Honors Chemistry Unit 3 Review: Atoms and the Periodic Table

Key Concepts and Definitions

• Matter consists of atoms, the fundamental building blocks of all substances
• Elements are pure substances made up of only one type of atom and cannot be broken down into simpler substances by chemical means
• Compounds are substances composed of two or more elements chemically combined in a specific ratio
• Atoms are composed of subatomic particles: protons, neutrons, and electrons
• Atomic number represents the number of protons in an atom's nucleus and uniquely identifies an element
• Mass number is the sum of the number of protons and neutrons in an atom's nucleus
• Isotopes are atoms of the same element with different numbers of neutrons, resulting in varying mass numbers

Atomic Structure and Subatomic Particles

• Atoms are composed of three main subatomic particles: protons, neutrons, and electrons
• Protons have a positive charge, neutrons have no charge, and electrons have a negative charge
• The nucleus, located at the center of an atom, contains protons and neutrons
  • Protons and neutrons are collectively called nucleons
  • The nucleus contains the majority of an atom's mass due to the relatively large masses of protons and neutrons compared to electrons
• Electrons orbit the nucleus in regions called electron shells or energy levels
• The number of protons in an atom's nucleus determines its atomic number and defines the element
• Atoms are electrically neutral when the number of protons equals the number of electrons
• The mass of an electron is approximately 1/1836 the mass of a proton or neutron

Atomic Models and Their Evolution

• Dalton's atomic model proposed that atoms were indivisible and indestructible particles
• Thomson's plum pudding model suggested that electrons were embedded in a positively charged "pudding" (the atom)
• Rutherford's gold foil experiment led to the discovery of the atomic nucleus and the nuclear model of the atom
  • Rutherford's model proposed that electrons orbit a small, dense, positively charged nucleus
  • The nuclear model introduced the concept of a mostly empty atom with a concentrated central mass
• Bohr's atomic model introduced the concept of stationary electron orbits or shells around the nucleus
  • Bohr proposed that electrons can only occupy specific energy levels and can jump between levels by absorbing or emitting energy
• The quantum mechanical model, based on Heisenberg's uncertainty principle and Schrödinger's wave equation, describes electrons as probability clouds around the nucleus
  • This model introduces the concept of atomic orbitals, which are regions in space where electrons are likely to be found

The Periodic Table: Organization and Trends

• The periodic table organizes elements based on their atomic number and chemical properties
• Elements are arranged in rows (periods) and columns (groups or families)
  • Periods represent the number of electron shells in an atom
  • Groups contain elements with similar chemical properties due to their electron configurations
• The periodic table is divided into four blocks based on the type of orbital being filled: s-block, p-block, d-block, and f-block
• Metals, located on the left side of the periodic table, are good conductors of heat and electricity and tend to form cations
• Nonmetals, located on the right side of the periodic table, are poor conductors of heat and electricity and tend to form anions
• Metalloids, located along the stair-step line between metals and nonmetals, have properties intermediate between those of metals and nonmetals
• Periodic trends, such as atomic radius, ionization energy, electron affinity, and electronegativity, vary predictably across the periodic table

Electron Configuration and Orbitals

• Electron configuration describes the arrangement of electrons in an atom's electron shells and subshells
• Electrons occupy orbitals, which are regions in space where electrons are likely to be found
• The four types of orbitals are s, p, d, and f, each with a distinct shape and energy level
  • s orbitals are spherical and can hold up to 2 electrons
  • p orbitals are dumbbell-shaped and can hold up to 6 electrons (3 orbitals with 2 electrons each)
  • d orbitals have complex shapes and can hold up to 10 electrons (5 orbitals with 2 electrons each)
  • f orbitals have even more complex shapes and can hold up to 14 electrons (7 orbitals with 2 electrons each)
• The order in which orbitals are filled follows the aufbau principle, which states that electrons occupy the lowest energy orbitals first
• Hund's rule states that electrons in the same subshell will occupy separate orbitals before pairing up, and all unpaired electrons will have the same spin
• The Pauli exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers (principal, angular momentum, magnetic, and spin)

Chemical Properties and Periodic Trends

• Chemical properties describe how an element behaves and reacts with other substances
• Periodic trends are patterns in the properties of elements that vary predictably across the periodic table
• Atomic radius generally decreases from left to right across a period and increases from top to bottom within a group
  • This trend is due to increasing nuclear charge across a period and increasing distance between the nucleus and valence electrons down a group
• Ionization energy, the energy required to remove an electron from a neutral atom, generally increases from left to right across a period and decreases from top to bottom within a group
  • This trend is due to increasing nuclear charge and decreasing atomic radius across a period, and increasing atomic radius and shielding effect down a group
• Electron affinity, the energy change when an electron is added to a neutral atom, generally becomes more negative from left to right across a period and less negative from top to bottom within a group
  • This trend is due to increasing nuclear charge and decreasing atomic radius across a period, and increasing atomic radius and shielding effect down a group
• Electronegativity, the ability of an atom to attract electrons in a chemical bond, generally increases from left to right across a period and decreases from top to bottom within a group
  • This trend is due to increasing nuclear charge and decreasing atomic radius across a period, and increasing atomic radius and shielding effect down a group

Isotopes and Nuclear Chemistry

• Isotopes are atoms of the same element with different numbers of neutrons, resulting in varying mass numbers
• The atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes
• Radioisotopes are unstable isotopes that undergo radioactive decay, emitting radiation in the form of alpha particles, beta particles, or gamma rays
  • Alpha decay involves the emission of a helium nucleus (2 protons and 2 neutrons)
  • Beta decay involves the emission of an electron or positron, resulting from the conversion of a neutron to a proton or vice versa
  • Gamma decay involves the emission of high-energy electromagnetic radiation
• Half-life is the time required for half of a given quantity of a radioisotope to decay
• 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
  • Nuclear fusion is the combining of light nuclei to form a heavier nucleus, releasing energy

Real-World Applications and Examples

• Carbon dating uses the radioactive decay of carbon-14 to determine the age of organic materials (fossils, artifacts)
• Medical imaging techniques, such as positron emission tomography (PET) scans, use radioisotopes to visualize metabolic processes in the body
• Radiation therapy uses targeted radiation from radioisotopes to treat cancer by damaging the DNA of cancer cells
• Nuclear power plants generate electricity through controlled nuclear fission reactions (uranium-235)
• Smoke detectors use a small amount of the radioisotope americium-241 to ionize the air and detect smoke particles
• Neon signs use electricity to excite electrons in neon atoms, causing them to emit light as they return to their ground state
• Fireworks utilize the characteristic colors produced by electrons in different elements returning to their ground state after being excited
• Forensic science uses atomic absorption spectroscopy to detect the presence and concentration of elements in evidence samples (lead in bullets, arsenic in poisoning cases)