‘Active Reading’

Noticed Electron theory(s), resonance, polar covalent bonding, formal charges, dipole moments.

Q’s

How does electron behavior in the view of organic chemistry differ from that of material science?
How important are ionic bonds, given that most attention is given to covalent bonds?
How does MO theory differ from VB theory?

Text, Problems

1.1 - Introduction

Organic Chemistry is the branch of science dealing (generally) with compounds containing carbon. The near-infinite configurations are resulting from carbon’s singular ability to form like-chains.

1.2 - Electrons in Atoms

Text

Chemistry happens because of the behavior of electrons in atoms and molecules. This basis is hinted at by the order of the periodic table.

Figure 1.2.1: Periodic Table, as seen by the organic chemist. (Naturwiki, CC BY-SA 3.0 https://creativecommons.org/licenses/by-sa/3.0, via Wikimedia Commons)

In a neutral atom of any given element, the number of protons equals that of electrons, and this number is the atomic number.

Distancing from the rather complicated quantum theory of electrons, it can be thought of that electrons reside in certain ‘shells’, and each shell of electrons corresponds to period (horizontal row) of the periodic table. The outermost shell consists (typically) of the ‘active’ electrons that are moved in reactions, bonding, etc. are are thus called the valance shell with valance electrons.

The number of valence electrons for any neutral atom in an a group of the periodic table (except helium) equals its group number. For example, Beryllium, being in Group 2, has two valance electrons.

Atoms can and do gain/lose electrons to form ions. An additional electron results in a net-negative anion, and a missing electron results in a net-positive cation.

IMPORTANT Atoms, particularly those of concern in organic chemistry, tend to react to gain/lose electrons to match the nearest noble gas. This is the octet rule, as the valance shell typically consists of eight electrons.

Problems

1.3 - Covalent Bonds, Lewis Structures

Text

Considering quantum behavior, electrons from a sort of probabilistic cloud around an atom’s nucleus. The density of this cloud is directly proportional to the probability that the electron is at that location when observed.

In covalently-bonded molecules, this cloud extends to all atoms in the molecule. Therefore, all valance electrons originating from any particular atom becomes delocalized, and can ‘move’ across all atoms. In essence, each atom’s valance electrons are shared across the molecule.

For more, see ‘molecular orbital theory’. However, for a more intuitive, mechanical approach, valance bond theory dictates that for every two shared electrons, a covalent bond forms between two atoms.

Lewis Structures, as proposed by Gilbert Newton Lewis (1875-1946), allow for visual analysis of these bonding behaviors. Each element is surrounded with dots (.), with each dot representing a valance electron. When shared, each group of two is represented by either (:), (..), or a line (—). Non-bonding electrons are shown as lone pairs.

![[equivalent_lewis_structures.png]] Figure 1.3.1 Water Lewis Structures

Importantly, the sum of all bonding and non-bonding valence electrons around each atom in many stable covalent compounds is eight (two for the hydrogen atom). This is the definition of the octet rule. When counting electrons, each atom ‘gets’ to account for every shared electron in it’s shared bonds.

![[totalling_valence_electrons.png]] Figure 1.3.2 Octet Rule Examples

In methane ($CH_4$), the carbon shares four single bonds to hydrogen. Since the neutral carbon has four electrons, it gains another four from all hydrogens to fufill it’s octet. Likewise, each hydrogen counts an electron from carbon to fufill it’s ‘duet’.

The same is said for double and triple bonds, where each account for two and three pairs of valance electrons respectively.

Problems

Lecture Notes

1 - Tuesday, 2026-03-31

How can I think like a scientist?

OK to fail!
Ask lots of questions!
Emphasize critical thinking.