Molecular Structure and Shapes

Molecular Structure and Shapes What it is/ Why we use it? Molecular Structures describes the type, arrangement, position, and direction of the bonds linking atoms within a molecule. It is the key to understanding the design and blueprints of molecules. Molecular shapes are determined by using the Valence Shell Electron Pair Repulsion (VSEPR). What compounds do we use this for? 1. A chemical bond is a mutual electrical attraction between the nuclei and valence electrons of different atoms that binds the atoms together. 2. Three major types of chemical bonding are: a. Ionic bonds: bonding that results from electrical attraction between anions and cations b. Covalent bonds: bonding that results from sharing of electrons between two atoms c. Polar covalent bonds: bonding in which the bonded atoms have an unequal attraction for shared electrons 3. The relationship between electronegativity and the ionic character of a chemical bond determines their affinity for electrons. If the electronegativity difference between two atoms forming a compound is > 1.7, then the bond formed has ionic character where one atom with higher electronegativity gains electrons and one with lower electronegativity gives up electrons. 4. Polar Bonds: an uneven distribution of charge Nonpolar covalent bonds are a type of chemical bond where two atoms share a pair of electrons with each other. Polar covalent bonding is a type of chemical bond where a pair of electrons is unequally shared between two atoms. 5. In general, the number of valence electrons determines whether the element will form bond with other atoms. If the element already has its outer orbital shells fully occupied then those elements are are unlikely to form bond. How to figure out/ draw? One can figure out the molecular shape of an ion by referring to the periodic table and creating the Lewis Dot Diagram for that ion. The sub number of the ion formula will show the number of atoms bonded to the central atom and the Lewis Dot Diagram will show the number of lone pairs of electrons. The Lewis Dot Diagram should match one of the shapes below, and that will show the molecular shape of the ions. straight line: bond lying approximately in the surface plane wedge bond: in front; thicker end would face the viewer (B) hatched bond: back of the plane (away from viewer) (D) dashed bond: partial bond, covalent bond that is partially formed or broken Example: The Lewis Structure of carbon dioxide shows two carbon-oxygen double bonds and no unshared electron pairs on the carbon atom. To simplify the molecule’s Lewis structure, we represent the covalent bonds with lines instead of dots. This is linear. How does the periodic table help determine these structures? VSEPR Theory: the repulsion between the sets of valence-level electrons surrounding an atom causes these sets to be oriented as far apart as possible Electronegativity: a measure of the ability of an atom in a chemical compound to attract electrons from another atom in the compound • The most electronegative elements are assigned an electronegativity value of four. • Electronegativities increase across each period of the periodic table, and get higher going down each column. • shared pairs will be as far away from each other as possible • central atoms follow the octet rule by sharing four electron pairs with B atoms • lone pairs of electrons: a pair of valence electrons that are not shared with another atom • atoms bonded to central atom: number of atoms attached to the central atom What does the shape tell us about the molecule/ substance properties? 1. This shape is dependent on the preferred spatial orientation of covalent bonds to atoms having two or more bonding partners. 2. The modern concept of the covalent bond has resulted in the ability to predict the geometry and hence the properties of matter such as reactivity, toxicity, and solubility.