Overview
This lecture explains the structure of the periodic table, its historical development, and the major periodic trends including atomic radius, ionization energy, electron affinity, and electronegativity.
Structure and History of the Periodic Table
- Dmitri Mendeleev organized elements into rows (periods) and columns (groups) based on similar properties.
- Mendeleev’s table predicted properties and existence of elements not yet discovered at his time.
- Elements in the same group have similar behavior due to having the same number of valence (outer shell) electrons.
- Group 1 elements have one valence electron; Group 2 elements have two, and so on.
- The periodic table organizes metals, metalloids, and nonmetals according to shared characteristics.
Periodic Trends
Atomic Radius
- Atomic radius increases down a group (more shells added).
- Atomic radius decreases across a period (left to right) due to increased nuclear charge pulling electrons closer.
- Ionic radius increases when electrons are added and decreases when electrons are removed.
Ionization Energy
- Ionization energy is the energy needed to remove an electron from the outer shell.
- Ionization energy decreases down a group and increases across a period (opposite to atomic radius trend).
- Successive ionization energies increase for each additional electron removed, with a large jump once a shell is emptied.
- Exceptions to the trend are explained by orbital symmetry (e.g., nitrogen and oxygen).
Electron Affinity
- Electron affinity measures how much an atom wants to gain an electron.
- Electron affinity increases across a period, with fluorine having the highest value (excluding noble gases).
- Exceptions follow the same pattern as ionization energy due to orbital symmetry.
Electronegativity
- Electronegativity is the atom's ability to hold electrons tightly.
- Increases across a period, highest in small atoms with high nuclear charge (like fluorine).
- Noble gases are generally not included in the electronegativity trend.
Key Terms & Definitions
- Valence Electrons — electrons in the outermost shell of an atom.
- Period — a horizontal row in the periodic table.
- Group — a vertical column in the periodic table.
- Atomic Radius — the size of an atom from the nucleus to the outermost electron.
- Ionization Energy — energy needed to remove an electron from an atom.
- Electron Affinity — the tendency of an atom to gain an electron.
- Electronegativity — the ability of an atom to attract and hold onto electrons.
Action Items / Next Steps
- Review trends of atomic radius, ionization energy, electron affinity, and electronegativity in the periodic table.
- Prepare for next lesson on chemical bonds.