Lewis Structures and Chemical Bonds Lecture 4 Lewis Structures and Chemical Bonds Chapter 7 Suggested HW 7.1, 7.3, 7.7, 7.15, 7.17, 7.23, 7.31, 7.37, 7.39, 7.59, 7.61, 7.63, 7.67, 7.69, 7.73, 7.75, 7.77, 7.81, 7.97, 7.99, 7.101, 7.105, 7.107, 7.109, 7.111, 7.115, 7.119, 7.121, 7.125, 7.127, 7.129, 7.139, 7.143, 7.145, 7.147

Electrons dictate the chemistry of an atom Review Electrons dictate the chemistry of an atom Electrons fill orbitals in order or increasing energy 1s  2s  2p  3s  3p  4s  3d  4p  5s  4d  5p … We can represent the electron configuration of an element by showing the orbital designation or orbital configuration Shell Number of electron in subshell 1s1 subshell S  1s2 2s22p6 3s23p4 [Ne]3s23p4

The Octet Rule – Atoms prefer to have 8 electrons in their outer shell Octet Rule and Ions The Octet Rule – Atoms prefer to have 8 electrons in their outer shell (or dublet for H and He). Valence electrons = outer shell Ions tend to form such that they satisfy the Octet Rule An ion is an atom that has gained or lost electrons This results in a net charge on the atom If an ion has a net (+) charge  cation If an ion has a net (-) charge  anion

Noble gas configuration ALWAYS satisfy the octet rule Forming Ions Noble gas configuration ALWAYS satisfy the octet rule Na  (Z = 11)  [Ne] 3s1 Mg  (Z = 12)  [Ne] 3s2 Cl  (Z = 17)  [Ne] 3s2 3p5 S  (Z = 17)  [Ne] 3s2 3p4

Lewis Representation American chemist G.N. Lewis had a profound impact on our understanding of chemical bonding. One of his many contributions was a simple way to represent chemical bonds. We call these Lewis Symbols. Valence Electrons are represented by dots. A single dot represents an electron A pair of dots represents two paired electrons sharing an orbital H He N O O2- K Mg2+ I- 1s1 1s2 2s22p3 2s22p4 2s22p6 3s1 [Ne] 5s25p6

Composed of Charged components Compounds Compound Bond Type Inorganic Compound Organic Compound Covalent Composed of Charged components Ionic Bonds Contains Carbon Cations Anions

Electronegativity X-Y Some atoms attract electrons more than others Electronegativity difference  2 Ionic Bond < 2 Covalent

Predict the type of bond that will form between: Electronegativity Predict the type of bond that will form between: Ca and F B and H C and F

Ionic Bonds Ionic bonds do not share electrons – bond forms through charge attraction NaCl Ion Na+ Cl- Electron Configuration [Ne] [Ne]2s22p6 Lewis Symbol CaCl2 Ion Ca2+ Cl- Electron Configuration [Ar] [Ne]2s22p6 Lewis Symbol

Electrons pair and are shared between both atoms Covalent Bonds Covalent bonds  electrons shared between two atoms. This involves an overlap of atomic orbitals Let’s consider H2 - two hydrogen atoms share electrons. Octet Rule NOT satisfied! Electrons pair and are shared between both atoms Commonly represented as: H H Solid line represents covalent bond

Covalent Bonding Energy Energy 1s1 1s1 s bond Hydrogen Hydrogen

Covalent Bonding – Fluorine (F2) Energy Energy Atomic Orbitals are NOT optimized for bonding 2p5 2p5 2s2 2s2 sp3 sp3 Fluorine Fluorine

Covalent Bonding Energy Energy sp3 sp3 s bond Fluorine Fluorine

Covalent Bonding

Covalent Bonding – CH4 2p2 sp3 2s2 1s1 Carbon Hydrogen (x4)

Covalent Bonding – CH4 sp3 1s1 Carbon Hydrogen (x4)

Covalent Bonding – CH4 sp3 1s1 Carbon Hydrogen (x4)

Covalent Bonding – CH4

Covalent Bonding – NH3

Covalent Bonding – NH3

Covalent Bonding – NH3

Covalent Bonding – NH3

VSEPR Valence Shell Electron Pair Repulsion Each region of electrons (bond or lone pair) counts as a balloon Balloons want to spread out as much as possible

Molecular Shapes

Will each of these molecules be polar? Molecule Polarity Will each of these molecules be polar? CH4 CH3F CH2F2 CHF3 CF4