Atomic Structure and Electron Configuration: Foundations for Organic Chemistry

Atomic Structure and Electron Configuration

Bohr Model of the Atom

The Bohr model was developed by Niels Bohr in 1913 to explain atomic structure and the emission of light by atoms. This model is foundational for understanding how electrons are arranged in atoms, which is essential for organic chemistry.

• Electrons revolve around the nucleus in fixed, circular orbits.

• Each orbit corresponds to a specific energy level or shell.

• Electrons can only occupy these fixed energy levels and cannot exist between them.

• Orbits of larger radii correspond to higher energy levels.

• Electrons can move between energy levels by absorbing or emitting light.

Example: The hydrogen atom emits light when its electron falls from a higher to a lower energy level.

Valence Shell

The valence shell is the outermost electron shell of an atom. The electrons in this shell, called valence electrons, play a crucial role in chemical bonding and reactions.

• Valence electrons require the least energy to be removed from the atom.

• They are directly involved in chemical reactions.

• Electrons in inner shells are called core electrons and do not participate in chemical reactions.

Example: In sodium (Na), the single electron in the third shell is the valence electron.

Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons to achieve eight electrons in their valence shell, resulting in a stable electron configuration similar to noble gases.

• Hydrogen and helium are exceptions, preferring two electrons in their outer shell.

• Atoms form chemical bonds to achieve the octet rule.

Example: Oxygen forms two bonds to complete its octet.

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom's shells, starting from the lowest energy shell.

• Each shell can hold a specific number of electrons:

Electron Shell	Number of Electrons
1	2
2	8
3	18
4	32

• Electrons fill the lowest energy shells first (Aufbau principle).

Example: Carbon (atomic number 6): Electron configuration is 2,4 (2 electrons in the first shell, 4 in the second).

Order of Filling Electron Shells

The order in which electron shells are filled depends on the number of electrons in the atom. For the first 20 elements, the sequence is straightforward, but for heavier elements, the filling order becomes more complex due to subshells.

Number of Electrons	Filling Sequence
Up to 20	2 → 8 → 8 → 2
21 to 38	2 → 8 → 18 → 8 → 2
39 to 56	2 → 8 → 18 → 18 → 8 → 2

Additional info: For organic chemistry, focus is typically on elements with atomic numbers up to 20 (main group elements).

Electron Configuration Examples

Electron configurations for selected elements illustrate how electrons are distributed among shells and how many valence electrons each element has.

Element	Atomic Number	Electron Configuration	Valence Electrons
Hydrogen	1	1	1
Carbon	6	2,4	4
Oxygen	8	2,6	6
Chlorine	17	2,8,7	7

Example: Fluorine (atomic number 9): Electron configuration is 2,7; it has 7 valence electrons.

Bohr Diagram

Bohr diagrams visually represent the arrangement of electrons in shells around the nucleus for different elements.

• Each shell is drawn as a circle around the nucleus, with dots representing electrons.

Example: Sodium (Na): 2 electrons in the first shell, 8 in the second, 1 in the third.

Lewis Dot Diagrams

Lewis dot diagrams show only the valence electrons for atoms or ions, which are crucial for understanding chemical bonding in organic molecules.

• Dots are placed around the element symbol to represent valence electrons.

• Helps visualize how atoms bond and form molecules.

Element	Electron Configuration	Valence Electrons
Ca	2,8,8,2	2
Al	2,8,3	3
O	2,6	6
Cl	2,8,7	7

Example: Oxygen is represented with six dots around the symbol "O".

Ions

Ions are atoms that have gained or lost electrons to achieve a full outer shell, often following the octet rule.

• Cation: Positively charged ion; has lost electrons.

• Anion: Negatively charged ion; has gained electrons.

• Electron configuration changes to reflect the loss or gain of electrons.

Symbol	Electron Configuration	Type
Na	2,8,1	Neutral
Na+	2,8	Cation
Cl	2,8,7	Neutral
Cl-	2,8,8	Anion

Example: Sodium loses one electron to become Na+ with a full outer shell.

Lewis Structures for Ions

Lewis structures for ions show the electrons present in the outermost shell. Cations typically have no dots, while anions have eight dots (an octet) and are enclosed in brackets to indicate the charge.

• Cations: No dots (no valence electrons).

• Anions: Eight dots, brackets, and charge indicated.

Ion	Lewis Structure
Ca2+	[Ca]2+ (no dots)
O2-	[••••••••]2- (eight dots)

Example: Chloride ion (Cl-) is shown with eight dots and a negative charge.

Summary Table: Key Terms and Concepts

Term	Definition	Example/Application
Bohr Model	Electrons in fixed orbits around nucleus	Explains atomic emission spectra
Valence Shell	Outermost electron shell	Determines chemical reactivity
Octet Rule	Atoms seek 8 electrons in valence shell	Formation of ionic/covalent bonds
Electron Configuration	Arrangement of electrons in shells	Na: 2,8,1
Lewis Dot Diagram	Shows valence electrons	Oxygen: six dots around "O"
Ion	Atom with net charge	Na+, Cl-

Equations and Principles

• Maximum electrons per shell: $2n^2$ where $n$ is the shell number.

• Electron configuration notation: For example, $1s^2 2s^2 2p^6$ for neon.

Additional info:

• These concepts form the basis for understanding molecular representations, bonding, and reactivity in organic chemistry.

• Organic chemistry focuses on the behavior of valence electrons in carbon, hydrogen, oxygen, nitrogen, and halogens.