Covalent Compounds

Covalent Compounds: Overview

• Formed when nonmetals bond with each other via covalent bonds (electron sharing).
• Ionic compounds form from transfer of electrons between a cation and an anion; covalent compounds rely on sharing to satisfy the octet.
• Examples (covalent):
  • $N<em>2O</em>4$ (dinitrogen tetroxide)
  • $SF_6$ (sulfur hexafluoride)
  • $SiCl_4$ (silicon tetrachloride)
  • Other covalent molecules include $H_2O$ and many nonmetal–nonmetal species.

Bonding and the Octet Rule

• Covalent bonds arise from electron sharing to achieve a stable valence shell.
• Octet Rule: most atoms prefer a full valence shell of 8 electrons.
• Hydrogen is an exception: it aims for 2 electrons (1s^1 + another electron from bonding).
• Why eight? Noble gas configuration; s can hold 2 e⁻, p can hold 6 e⁻, totaling 8 for main-group elements.

Writing Formulas from Names (Covalent Compounds)

• Greek prefixes indicate the number of atoms:
  • mono-, di-, tri-, tetra-, penta-, hexa-, hepta-, octa-, nona-, deca-
• Important rules:
  • The first element’s prefix mono- is not written if there is only one atom.
  • The second element ends with -ide and uses the appropriate prefix.
  • When certain prefixes meet with vowel sounds, the final 'a' of the prefix may be dropped (e.g., diphosphorus pentoxide instead of disphosphorus pentaoxide).
• Examples:
  • Carbon dioxide: $CO_2$ (name: carbon dioxide)
  • Diphosphorus pentoxide: $P<em>2O</em>5$
  • Dinitrogen monoxide: $N_2O$
  • Iodine pentafluoride: $IF_5$
  • Tetraphosphorus decoxide: $P<em>4O</em>{10}$
  • Tetrasulfur tetranitride: $S<em>4N</em>4$

Writing Names from Formulas (Covalent Formulas)

• For binary covalent compounds, name the element farther left in the periodic table first. If in the same group, name the lower (more distant down the table) element first.
• Use prefixes to denote the number of each atom; the first element’s prefix is omitted if it is 1, the second element uses its full prefix plus -ide.
• Examples:
  • $CO$ → carbon monoxide
  • $P<em>2O</em>5$ → diphosphorus pentoxide
  • $NO$ → dinitrogen monoxide
  • $IF_5$ → iodine pentafluoride
• Note: when naming, the second element always ends with -ide.

Greek Prefixes and Numerals (quick reference)

• mono- : 1
• di- : 2
• tri- : 3
• tetra-: 4
• penta-: 5
• hexa- : 6
• hepta-: 7
• octa- : 8
• nona- : 9
• deca- : 10

Practice/Examples (from the slides)

• Name from formula: $N_2O$ → dinitrogen monoxide
• From name: $N<em>2O</em>5$ → diphosphorus pentoxide
• Formula from name: carbon monoxide → $CO$
• Formula from name (example with vowels rule): P2O5 → $P<em>2O</em>5$
• Other common formulas:
  • $N O$ → NO (nitrogen monoxide)
  • $SeF_6$ → selenium hexafluoride
  • $P<em>4S</em>6$ → tetraphosphorus hexasulfide
  • $P<em>4O</em>{10}$ → tetraphosphorus decoxide
  • $S<em>4N</em>4$ → tetrasulfur tetranitride

Quick Q&A (sample from the slides)

• What is the name of the covalent compound with formula $CCl_2$? → carbon dichloride
• What is the name of the covalent compound with formula $CCl_4$? → carbon tetrachloride
• What is the name of the covalent compound with formula $SO_3$? → sulfur trioxide
• What is the name of the covalent compound with formula $N_2O$? → dinitrogen monoxide
• If given a name, e.g., phosphorus tribromide, the formula is $PBr<em>3$; if given a formula, e.g., $PBr</em>3$, the name is phosphorus tribromide