Electronegativity and Electron Affinity电负性与电子亲和能

Who pulls harder on a shared pair?

In a hydrogen chloride molecule, the hydrogen and chlorine atoms share a pair of electrons (chapter 2.4 covers this in detail). Do the electrons sit exactly halfway between them? Experiments show they sit closer to chlorine: the chlorine end carries a partial negative charge and the hydrogen end a partial positive charge. Atoms of different elements pull on electrons with different strengths, and that pull can be compared with a single number.

Electronegativity

Electronegativity电负性 measures how strongly an atom in a compound attracts the electrons in a bond. The higher the electronegativity, the harder the atom pulls. Linus Pauling introduced electronegativity in 1932 as a relative scale with no units, with fluorine at the top (about 4.0) and the other elements compared with it.

• Electronegativity increases across a period and decreases down a group. The reason is the same as for ionization energy: the larger Z_eff and the smaller the atom, the more strongly the nucleus attracts bonding electrons.
• Fluorine is the most electronegative element (3.98), followed by oxygen (3.44); cesium, at the lower left, is the least electronegative element in the table below (0.79).
• Metals generally have electronegativities below about 1.8 and nonmetals above it; metalloids near the staircase line, such as boron, silicon, and germanium, fall around 1.8 to 2.2. The lower the electronegativity, the more metallic the element; the higher, the more nonmetallic.
• The noble gases helium, neon, and argon form essentially no compounds, so they have no electronegativity values.
• Elements on a diagonal of the table, such as lithium and magnesium or beryllium and aluminum, have similar electronegativities and somewhat similar properties. This is called the diagonal relationship.

About the data: this site uses Pauling’s revised values (F 3.98, O 3.44, Cl 3.16, N 3.04, C 2.55, H 2.20), the set most US textbooks use. Chinese textbooks use Pauling’s older one-decimal values (F 4.0, O 3.5, Cl 3.0, N 3.0, C 2.5, H 2.1). The numbers differ slightly, but the trends are identical.

Using electronegativity difference to classify bonds

The larger the electronegativity difference between two elements, the more the bonding electrons shift toward the more electronegative atom. Roughly:

• a difference below 0.4 means the electrons are shared almost equally, usually a nonpolar covalent bond, as in H–H or C–H;
• a difference between 0.4 and 1.7 means the electrons are pulled toward one atom, usually a polar covalent bond, as in H–Cl or O–H;
• a difference above 1.7 means the electrons are essentially transferred, usually an ionic bond, as in NaCl.

These cutoffs are only rules of thumb, and different books use 1.7, 1.8, or 2.0. Lesson 2.4.7 covers bond polarity in detail.

Electron affinity

Electron affinity电子亲和能 is the energy change when a gaseous atom gains an electron to form a gaseous anion. When a chlorine atom gains an electron, it releases 349 kJ/mol of energy:

• The more energy released, the more readily the atom gains an electron. The halogens have the largest electron affinities, because one more electron completes their octet.
• Electron affinity generally increases across a period and decreases down a group, with many exceptions. Group 2, nitrogen, and the noble gases actually require energy to gain an electron, so their anions are unstable.
• Fluorine’s electron affinity (328 kJ/mol) is smaller than chlorine’s (349 kJ/mol): the fluorine atom is tiny, its 2p sublevel already holds 5 electrons, and an incoming electron is strongly repelled.
• Two sign conventions. Data tables often list the energy released as a positive number (349 kJ/mol for Cl). Many US textbooks instead write it as the energy change of the system, ΔE, which is negative when energy is released, so chlorine’s electron affinity is −349 kJ/mol. Check which convention a question uses before you compare values.

Chinese textbooks focus on ionization energy and electronegativity; electron affinity is a property that US Honors and AP courses add. It is not the same as electronegativity. Electron affinity is the energy change for an isolated gas-phase atom, in kJ/mol; electronegativity describes how strongly an atom in a compound pulls on bonding electrons, a relative number with no units.

Switch between properties and watch the color sweep from one corner of the table to the other. In Electronegativity mode, click two elements to see what kind of bond they’d roughly form:

Example 1: Classifying bonds

Use electronegativity differences to classify the H–Cl, Na–Cl, C–H, and O–H bonds.

Example 2: Ranking electronegativity

Without a table, rank P, S, O, and F from lowest to highest electronegativity, then check with data.

Common mistakes

Quick check

Summary

• Electronegativity is how strongly an atom in a compound attracts bonding electrons. It increases across a period and decreases down a group; fluorine is highest, and the elements at the lower left (cesium and francium) are lowest. Metals are generally below about 1.8, nonmetals above.
• The electronegativity difference roughly classifies a bond: below 0.4 nonpolar covalent, 0.4 to 1.7 polar covalent, above 1.7 usually ionic.
• Electron affinity is the energy change when a gaseous atom gains an electron. The halogens have the largest values, and Cl is larger than F. Tables often list it as positive energy released; many US textbooks write it as a negative ΔE.