Ligações Covalentes e Estruturas de Lewis

Nonmetals have high ionization energies, making it difficult to transfer valence electrons to other atoms. Instead, they share electrons with other nonmetals to achieve stable electron configurations. This mutual electron sharing is energetically favorable because both atoms benefit from the shared electrons, creating a covalent bond.

Single bonds involve one shared electron pair, double bonds involve two shared pairs, and triple bonds involve three shared pairs. Triple bonds are shorter and stronger than double bonds, which are shorter and stronger than single bonds. Bond strength increases with multiplicity, which is why triple bonds like those in nitrogen are particularly difficult to break.

Bonding pairs are electrons shared between two atoms in a covalent bond, while lone pairs are valence electrons not participating in bonding. In Lewis structures, bonding pairs are shown as lines or dots between atoms, and lone pairs are shown as dots on individual atoms. Both contribute to an atom's valence electron count.

The Lewis model predicts that molecules are stable when all atoms achieve noble gas electron configurations, typically an octet of valence electrons. For example, water is stable because both oxygen and hydrogen reach stable configurations through electron sharing. If an atom cannot achieve an octet through bonding, the resulting molecule or ion is unstable unless additional electrons are added.

Covalent bonds are directional because shared electrons link two specific atoms, creating distinct molecular units. Ionic bonds are nondirectional and hold multiple ions together in a lattice structure. This directional nature of covalent bonding means individual molecules are the fundamental units in covalent compounds, unlike ionic compounds where the lattice is the basic unit.

Covalent compounds generally have lower melting and boiling points than ionic compounds because attractions between neutral molecules are weaker than those between charged ions. Many covalent compounds are liquids or gases at room temperature and are typically softer as solids. Unlike ionic compounds, most covalent compounds are insoluble in water and poor electrical conductors.

The periodic table shows how many valence electrons an atom needs to reach an octet. Group 17 elements need one electron and form one bond, Group 16 elements need two and form two bonds, and Group 14 elements like carbon need four and form four bonds. Group 18 elements have filled configurations and rarely bond. This pattern helps predict bonding behavior for lewis structures of molecular compounds and polyatomic ions.