Overview
This lecture provides an overview of the periodic table, its historical development, and key periodic trends such as atomic radius, ionization energy, electron affinity, and electronegativity.
Historical Development of the Periodic Table
- Dmitri Mendeleev arranged elements in a table based on observed patterns, creating periods (rows) and groups (columns).
- Elements in the same group have similar behaviors, which allowed prediction of undiscovered elements.
- Mendeleev's table was successful because it organized known data and predicted unknown elements' properties.
Structure and Organization of the Periodic Table
- Groups are vertical columns; periods are horizontal rows.
- Elements in the same group have the same number of valence (outer shell) electrons.
- Group 1 elements have one valence electron, group 2 elements have two, and so on.
Periodic Trends
Atomic Radius
- Atomic radius increases down a group (more electron shells).
- Atomic radius decreases across a period (left to right) due to increasing nuclear charge pulling electrons closer.
Ionic Radius
- Adding electrons increases ionic radius (more electron repulsion); removing electrons decreases it.
- For ions with the same electron configuration, radius decreases as atomic number increases.
Ionization Energy
- Ionization energy is the energy required to remove an outermost electron.
- It decreases down a group (outer electrons further from nucleus) and increases across a period (electrons held tighter).
- Second and successive ionization energies are higher than the first, especially after removing all electrons from an outer shell.
- Exceptions, such as lower ionization energy for oxygen compared to nitrogen, are explained by orbital symmetry and stability.
Electron Affinity
- Electron affinity is the energy change when an atom gains an electron.
- Increases across a period (excluding noble gases); highest for fluorine.
- Exceptions in trend are due to similar orbital stability factors as ionization energy.
Electronegativity
- Electronegativity is the ability of an atom to attract electrons in a bond.
- Increases across a period and up a group; highest in fluorine (noble gases are excluded).
Key Terms & Definitions
- Group — vertical column in the periodic table; elements share similar properties.
- Period — horizontal row in the periodic table.
- Valence electrons — electrons in the outermost shell of an atom.
- Atomic radius — size of an atom from nucleus to outer electron shell.
- Ionization energy — energy needed to remove an electron from an atom.
- Electron affinity — energy change when an atom gains an electron.
- Electronegativity — tendency of an atom to attract shared electrons in a bond.
- Orbital symmetry — stability related to electron arrangements in atomic orbitals.
Action Items / Next Steps
- Review periodic trends and their directions across the table.
- Prepare for discussion on chemical bonds in the next lesson.