Lewis Structures and Molecular Geometry

Valence Electrons Electrons fill orbitals that are on the outside of the nucleus of the atom The number of electrons in an orbital varies depending on the atom, the number of electrons, etc. The electrons in the outside orbital are called Valence Electrons

Valence Electrons Draw a Bohr Diagram for the following atoms HLiNa How many valence electrons in each atom?

Valence Electrons The number of valence electrons is the same for every atom in a group/family (column on the periodic table) We do not include the transition metals for this as they act very differently

Lewis Dot Diagrams Valence electrons are often shown in Lewis Dot Diagrams In these the nucleus is written in the middle with dots representing each valence electron Electrons MUST be drawn as single electrons before they can be paired up Example Draw a Lewis Dot Diagram for the following atoms FluorineSulphur

Lewis Dot Diagram Examples Draw a Lewis Dot Diagram for the following atoms Carbon Beryllium Arsenic Krypton

Lewis Dot Diagrams for Molecules As well as Lewis Dot Diagrams for atoms, we can draw them for molecules as well There is a lot of background we need to learn first to know how to draw these properly:

Fundamental Requirements: All valence electrons of the atoms involved must be represented & accounted for Usually, all electrons are paired Usually, each atom achieves a stable octet Octet – 8 electrons around the atom Sometimes multiple covalent bonds (double & triple) are necessary. These are formed most readily by C, N, O, P, and S atoms

Skeletal Structure: Refers to all the atoms involved arranged in the order in which they are bonded to one another When more than 2 atoms are involved: Central atom: bonded to 2 or more atoms Terminal atom: bonded to only 1 atom

Rules regarding central & terminal atoms: 1. Hydrogen atoms are always terminal atoms 2. Carbon atoms are always central atoms 3. When carbon isn’t involved, the central atom is determined by a characteristic known as electronegativity

Electronegativity To know how to come up with the correct structure you will need to know about ELECTRONEGATIVITY An atom’s ability to attract or compete for other electrons with other atoms to which it is bonded Electronegativity increases from left to right and bottom to top on the periodic table Noble Gases are excluded from this rule

Lewis Dot Diagrams for Molecules Steps 1.Count TOTAL valence electrons in the compound 2.Draw a Central Atom Carbon atoms are always central atoms Central atoms are those with the lowest electronegativity value compared to other atoms 3.Connect the central atoms to the terminal atoms with a single bond 4.Count remaining electrons Remember that a bond is 2 electrons 5.Add lone pairs to the terminal atoms first 6.Look for a way to complete the octet of the central atom (lone pairs or double/triple bonds)

Common Bonding Patterns When looking at a Lewis structure, look for common bonding patterns. This is a big clue if you got the structure correct.

Examples CF 4 CO 2 H 2 S NH 3