The strength with which an atom attracts electrons in a covalent bond is called electronegativity. It also changes regularly across the periodic table and determines the nature of chemical bonds.
Basic Knowledge
Electronegativity (Pauling definition) is the relative value of the strength with which an atom in a covalent bond attracts the shared electron pair. Representative values:
・F = 4.0 (maximum)
・O = 3.5
・N = 3.0
・Cl = 3.0
・C = 2.5
・H = 2.1
・Na = 0.9
・K = 0.8 (small)
The trend is clear: it is larger toward the upper right of the periodic table (excluding noble gases) and smaller toward the lower left. F is the largest, and the region around Fr is the smallest.
The type of bond changes according to the difference in electronegativity:
・Large difference (about 1.7 or more) → ionic bond (such as Na-Cl)
・Moderate difference → polar covalent bond (H-O, H-Cl)
・Almost no difference → nonpolar covalent bond (H-H, Cl-Cl)
This is an important concept that introduces chemical bonding, which will be covered in the next lesson.
Electronegativity（Strength of attracting electrons in a covalent bond. F is the maximum at 4.0）
Polarity（A separation of positive and negative charge in a bond or molecule caused by uneven electron distribution）
Nonpolar bond（Almost no difference in electronegativity. H-H, Cl-Cl）
Polar bond（A difference in electronegativity. Charge distribution of δ+ and δ-）
Metallic and nonmetallic elements（Separated by a line on the periodic table. Metals are lower left; nonmetals are upper right）
Metalloids（Elements with both metallic and nonmetallic properties. Al, Zn, Sn, Pb）
Further Study (Background and Significance)
Electronegativity was proposed in 1932 by Pauling, a twentieth-century chemist who received two Nobel Prizes. It made it possible to understand chemical bonding as a continuous spectrum from ionic bonding to polar covalent bonding to nonpolar covalent bonding.
The overall polarity of a molecule is determined by both bond polarity and molecular shape (symmetry). For example, CO2 has polar C=O bonds, but because it is linear, the dipole moments cancel and it is a nonpolar molecule. H2O also has polar O-H bonds, but its bent shape makes it a polar molecule. Polarity affects solubility (like dissolves like), boiling point, surface tension, and many other properties.
In organic chemistry, predicting reactivity from the polarity of functional groups becomes routine. Because electronegativity is also a starting point for organic reactions, mastering its basic trends here is extremely important.
- Electronegativity = strength of attracting a shared electron pair
- F=4.0 is the maximum; it increases toward the upper right
- The difference determines the type of bond (ionic or covalent)
- Large difference → ionic; moderate → polar covalent; 0 → nonpolar covalent
- Pauling proposed it in 1932
- Molecular polarity = bond polarity + shape (symmetry)
- CO2 is nonpolar; H2O is polar
- Solubility follows “like dissolves like”
Points to Note (Common Confusions)
① Do not confuse electronegativity (in a covalent bond), ionization energy (releasing an electron), and electron affinity (receiving an electron). They have similar trends but different definitions. ② Bond polarity (uneven electron distribution between atoms) and molecular polarity (uneven distribution throughout the molecule) are different. A symmetrical molecule may be nonpolar even when its bonds are polar. ③ Noble gases rarely form covalent bonds, so electronegativity values are often not listed for them. ④ Electronegativity is a relative value, not an absolute value (Pauling set F=4.0 as the reference).
Practice
- Which element has the greatest electronegativity? Give its value.
- Are CO2 and H2O polar or nonpolar molecules? Briefly explain why.
- What type of bond is likely to form between atoms with a large electronegativity difference?