Covalent Bonds and Molecular Compounds

Covalent Bonds

  • Covalent bonds are the type of bonds that hold elements together by sharing electrons instead of transferring them completely.

    • Definition: Covalent Bond - A chemical bond formed by the sharing of electrons between atoms.

Molecules

  • When elements are held together by covalent bonds, they form discrete units known as molecules.

    • Example: Two individual hydrogen atoms can bond to form a hydrogen molecule (H₂).

Valence Electrons and Noble Gas Configuration

  • Each hydrogen atom has one valence electron.

  • To achieve the nearest noble gas configuration (helium configuration), hydrogen atoms pair with another hydrogen atom, sharing their valence electrons.

    • Result: Each hydrogen atom effectively has two electrons (due to sharing).

Representation of Covalent Bonds

  • Covalent bonds can be represented visually in various ways, but most commonly:

    • Shared electrons can be shown using dots (•) between the element symbols.

    • More commonly, a line (—) is used to denote a covalent bond: each line represents a pair of shared electrons.

Chlorine Example

  • Chlorine is often referred to in covalent bond examples. It is part of a group that needs electrons to form a complete octet.

    • Chlorine’s Lewis Structure contains a total of eight electrons (two in each bond shared with another atom).

Types of Covalent Bonds

  • If one pair of electrons is shared between two atoms, it creates a single covalent bond.

  • If two pairs of electrons are shared, it creates a double covalent bond.

  • Example: Oxygen atoms can share two pairs of electrons to form a double bond.

Naming and Writing Formulas for Molecular Compounds

  • Molecular compounds consist of nonmetals that can combine in various ratios.

    • Example: Compounds formed between nitrogen and oxygen can vary widely (such as NO, N₂O, N₂O₅, etc.).

  • To distinguish different compounds formed from the same elements, Greek prefixes are used:

    • Mono- (1), Di- (2), Tri- (3), Tetra- (4), Penta- (5), Hexa- (6), Hepta- (7), Octa- (8), Nona- (9), Deca- (10).

Prefixes in Compound Nomenclature

  • Rules for naming molecular compounds:

    • The first element keeps its entire name.

    • The second element is named using its root and an ending of -ide.

    • Prefixes are added based on the number of atoms present.

    • If the first element has a subscript of one, no prefix is needed.

    • Ensure that prefixes do not create awkward pronunciations by dropping the last vowel when needed; for example, CO becomes carbon monoxide.

Drawing Lewis Structures

  • Objective: To visualize the distribution of valence electrons in a molecule.

  • Identify the central atom (the one with the lowest group number on the periodic table).

    • Exception: Hydrogen cannot be a central atom as it only needs one electron to complete its valence shell.

Example of Lewis Structure for CF₄

  1. Identify central atom (Carbon, C).

  2. List valence electrons for each atom:

    • Carbon: 4 valence electrons.

    • Fluorine: 7 valence electrons (4 Fluorines = 28 valence electrons).

  3. Total valence electrons = 4 (C) + 28 (F) = 32 electrons.

  4. Drawing connections: Each bond uses 2 electrons, so from 32 total, subtract 8 used in bonding (4 bonds), leaving 24 electrons for lone pairs on Fluorine.

  5. Each Fluorine needs 6 more electrons to complete its octet (totaling 8 per atom).

  6. Therefore, structurally represented, CF₄ has complete octets.

Electron Assignments and Octet Rule

  • Review how electrons are assigned within a structure:

    • Each bond accounts for two electrons.

    • Individual elements must satisfy the octet rule (8 electrons), except for exceptions:

    • Hydrogen needs 2.

    • Beryllium needs 4.

    • Boron needs 6.

    • All other elements generally need 8 electrons for stability.

Validation of Electron Structures

  • Verify that individual elements in a molecule have satisfied their necessary electron requirements.

    • Example Validation: In CF₄, all Fluorine atoms should have 8 electrons (octets completed) and the Carbon atom should also have its 8 electrons (4 bonds create 8).

Examples of Other Molecules

  • Transition from CF₄ to SiH₄ and others by determining central atoms, electron counts, and verifying completed octets for all parts of the structure.

  • Practice identifying central atoms based on their capabilities and the presence of hydrogen as an exception.