Chapter 3: Chemical Bonding, Electron Arrangement, and Molecular Structure

Compounds – How Elements Combine

Introduction to Chemical Compounds

Chemical compounds are formed when atoms of different elements combine through chemical bonds. The way atoms bond depends on their electron arrangements and the drive to achieve stable electron configurations.

• Atoms are the basic units of matter, consisting of protons, neutrons, and electrons.

• Compounds are substances formed from two or more different elements bonded together.

• Bonding is primarily determined by the behavior of electrons, especially those in the outermost shell (valence electrons).

• Example: Water (H2O) is a compound formed from hydrogen and oxygen atoms.

Electron Arrangements and the Octet Rule

Structure of the Atom and Electron Cloud

Electrons occupy regions of space called the electron cloud, which surrounds the nucleus. Their arrangement is crucial for understanding chemical reactivity.

• Electrons are in constant motion and possess energy.

• They are organized into energy levels (also called shells), each with a specific energy.

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

Energy Levels and Electron Distribution

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): up to 2 electrons

• Second energy level (n=2): up to 8 electrons

• Third energy level (n=3): up to 18 electrons

• Fourth energy level (n=4): up to 32 electrons

Electrons fill these levels in order of increasing energy.

Table: Number of Electrons in Energy Levels for Selected Elements

Element	Group Number	Total Number of Electrons	n = 1	n = 2	n = 3	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
Mg	2A	12	2	8	2
Al	3A	13	2	8	3
Si	4A	14	2	8	4
P	5A	15	2	8	5
S	6A	16	2	8	6
Cl	7A	17	2	8	7
Ar	8A	18	2	8	8
K	1A	19	2	8	8	1
Ca	2A	20	2	8	8	2

Valence Electrons and Reactivity

Valence Electrons

The chemical properties and reactivity of an element are determined by its valence electrons (electrons in the outermost energy level).

• Valence electrons are involved in chemical bonding.

• Example: Oxygen in H2O has 6 valence electrons.

• Atoms with a full set of valence electrons (usually 8) are stable (noble gases).

Noble Gases and the Octet Rule

Noble Gases

Noble gases are elements in Group 8A of the periodic table. Their atoms have completely filled valence shells, making them very stable and unreactive.

• All noble gases (except helium) have 8 valence electrons.

• Other elements tend to react to achieve a similar stable configuration (the octet rule).

• Stability and reactivity are inversely related: the more stable the electron configuration, the less reactive the atom.

The Octet Rule

The octet rule states that atoms tend to gain, lose, or share electrons to achieve a full set of eight valence electrons, similar to the noble gases.

• Having 8 valence electrons is highly stable.

• Atoms will combine to achieve an octet, forming different types of compounds.

• Metals tend to lose electrons; nonmetals tend to gain or share electrons.

Ion Formation and Isoelectronic Species

Formation of Ions

An atom becomes an ion when it gains or loses electrons to achieve a stable electron configuration.

• If protons = electrons, the atom is neutral.

• If electrons are gained, the atom becomes a negatively charged anion.

• If electrons are lost, the atom becomes a positively charged cation.

• Example: Chlorine atom (Cl) gains one electron to become chloride ion (Cl-).

Atoms often become isoelectronic with noble gases (having the same number of electrons).