🧪AP Chemistry Unit 1 – Atomic Structure and Properties

Fundamental Concepts

• Matter consists of atoms, the smallest unit of an element that retains its properties
• Atoms are composed of subatomic particles: protons, neutrons, and electrons
• Protons have a positive charge, neutrons have no charge, and electrons have a negative charge
  • The number of protons in an atom determines its atomic number and element identity
  • Atoms are electrically neutral when the number of protons equals the number of electrons
• Elements are pure substances composed of only one type of atom (gold, oxygen)
• Compounds are substances made up of two or more elements chemically combined in a specific ratio (water, salt)
• Mixtures contain two or more substances that are not chemically combined and can be separated by physical means (air, salt water)
• Chemical reactions involve the rearrangement of atoms to form new substances with different properties

Atomic Theory and Models

• Dalton's atomic theory proposed that atoms are indivisible and indestructible particles
  • This theory laid the foundation for modern atomic understanding but had limitations
• Thomson's plum pudding model suggested that electrons are embedded in a positively charged "pudding"
  • Experiments with cathode rays led to the discovery of electrons
• Rutherford's gold foil experiment provided evidence for the nuclear model of the atom
  • Most of an atom's mass and positive charge is concentrated in a small, dense nucleus
  • Electrons orbit the nucleus in a relatively large space
• Bohr's model introduced the concept of energy levels for electrons
  • Electrons can only occupy specific energy levels and can move between them by absorbing or emitting energy
• The quantum mechanical model describes electrons as having wave-like properties and existing in orbitals
  • This model incorporates the Heisenberg uncertainty principle and Schrödinger's wave equation

Subatomic Particles

• Protons are positively charged particles located in the nucleus of an atom
  • The number of protons determines an element's atomic number and identity
  • Protons have a mass of approximately 1 atomic mass unit (amu)
• Neutrons are electrically neutral particles found in the nucleus alongside protons
  • Neutrons contribute to the mass of an atom but do not affect its atomic number
  • Like protons, neutrons have a mass of about 1 amu
• Electrons are negatively charged particles that orbit the nucleus in energy levels or orbitals
  • Electrons are much lighter than protons and neutrons, with a mass of approximately 1/1836 amu
  • The number of electrons in an atom determines its chemical properties and reactivity
• Quarks are even smaller particles that make up protons and neutrons
  • Protons consist of two up quarks and one down quark
  • Neutrons consist of one up quark and two down quarks
• Leptons are another type of subatomic particle that includes electrons and neutrinos
  • Neutrinos have very little mass and rarely interact with other particles

Electron Configuration

• Electron configuration describes the arrangement of electrons in an atom's orbitals
• Electrons occupy orbitals in order of increasing energy, following the Aufbau principle
  • The Aufbau principle states that electrons fill orbitals starting with the lowest energy level
• Orbitals are designated by their principal quantum number (n), angular momentum quantum number (l), and magnetic quantum number (m)
  • The principal quantum number represents the energy level or shell (1, 2, 3, etc.)
  • The angular momentum quantum number represents the subshell (s, p, d, f)
  • The magnetic quantum number represents the orientation of the orbital in space
• The Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers
  • This means that each orbital can hold a maximum of two electrons with opposite spins
• Hund's rule states that electrons in the same subshell will occupy separate orbitals with parallel spins before pairing up
• Electron configurations can be written using the noble gas notation, which abbreviates the configuration using the previous noble gas (Ne, Ar, Kr)

Periodic Table Organization

• The periodic table arranges elements in order of increasing atomic number
• Elements are organized into periods (rows) and groups (columns) based on their electron configurations and properties
  • Periods represent the number of electron shells in an atom
  • Groups contain elements with similar electron configurations and chemical properties
• The periodic table is divided into four blocks based on the type of subshell being filled: s-block, p-block, d-block, and f-block
  • The s-block includes alkali metals and alkaline earth metals
  • The p-block includes nonmetals, halogens, and noble gases
  • The d-block contains transition metals
  • The f-block includes lanthanides and actinides
• Metals are located on the left side of the periodic table and tend to be shiny, malleable, and good conductors of heat and electricity
• Nonmetals are located on the right side of the periodic table and are generally poor conductors of heat and electricity
• Metalloids, located along the stair-step line, have properties intermediate between metals and nonmetals (silicon, germanium)

Atomic Properties and Trends

• Atomic radius is the distance from the nucleus to the outermost electron shell
  • Atomic radius generally decreases from left to right across a period due to increasing effective nuclear charge
  • Atomic radius increases down a group because of additional electron shells
• Ionization energy is the energy required to remove an electron from a neutral atom in the gaseous state
  • Ionization energy generally increases from left to right across a period due to increasing effective nuclear charge
  • Ionization energy decreases down a group because of increased atomic radius and shielding effect
• Electron affinity is the energy change that occurs when an electron is added to a neutral atom in the gaseous state
  • Electron affinity generally becomes more negative from left to right across a period
  • Electron affinity becomes less negative down a group due to increased atomic radius and shielding effect
• Electronegativity is the ability of an atom to attract electrons in a chemical bond
  • Electronegativity increases from left to right across a period and decreases down a group
  • Electronegativity values are used to predict the type of bonding between elements (ionic, covalent, or polar covalent)

Isotopes and Nuclear Chemistry

• Isotopes are atoms of the same element with different numbers of neutrons
  • Isotopes have the same atomic number but different mass numbers
  • The mass number is the sum of the number of protons and neutrons in an atom
• Isotopes can be stable or unstable (radioactive)
  • Stable isotopes do not undergo radioactive decay and have a constant number of protons and neutrons over time
  • Unstable isotopes, or radioisotopes, undergo radioactive decay to achieve a more stable configuration
• Radioactive decay is the process by which an unstable nucleus emits particles or energy to form a more stable nucleus
  • Alpha decay involves the emission of an alpha particle (two protons and two neutrons)
  • Beta decay involves the emission of a beta particle (an electron) and the conversion of a neutron to a proton
  • Gamma decay involves the emission of high-energy photons (gamma rays) to release excess energy
• Half-life is the time required for half of a given quantity of a radioactive isotope to decay
  • The half-life is constant for a specific isotope and can range from fractions of a second to billions of years
• Nuclear reactions, such as fission and fusion, involve changes in the nucleus of an atom
  • Nuclear fission is the splitting of a heavy nucleus into lighter nuclei, releasing energy (used in nuclear power plants)
  • Nuclear fusion is the combining of light nuclei to form a heavier nucleus, releasing large amounts of energy (occurs in the Sun)

Real-World Applications

• Atomic structure and properties play a crucial role in various fields, including materials science, energy production, and medicine
• Understanding electron configurations helps predict chemical bonding and reactivity, which is essential for designing new materials (semiconductors, catalysts)
• Radioisotopes are used in nuclear medicine for diagnostic imaging and cancer treatment
  • Technetium-99m is commonly used in bone scans and heart imaging
  • Iodine-131 is used to treat thyroid cancer and hyperthyroidism
• Carbon dating, which relies on the radioactive decay of carbon-14, is used to determine the age of organic materials (fossils, artifacts)
• Atomic spectroscopy techniques, such as atomic absorption and emission spectroscopy, are used for elemental analysis in environmental monitoring and quality control
• Quantum dots, which are nanoscale semiconductor crystals with size-dependent electronic properties, are used in LED displays and solar cells
• Nuclear power plants harness the energy released from nuclear fission reactions to generate electricity
• Research in nuclear fusion aims to develop a clean and virtually limitless energy source for the future