7.2 Covalent Bonding

Covalent bonding is all about sharing electrons between atoms. This type of bond forms when nonmetals team up, sharing electrons to achieve stability. The strength of these bonds depends on how tightly the atoms hold onto their shared electrons.

Covalent bonds can be polar or nonpolar, affecting a molecule's properties. This impacts how compounds behave, from their melting points to their ability to conduct electricity or dissolve in different substances. Understanding covalent bonding helps explain why materials act the way they do.

Covalent Bonding

Formation of covalent bonds

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• Covalent bonds form when atoms share electrons to achieve a stable octet (8 electrons) in their outer shell, allowing them to fill their valence shells and become more stable
• Covalent bonds typically form between nonmetals, which have high electronegativity and tend to gain electrons rather than lose them (carbon, oxygen, nitrogen)
• The shared electrons in a covalent bond are attracted to the nuclei of both atoms, creating a stable molecule held together by this attraction
• The strength of a covalent bond is measured by its bond energy, which is the energy required to break the bond

Polarity in covalent bonds

• Electronegativity measures an atom's ability to attract electrons in a chemical bond, with atoms of higher electronegativity more strongly attracting electrons
• Covalent bonds can be nonpolar or polar depending on the electronegativity difference between the bonded atoms
  • Nonpolar covalent bonds form when the electronegativity difference is close to zero, resulting in equal sharing of electrons between the atoms ($\ce{H2}$, $\ce{Cl2}$, $\ce{O2}$)
  • Polar covalent bonds form when there is a significant electronegativity difference between the atoms, causing unequal sharing of electrons
    • The more electronegative atom attracts the electrons more strongly, developing a partial negative charge ($\delta-$)
    • The less electronegative atom develops a partial positive charge ($\delta+$) ($\ce{HCl}$, $\ce{H2O}$, $\ce{NH3}$)

Covalent vs ionic compounds

• Covalent compounds have different properties compared to ionic compounds due to the nature of their bonding
• Melting and boiling points
  • Covalent compounds generally have lower melting and boiling points than ionic compounds because covalent bonds are relatively weak compared to the strong electrostatic forces in ionic compounds (sugar vs salt)
• Electrical conductivity
  • Covalent compounds are typically poor conductors of electricity in both solid and molten states because they lack free electrons or mobile ions to conduct electricity (plastic, oil)
  • Ionic compounds are good conductors of electricity when molten or dissolved in water because they have mobile ions that can carry electrical current (salt water, molten NaCl)
• Solubility
  • Covalent compounds are often soluble in organic solvents but insoluble in water because they are typically nonpolar and do not interact strongly with polar water molecules (oil, wax)
  • Ionic compounds are usually soluble in water but insoluble in organic solvents because they are held together by strong electrostatic forces that can be disrupted by polar water molecules (NaCl, KCl)

Molecular Structure and Representation

• Lewis structures are diagrams used to represent the arrangement of valence electrons in molecules, showing both bonding and non-bonding electron pairs
• The molecular geometry of a compound is determined by the arrangement of atoms in three-dimensional space, which affects its properties and reactivity
• Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals, which influences the shape and bonding capabilities of molecules