Rucete ✏ AP Chemistry In a Nutshell
2. The Periodic Table
In this chapter, you'll learn about the periodic table, atomic symbols, isotopes, atomic and ionic radii, ionization energies, electron affinity, electronegativity, and other periodic trends.
The Modern Periodic Table
• Elements are arranged by increasing atomic number.
• Rows (periods) contain from 2 to 32 elements. Periods with 32 elements place 14 elements below the main table.
• Columns (groups) contain elements with similar chemical and physical properties.
Atomic Symbols
• Elements have one- or two-letter symbols, often abbreviations of their English or Latin names.
• Symbol corners represent isotopic mass (top-left), atomic number (bottom-left), ionic charge (top-right), and atom count (bottom-right).
Electrons, Protons, and Neutrons
• Protons: number equals atomic number (Z).
• Electrons: number equals protons in neutral atoms; ions gain or lose electrons.
• Neutrons: calculated by isotope mass number (A) minus atomic number (Z).
Isotopes and Radioactivity
• Isotopes have the same number of protons but different numbers of neutrons.
• Radioactivity involves unstable nuclei spontaneously emitting particles (alpha, beta, neutron, positron) or energy (gamma rays) to become stable.
Mass Spectrometry
• Measures the mass-to-charge (m/z) ratio of ions to identify isotopes and determine molar masses.
• Peaks represent ions; the largest peak usually indicates the molar mass.
Atomic Masses
• Atomic masses are weighted averages based on isotope abundances relative to carbon-12 (exactly 12 amu).
• Calculations involve multiplying isotope masses by their relative abundances and summing the results.
Periodic Properties of the Elements
• Elements with similar electronic structures exhibit similar chemical properties.
• Alkali metals, alkaline earth metals, transition metals, halogens, and noble gases show clear periodic relationships.
Physical Properties of the Elements
• Mercury and bromine are liquid at room temperature; noble gases, H₂, N₂, O₂, F₂, Cl₂ are gases; most others are solids.
• Some elements naturally exist as diatomic (H₂, O₂, N₂, halogens) or polyatomic (S₈, P₄) molecules.
Metals and Metalloids
• Metals dominate the periodic table; metalloids lie along the metal/nonmetal boundary, showing mixed properties.
• Metallic character increases down a group.
Allotropes
• Different structural forms of the same element (O₂ vs O₃, graphite vs diamond).
Variation of Physical Properties
• Metallic properties increase down a group; melting and boiling points generally decrease in metals, increase in nonmetals down a group.
Atomic Radii
• Atomic radius increases down a group due to larger energy levels; decreases across a period due to increased effective nuclear charge.
Effective Nuclear Charge
• Core electrons shield valence electrons, affecting attraction between nucleus and electrons.
• Effective nuclear charge increases across a period, decreasing atomic radius.
Ionization Energy
• Energy required to remove electrons; increases across a period, decreases down a group.
• Core electrons have significantly higher ionization energies compared to valence electrons.
Photoelectron Spectroscopy (PES)
• Uses high-energy beams to eject electrons and measure kinetic energy to determine electron binding energies.
• Peak intensities in spectra correlate to electrons in specific orbitals.
Electron Affinity
• Energy change associated with the addition of an electron to an atom in the gas phase.
• Generally becomes more negative (more favorable) across a period; becomes less negative (less favorable) down a group.
Electronegativity
• Tendency of an atom to attract electrons in a chemical bond.
• Increases across a period due to increased nuclear charge; decreases down a group due to increased atomic radius.
• Fluorine is the most electronegative element.
Metallic and Nonmetallic Character
• Metallic character increases down a group and decreases across a period.
• Nonmetallic character decreases down a group and increases across a period.
Ionic Radii
• Cations (positive ions) are smaller than their parent atoms due to electron removal and reduced electron-electron repulsion.
• Anions (negative ions) are larger than their parent atoms due to electron addition and increased electron-electron repulsion.
• Ionic radius increases down a group and generally decreases across a period.
Isoelectronic Species
• Atoms and ions with identical electron configurations.
• Ionic size decreases with increasing nuclear charge (increasing atomic number).
Acid-Base Behavior of Oxides
• Metallic oxides react with water to form basic solutions (e.g., Na₂O).
• Nonmetallic oxides react with water to form acidic solutions (e.g., SO₃).
• Amphoteric oxides (e.g., Al₂O₃) react with both acids and bases.
Summary of Periodic Trends
• Atomic radius: increases down, decreases across.
• Ionization energy: decreases down, increases across.
• Electron affinity: less negative down, more negative across.
• Electronegativity: decreases down, increases across.
• Metallic character: increases down, decreases across.
In a Nutshell
The periodic table arranges elements by atomic number, reflecting periodic trends in atomic radius, ionization energy, electron affinity, electronegativity, and ionic sizes. Understanding these trends helps predict element properties and chemical reactivity.
Practice Questions
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