The Structure of an Atom and the Periodic Table

Composition of the Atom

• Atoms consist of protons (+), neutrons (0), and electrons (-).

• Atomic number (Z): number of protons.

• Mass number (A): total of protons and neutrons (A = Z + N).

Subatomic Particles

• Electrons occupy the atomic volume and determine chemical behavior.

• Proton charge equals electron charge but opposite in sign.

Isotopes

• Isotopes have the same number of protons but different neutrons.

• Isotopic mass is calculated based on abundance.

• Average atomic mass is a weighted average of isotopic masses.

Development of Atomic Theory

• Key figures: Dalton, Thomson, Rutherford, Bohr, Schrödinger.

• Dalton's postulates: atoms are indivisible, all matter consists of atoms, atoms of the same element are identical, compounds form by combining whole atoms.

Bohr Model

• Electrons are in fixed energy levels; energy is quantized.

• Limitations include inability to explain multi-electron atoms.

Modern Atomic Theory

• Electrons exist in probability regions (orbitals) rather than defined orbits.

• Quantum mechanics describes electron behavior.

Periodic Table

• Elements are organized by atomic number.

• Classified into metals, nonmetals, and metalloids.

• Groups (columns) share similar properties; periods (rows) show trends in properties across the table.

Electron Configuration

• Electron arrangement affects reactivity; determined by Aufbau principle, Pauli exclusion, and Hund's rule.

• Valence electrons influence chemical bonding and stability.

Trends in the Periodic Table

• Atomic and ion size varies across periods and groups.

• Ionization energy generally increases across a period and decreases down a group.

• Electron affinity varies, indicating tendency to gain electrons.

Octet Rule

• Atoms tend to gain, lose, or share electrons to achieve a Noble gas configuration for stability.