Chapter 3: Compounds—How Elements Combine (GOB Chemistry Study Notes)

Chapter 3: Compounds—How Elements Combine

Outline of Topics

• Electron Arrangements and the Octet Rule

• In Search of an Octet, Part 1: Ion Formation

• Ionic Compounds—Electron Give and Take

• In Search of an Octet, Part 2: Covalent Bond Formation

• The Mole: Counting Atoms and Compounds

• Getting Covalent Compounds into Shape

• Electronegativity and Molecular Polarity

Electron Arrangements and the Octet Rule

Atomic Structure and Electron Clouds

Atoms form compounds to achieve more stable electron arrangements. Electrons are found in regions around the nucleus called electron clouds, and their exact location is indeterminate. Because electrons are charged and in constant motion, they possess energy.

• Electron cloud: The region where electrons are likely to be found around the nucleus.

• Energy levels: Electrons occupy specific energy levels, not positions in between.

• Electrons fill the lowest energy levels first, which are closest to the nucleus.

Energy Levels and Electron Capacity

Each energy level can hold a specific maximum number of electrons, determined by the formula $2n^2$, where $n$ is the energy level number.

• First energy level ($n=1$): Maximum of 2 electrons

• Second energy level ($n=2$): Maximum of 8 electrons

• Third energy level ($n=3$): Maximum of 18 electrons

• Fourth energy level ($n=4$): Maximum of 32 electrons

Formula: $2n^2$

Electron Configuration Table

The following table summarizes the distribution of electrons in the first four energy levels for selected elements:

Element	Group Number	Total Number of Electrons	Electrons in n=1	Electrons in n=2	Electrons in n=3	Electrons in n=4
H	1A	1	1
He	8A	2	2
Li	1A	3	2	1
Be	2A	4	2	2
B	3A	5	2	3
C	4A	6	2	4
N	5A	7	2	5
O	6A	8	2	6
F	7A	9	2	7
Ne	8A	10	2	8
Na	1A	11	2	8	1

Additional info: Table continues for elements up to Ca, showing electron distribution in higher energy levels.

Valence Electrons and the Octet Rule

The valence shell is the highest energy level containing electrons, and the electrons in this shell are called valence electrons. These electrons are involved in chemical bonding.

• Elements in the same group have the same number of valence electrons.

• Groups (columns) indicate the number of valence electrons for main-group elements.

• Periods (rows) indicate the outermost energy level containing electrons.

Atoms tend to react to achieve eight electrons in their valence shell, known as the octet rule. Noble gases (Group 8A) are stable because they have a complete octet (except helium, which has two).

Summary Table: Electron Arrangements

Element	Valence Electrons	Energy Level of Valence Electrons
Boron (B)	3	2
Aluminum (Al)	3	3

Additional info: This pattern applies to all main-group elements.

Key Points

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve eight valence electrons.

• Noble gases: Have a full valence shell and are chemically inert.

• Valence electrons: Determine chemical reactivity and bonding.

Example

How many valence electrons are present in the following atoms?

• Barium (Ba): 2 valence electrons

• Silicon (Si): 4 valence electrons

• Phosphorus (P): 5 valence electrons