Chemical Bonds Quick Reference

Covalent Bonds

• Atoms attract by sharing electrons (not transferring). Covalent bonds can be polar or nonpolar.
• Polar covalent: electrons shared unequally.
• Nonpolar covalent: electrons shared fairly.
• Examples:
  • Water, H$_2$O, polar covalent.
  • Diesel fuel, C$<em>{16}$H$</em>{34}$, nonpolar covalent.

Ionic Bonds

• Electrons transferred between atoms; resulting ions attract (electrostatic).
• Example: NaCl (table salt).

Metallic Bonds (brief note)

• Found in metals; electrons delocalized in a ‘sea’; contributes to hardness and conductivity. Not covered in depth here.

Octet Rule (brief analogy)

• Atoms seek 8 valence electrons in their outer shell.
• Example: Oxygen has 6 valence electrons and tends to share to reach 8; carbon with 4 shares to reach 8 (as in CO$_2$).

Electronegativity

• Definition: tendency of an atom to attract electrons.
• Trends:
  • Increases across a period.
  • Increases up a group.
• Highest: Fluorine, $EN_F \,=\, 3.98$.
• Lowest: Francium, $EN_{Fr} \,=\, 0.7$.
• More electronegative atoms pull shared electrons more strongly, affecting bond type.

Bond classification by electronegativity difference

• Define difference: $\Delta EN = |EN<em>A - EN</em>B|$.
• Thresholds:
  • Nonpolar covalent: $\Delta EN < 0.5$.
  • Polar covalent: $0.5 \le \Delta EN \le 1.7$.
  • Ionic: $\Delta EN > 1.7$.

Examples (apply the thresholds)

• Water: H$_2$O
  • $EN<em>H = 2.2,\ EN</em>O = 3.44$
  • $\Delta EN = |3.44 - 2.2| = 1.24$ → polar covalent.
• Diesel fuel: C$<em>{16}$H$</em>{34}$
  • $EN<em>C = 2.55,\ EN</em>H = 2.2$
  • $\Delta EN = |2.55 - 2.2| = 0.35$ → nonpolar covalent.
• Salt: NaCl
  • $EN<em>{Na} = 0.93,\ EN</em>{Cl} = 3.16$
  • $\Delta EN = |3.16 - 0.93| = 2.23$ → ionic.
• Ammonia: NH$_3$
  • $EN<em>N = 3.04,\ EN</em>H = 2.2$
  • $\Delta EN = |3.04 - 2.2| = 0.84$ → polar covalent.