Inorganic Chemistry I Unit 1 Review: Atomic Structure & Periodic Trends

Fundamental Concepts of Atomic Structure

• Atoms consist of a dense, positively charged nucleus surrounded by negatively charged electrons in motion
• The nucleus contains protons (positively charged) and neutrons (no charge), while electrons orbit the nucleus in shells or orbitals
• Atomic number (Z) represents the number of protons in an atom and determines its chemical identity (e.g., carbon has 6 protons)
• Mass number (A) is the sum of the number of protons and neutrons in an atom
  • Isotopes are atoms of the same element with different numbers of neutrons (same Z, different A)
• Atomic mass is the weighted average of the masses of all isotopes of an element
• Electrons exhibit both particle and wave properties (wave-particle duality)
• The Heisenberg Uncertainty Principle states that the position and momentum of an electron cannot be simultaneously determined with perfect accuracy

Quantum Mechanical Model of the Atom

• The quantum mechanical model describes the behavior of electrons in an atom using mathematical equations
• Electrons occupy specific energy levels or shells (n = 1, 2, 3, ...) around the nucleus
  • Subshells (s, p, d, f) exist within each principal energy level
• Orbitals are specific regions in space where electrons are likely to be found, characterized by a unique set of quantum numbers
  • Principal quantum number (n) defines the energy level and size of the orbital
  • Angular momentum quantum number (l) describes the shape of the orbital (s, p, d, f)
  • Magnetic quantum number (m_l) specifies the orientation of the orbital in space
  • Spin quantum number (m_s) indicates the intrinsic angular momentum (spin) of an electron (±1/2)
• The Aufbau principle states that electrons fill orbitals in order of increasing energy (1s, 2s, 2p, 3s, 3p, 4s, 3d, ...)
• Hund's rule specifies that electrons occupy orbitals of the same energy singly before pairing up with opposite spins
• The Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers

Electron Configuration and Orbital Diagrams

• Electron configuration is a shorthand notation describing the distribution of electrons in an atom's orbitals
  • Example: carbon (Z = 6) has the electron configuration 1s² 2s² 2p²
• Orbital diagrams visually represent the arrangement of electrons in an atom's orbitals using arrows (↑ or ↓) to indicate electron spin
• Valence electrons are those in the outermost shell of an atom and are involved in chemical bonding
  • Elements in the same group have the same number of valence electrons (e.g., alkali metals have 1, halogens have 7)
• Core electrons are those in the inner shells of an atom and are not typically involved in bonding
• Excited states occur when electrons absorb energy and move to higher energy orbitals
  • Electrons in excited states are denoted with an asterisk (e.g., 1s² 2s² 2p³ 3s^1*)
• Isoelectronic species are atoms or ions with the same electron configuration (e.g., N^3-, O^2-, F^-, Ne)

Development of the Periodic Table

• The periodic table organizes elements based on their atomic number and electron configuration
• Dmitri Mendeleev arranged elements by increasing atomic mass and similar chemical properties, leaving gaps for undiscovered elements
• Henry Moseley later discovered that the atomic number, not mass, was the fundamental property for organizing elements
• The modern periodic table consists of 7 periods (rows) and 18 groups (columns)
  • Periods represent the principal energy levels (n = 1, 2, 3, ...)
  • Groups contain elements with similar electron configurations and chemical properties
• The periodic table is divided into four blocks based on the type of orbital being filled: s-block, p-block, d-block (transition metals), and f-block (lanthanides and actinides)
• Metals, located on the left side of the periodic table, are typically shiny, malleable, and good conductors of heat and electricity
• Nonmetals, located on the right side, are typically dull, brittle, and poor conductors
• Metalloids, located along the stair-step line, have properties intermediate between metals and nonmetals (e.g., silicon, germanium)

Periodic Trends in Atomic Properties

• Atomic radius decreases from left to right across a period and increases down a group
  • Effective nuclear charge (Z_eff) increases across a period, pulling electrons closer to the nucleus
  • Additional shells are added down a group, increasing distance between the nucleus and valence electrons
• Ionization energy, the energy required to remove an electron from a gaseous atom, increases across a period and decreases down a group
  • Higher Z_eff across a period makes electrons harder to remove
  • Larger atomic radius down a group makes electrons easier to remove
• Electron affinity, the energy released when an electron is added to a gaseous atom, generally increases across a period and decreases down a group
  • Exceptions occur due to electron configuration stability (e.g., half-filled and fully-filled orbitals)
• Electronegativity, the ability of an atom to attract electrons in a chemical bond, increases across a period and decreases down a group
  • Fluorine is the most electronegative element, while francium is the least electronegative
• Metallic character increases down a group and decreases across a period
  • Metals tend to lose electrons easily and form cations, while nonmetals tend to gain electrons and form anions

Bonding and Molecular Structure Basics

• Chemical bonds form when atoms share or transfer electrons to achieve a more stable electron configuration
• Ionic bonds involve the transfer of electrons from a metal to a nonmetal, resulting in oppositely charged ions (e.g., NaCl)
  • Ionic compounds form lattice structures and have high melting points and electrical conductivity when molten or in solution
• Covalent bonds involve the sharing of electrons between nonmetals, resulting in molecules or network solids (e.g., H₂O, diamond)
  • Covalent compounds have lower melting points and are typically poor conductors of electricity
• Metallic bonds involve the delocalization of valence electrons across a lattice of metal cations, resulting in properties such as malleability and electrical conductivity
• Lewis structures represent the arrangement of atoms and electrons in a molecule using dots for valence electrons and lines for shared pairs
  • Formal charge helps determine the most stable Lewis structure by minimizing charge separation
• VSEPR (Valence Shell Electron Pair Repulsion) theory predicts the geometry of a molecule based on the number of electron pairs around the central atom
  • Electron pairs arrange themselves to minimize repulsion, leading to geometries such as tetrahedral (4 pairs), trigonal bipyramidal (5 pairs), and octahedral (6 pairs)
• Hybridization describes the mixing of atomic orbitals to form new hybrid orbitals with specific geometries (e.g., sp³, sp², sp)

Applications in Inorganic Chemistry

• Inorganic chemistry studies the properties and reactions of all elements and their compounds, excluding most carbon-based molecules
• Solid-state chemistry investigates the structure, bonding, and properties of solid materials, including ceramics, semiconductors, and superconductors
  • Perovskites (e.g., CaTiO₃) have applications in solar cells, catalysis, and electronic devices
• Coordination chemistry explores the formation and properties of metal complexes with ligands
  • Hemoglobin, an iron-containing protein, transports oxygen in the blood using a porphyrin ligand
  • Cisplatin, a platinum-based anticancer drug, forms complexes with DNA to inhibit cell division
• Bioinorganic chemistry studies the role of metal ions and their complexes in biological systems
  • Enzymes often require metal cofactors for catalytic activity (e.g., zinc in carbonic anhydrase)
  • Photosynthesis relies on a manganese-containing complex in photosystem II to split water and release oxygen
• Materials science applies inorganic chemistry principles to develop new materials with desired properties
  • Quantum dots, nanoscale semiconductor crystals, have size-dependent optical and electronic properties for use in displays and solar cells
  • Metal-organic frameworks (MOFs) are porous crystalline materials with applications in gas storage, separation, and catalysis

Key Experiments and Discoveries

• Cathode ray tube experiments by J.J. Thomson led to the discovery of the electron and the plum pudding model of the atom
• Ernest Rutherford's gold foil experiment revealed the existence of a small, dense, positively charged nucleus
  • Most alpha particles passed through the foil undeflected, indicating that atoms are mostly empty space
• Niels Bohr proposed a model of the atom with electrons orbiting the nucleus in fixed, quantized energy levels
  • The Bohr model explained the discrete emission spectrum of hydrogen but failed for more complex atoms
• Louis de Broglie's hypothesis of the wave nature of electrons led to the development of the quantum mechanical model
  • The Davisson-Germer experiment demonstrated electron diffraction, confirming the wave-particle duality of matter
• Erwin Schrödinger developed the wave equation to describe the behavior of electrons in an atom
  • The Schrödinger equation treats electrons as standing waves and yields the probability of finding an electron at a given point in space
• Max Born interpreted the wave function as a probability amplitude, leading to the probabilistic interpretation of quantum mechanics
• James Chadwick discovered the neutron, completing the modern understanding of atomic structure
• Glenn Seaborg and colleagues synthesized transuranium elements (atomic number > 92) through nuclear reactions, expanding the periodic table
  • Seaborgium (Sg, element 106) was named in his honor