Covalent bonding and small molecules
Bonding, structure and the properties of matter • Chemical bonds
Flashcards
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Key concepts
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Covalent bonding: definition and cause
Covalent bonding occurs when two non-metal atoms share pairs of electrons. Sharing provides both atoms with a more stable electron arrangement by filling their outer electron shells (often to a noble gas configuration). Covalent bonds form between atoms whose electronegativities allow mutual attraction of the shared electrons. The number of shared pairs equals the number of covalent bonds between the atoms. Single bonds share one pair, double bonds share two pairs and triple bonds share three pairs. The bond type affects bond strength and bond length: more shared pairs produce stronger, shorter bonds.
Dot-and-cross diagrams (electrons shown)
Dot-and-cross diagrams show the outer electrons of each atom using different symbols (dots versus crosses) so that the origin of each electron is clear. The shared pairs appear between atoms and non-shared (lone) pairs remain beside a single atom. Diagrams must show only outer-shell electrons for clarity and relevance to bonding. Examples: H2 shows one shared pair between two H atoms; Cl2 shows one shared pair between two Cl atoms with three lone pairs on each chlorine; O2 shows two shared pairs (a double bond) and two lone pairs on each oxygen atom; N2 shows three shared pairs (a triple bond). For molecules with different elements, such as HCl, the shared pair sits between H and Cl and chlorine retains three lone pairs.
Line representation of covalent bonds
A single covalent bond commonly uses a single straight line between atom symbols to represent a shared pair of electrons. Double and triple bonds use two or three lines respectively. Line diagrams simplify structures by omitting separate electron symbols while keeping the number and arrangement of bonds clear. Line-bond diagrams are useful for drawing larger structures or for quick recognition of molecular formulae. For example, H2O can be written H–O–H and CH4 as H–C–H with the remaining three H atoms attached to carbon (usually shown with lines radiating from C).
Deducing molecular formula from models and diagrams
A molecular formula lists the number and types of atoms in a molecule (for example H2O or CH4). Counting atom symbols in a model or diagram gives the molecular formula directly. In dot-and-cross or line diagrams, each atom symbol counts once per occurrence. When presented with a ball-and-stick or 2D diagram, identify each atom type and count the total atoms of each element. The simplest whole-number ratio of these counts provides the molecular formula for a discrete molecule (not for giant covalent lattices).
Common small molecules and typical bonding patterns
Common small molecules follow predictable bonding patterns: hydrogen (H) forms 1 bond; oxygen (O) typically forms 2 bonds; nitrogen (N) typically forms 3 bonds; carbon (C) typically forms 4 bonds; chlorine (Cl) typically forms 1 bond. Typical examples: H2 (hydrogen), Cl2 (chlorine), O2 (oxygen), N2 (nitrogen), HCl (hydrogen chloride), H2O (water), NH3 (ammonia), CH4 (methane). Small molecules usually have low melting and boiling points because intermolecular forces (such as van der Waals forces or hydrogen bonding) are weaker than ionic or covalent network bonds. Presence of hydrogen bonding (for H2O and NH3) raises boiling points relative to similar-sized molecules without hydrogen bonding.
Key notes
Important points to keep in mind