Lecture_13-14

Lecture_13-14 - Next Week Lab: Does Human Breathing...

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Next Week Lab: Does Human Breathing Contribute to Global Warming? Handout HW: Due Friday as usual Reading assignments and learning objectives are available
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Formal charge Formal charge: The charge an atom would have if its bonding electrons were distributed equally between the atom and its bonded partners (homolytic cleavage of the bonds). Minimize formal charge. To determine formal charge: Assign half of the electrons in a bond to each of the two atoms sharing them. Assign unshared pairs of electrons to the atom to which they belong. Determine the formal charge for each atom by comparing the number of electrons assigned to it to the number of valence electrons for the neutral atom. SCN or CSN ?
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Finding Formal Charge Formal charge = # of valence e- around atom in its neutral, unbound state minus # of electrons around the atom in the molecule (1/2 of electrons in each bond plus all unpaired electrons) Σ all formal charges = charge on ion or molecule.
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The SCN Ion SCN or CSN ? -1 0 0 -2 +2 -1 What is the charge on the following SCN ion?
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Valence Shell Electron Pair Repulsion Theory (VSEPR Theory) Most important factor in determining geometry Most important factor in determining geometry is relative repulsion between electron pairs. is relative repulsion between electron pairs. M M olecule adopts the shape that minimizes the olecule adopts the shape that minimizes the electron pair repulsions. electron pair repulsions.
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Ideal Electron Pair Geometries Ideal Electron Pair Geometries Electron unit = bond pair, multiple bond, lone pair
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VSEPR - Valence Shell Electron Pair Repulsion Theory Each unit of valence electrons around a central atom (single bonds, multiple bonds, and lone pairs) is located as far away as possible from the others in order to maximize repulsions. Repulsions maximize the space that each electron unit attached to the central atom occupies. One of the five electron-unit arrangements usually result Electron-units define the object arrangement, Positions of the atomic nuclei define molecular shape. Because valence electrons can be bonding or nonbonding, the same electron-group arrangement can give rise to different molecular shapes.
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Perspective
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The VSEPR Method The VSEPR Method 1. Draw the Lewis structure. 2. Determine the number of electron units (single bonds,
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Lecture_13-14 - Next Week Lab: Does Human Breathing...

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