Organizing the Elements

Historical Context

• 1916: First self-service grocery store in Memphis, Tennessee
• Organizing products by similar characteristics makes shopping easier
• Similarly, elements are arranged in the periodic table by their properties

Early Discovery and Classification

• Known elements before 1700: 13
• 1765-1775: Discovery of hydrogen, nitrogen, and oxygen
• 1829: J.W. Dobereiner's triad system, grouping elements with similar properties
• Triads: Groups of three elements with similar properties

Mendeleev's Contribution

• 1869: Dmitri Mendeleev develops periodic table
• Elements arranged by increasing atomic mass
• Left spaces predicting undiscovered elements and their properties
• Mendeleev's table gained acceptance due to predictive power

Modern Periodic Table

• Organized by increasing atomic number (Henry Moseley, 1913)
• Periodic Law: Periodic repetition of element properties when arranged by atomic number
• Seven periods, varying numbers of elements
• Elements within a group have similar properties

Classes of Elements

• Metals: Good conductors, malleable, mostly solid at room temperature
• Nonmetals: Poor conductors, more variation in states (gases, solids, liquids)
• Metalloids: Properties of both metals and nonmetals

Element Classifications

• Noble Gases: Group 9A, rarely react, full s and p sublevels
• Representative Elements: Varied properties, incomplete s and p sublevels
• Transition Metals: d orbitals with electrons, metals like copper and iron
• Inner Transition Metals: f orbitals with electrons, once considered rare

Key Terms

• Periodic Law, Metals, Nonmetals, Metalloids

Guide for Reading

• Compare and contrast periodic tables (Figures 6.4 and 6.5)
• Understand the significance of Mendeleev's and modern periodic tables

Periodic Trends

Atomic Size

• Measured as atomic radius (half the distance between nuclei)
• Trend: Increases down a group, decreases across a period

Ions

• Form when atoms gain or lose electrons
• Cation: Positive ion (loses electrons)
• Anion: Negative ion (gains electrons)
• Ionization Energy: Energy required to remove an electron, decreases down a group, increases across a period

Ionization Energy Trends

• First ionization energy decreases down a group, increases across a period
• Elements with low ionization energy tend to form cations

Ionic Size

• Cations smaller, anions larger than their neutral atoms
• Trend: Ionic size increases down a group, decreases across a period

Electronegativity

• Ability of an atom to attract electrons in a bond
• Trend: Decreases down a group, increases across a period
• Highest in fluorine, lowest in cesium

Summary of Trends

• Trends include: Atomic size, ionization energy, ionic size, and electronegativity
• Explained by variations in atomic structure, nuclear charge, and shielding effect

Review Questions

• How elements are organized and classified
• Trends in atomic size, ionization energy, and electronegativity