In reactions involving covalent compounds, a complete electron transfer does not occur. Instead, there is a shift of electrons toward the more electronegative atom. Some atoms have a partial gain of electrons; others have a partial loss of electrons. Charge is called oxidation numbers or oxidation state for an individual atom of one type Used to indicate approximate electron distribution in covalent bonding What contains covalent bonding? molecular compounds polyatomic ions Not “real” charges since electrons are shared
Oxidation States • Shared electron are counted as “belonging” to the more electronegative element • Since it has a greater attraction for the electrons • The electrons are actually located closer to that atom • Periodic Trend for electronegativity: increases as you go up and go to the right
Assigning Oxidation States • Fluorine always has a -1 oxidation state • Since it is the most electronegative element • Oxygen usually has a -2 • Except in peroxides where it has a -1 • Hydrogen usually has a +1 • But when it is paired with a metal, it has a -1: ex. LiH
Assigning Oxidation States • Atoms in a pure element have an oxidation state of zero. • Na, O2, Fe, etc. • The more electronegative element is assigned a negative oxidation state while the less electronegative has a positive state • CF4: • C is less electronegative: + • F is more electronegative: -
Sample Problem: Assigning Oxidation Numbers to Atoms What is the oxidation number of each atom in SO2? AnalyzeIdentify the relevant concepts. Use the set of rules in Table 2 to assign and calculate oxidation numbers. SolveApply concepts to this situation.In SO2, there are two oxygen atoms, and each one has an oxidation number of –2 (Rule 3). The sum of the oxidation numbers for the neutral compound must be 0 (Rule 5). Therefore, the oxidation number of sulfur is +4, because: +4 + (2 x (–2) = 0. Oxidation numbers are often written above the chemical symbols in a formula. For SO2, the oxidation numbers would be written as shown to the right.
Assigning Oxidation States • Sum of the oxidation states must equal zero in a neutral compound • N2O5 • O: -2 x 5 = -10 • N: +5 x 2 = +10 • Sum of states in a monatomic/polyatomic ion must equal the charge of the ion • NO3-1 • O: -2 x 3 = -6 • N: + 5 x 1 = +5
UF6 F: -1 0 overall U: +6 to balance 6x-1 H2SO4 H: +1 O: -2 0 overall S: +6 to balance (4x-2)+(2x+1) ClO31- O: -2 -1 overall Cl: +5 to balance 3x-2 CO2 O: -2 0 overall C: +4 Practice
Br2 Br: 0 NH3 H: +1 N: -3 CaSO3: Ca: +2 O: -2 S: +4 HSO3- H: +1 O: -2 -1 overall S: +4 B2H6 H: +1 0 overall B: +3 Determine the Oxidation States