Atomic Structure, Periodic Trends, and Electron Configuration Study Notes

Atoms & Elements

Bohr Model and Quantum Models of the Atom

The structure of the atom has been described using various models, including the Bohr model and more advanced quantum mechanical models. These models help explain the arrangement of electrons and the behavior of atoms.

• Bohr Model: Proposes that electrons orbit the nucleus in fixed energy levels (shells) without radiating energy. Electrons can move between levels by absorbing or emitting energy.

• Quantum Mechanical Model: Describes electrons as existing in orbitals, regions of space with a high probability of finding an electron, rather than fixed paths.

• Energy Levels: Each shell corresponds to a principal quantum number (n = 1, 2, 3, ...).

Example: In the Bohr model of hydrogen, the electron occupies the n=1 energy level in its ground state.

Periodic Properties of the Elements

Trends in Ionization Energy

Ionization energy is the energy required to remove an electron from a gaseous atom or ion. Periodic trends in ionization energy are observed across periods and down groups in the periodic table.

• Across a Period: Ionization energy increases from left to right due to increasing nuclear charge and decreasing atomic radius.

• Down a Group: Ionization energy decreases due to increasing atomic radius and electron shielding.

Equation: $\text{X}(g) \rightarrow \text{X}^+(g) + e^-$

Trends in Atomic Radius

The atomic radius is the distance from the nucleus to the outermost electron shell. It varies predictably across the periodic table.

• Across a Period: Atomic radius decreases from left to right due to increased nuclear charge pulling electrons closer.

• Down a Group: Atomic radius increases as additional electron shells are added.

Atoms & Elements

Valence Electrons

Valence electrons are the outermost electrons of an atom and are important in determining chemical reactivity and bonding.

• Group Number: For main group elements, the group number indicates the number of valence electrons.

• Lewis Dot Structures: Represent valence electrons as dots around the element symbol.

Example: Carbon (C) has 4 valence electrons.

Charge of Ions Based on Valence Electrons

Atoms gain or lose electrons to achieve a stable electron configuration, often resulting in the formation of ions.

• Cations: Formed when atoms lose electrons (typically metals).

• Anions: Formed when atoms gain electrons (typically nonmetals).

• Octet Rule: Atoms tend to gain, lose, or share electrons to achieve 8 valence electrons.

Example: Sodium (Na) loses one electron to form Na+; chlorine (Cl) gains one electron to form Cl-.

Atoms & Elements

Electron Configuration

Electron configuration describes the arrangement of electrons in an atom's orbitals. It follows the Aufbau principle, Pauli exclusion principle, and Hund's rule.

• Aufbau Principle: Electrons fill the lowest energy orbitals first.

• Pauli Exclusion Principle: No two electrons in an atom can have the same set of four quantum numbers.

• Hund's Rule: Electrons occupy degenerate orbitals singly before pairing.

Example: The electron configuration of oxygen (O) is 1s2 2s2 2p4.

Vocabulary

• Ionization Energy: The energy required to remove an electron from a gaseous atom.

• Atomic Radius: The size of an atom, typically measured from the nucleus to the outermost electron shell.

• Valence Electrons: Electrons in the outermost shell of an atom.

• Periodic Trends: Patterns in properties of elements across periods and groups in the periodic table.

• Electron Configuration: The arrangement of electrons in an atom's orbitals.