy change can be understood using energy levels. If the atom becomes more stable after receiving an electron, energy leaves the atoms and appears as heat or light in the surroundings. This is why electron affinity values are often reported as negative for favorable electron gain.
Some textbooks use a different sign convention and report the amount of energy released as a positive number. Students should always check the definition used by the source before comparing values.
The physical idea stays the same. A large energy release means the added electron is strongly bound in the negative ion.
Electron affinity helps explain how atoms behave during the formation of ionic substances. In sodium chloride, sodium loses an electron more easily than it gains one, while chlorine has a strong tendency to gain an electron. The resulting positive and negative ions attract each other.
Electron affinity alone does not predict whether the whole process happens. Energy is required to remove an electron from sodium, and more energy is released when the ions form a crystal lattice. Chemists consider all of these energy changes together.
When learning this topic, separate electron affinity from ionization energy and electronegativity. Electron affinity concerns an isolated gaseous atom gaining an electron. Electronegativity describes an atom's pull on shared electrons within a bond.
Key Facts
- Electron affinity reaction: X(g) + e- -> X-(g) + energy for many nonmetals.
- Electron affinity is the energy change when 1 mole of gaseous atoms gains 1 mole of electrons.
- A more negative electron affinity value means more energy is released when the electron is added.
- Electron affinity generally becomes more negative from left to right across a period.
- Electron affinity generally becomes less negative down a group because the added electron is farther from the nucleus.
- Ionization energy removes an electron: X(g) -> X+(g) + e-, while electron affinity adds an electron: X(g) + e- -> X-(g).
Vocabulary
- Electron affinity
- Electron affinity is the energy change that occurs when a gaseous neutral atom gains an electron.
- Anion
- An anion is a negatively charged ion formed when an atom or group of atoms gains one or more electrons.
- Ionization energy
- Ionization energy is the energy required to remove an electron from a gaseous atom or ion.
- Effective nuclear charge
- Effective nuclear charge is the net positive pull felt by an electron after accounting for shielding by other electrons.
- Shielding
- Shielding is the reduction of nuclear attraction on outer electrons caused by inner electron shells.
Common Mistakes to Avoid
- Confusing electron affinity with ionization energy. Electron affinity involves adding an electron, while ionization energy involves removing an electron.
- Assuming every atom releases energy when it gains an electron. Some atoms resist gaining electrons because the added electron enters an unfavorable orbital or increases repulsion too much.
- Saying electron affinity always increases smoothly across the periodic table. There are exceptions due to filled and half-filled subshell stability, such as noble gases and some group 2 or group 15 elements.
- Forgetting that electron affinity is defined for gaseous atoms. Values for atoms in solids, liquids, or solutions include other energy effects and are not the same quantity.
Practice Questions
- 1 Write the electron affinity equation for chlorine forming chloride, including physical states and energy on the correct side for an energy-releasing process.
- 2 An atom has an electron affinity of -349 kJ/mol. How much energy is released when 2.00 mol of gaseous atoms each gain one electron?
- 3 Compare fluorine, chlorine, and bromine. Which is expected to have a very favorable electron affinity, and why does atomic size affect the trend down the group?