Periodic Table Organization and Periodic Trends

Dmitry Mendeleev organized the periodic table into rows called periods and columns called groups based on elemental behavior and predictive capabilities.
Elements in the same group exhibit similar chemical properties because they share the same number of valence electrons in their outermost shell.
Moving down a group increases the principal quantum number $n$ , adding an electron shell at each step.

Atomic radius increases moving down a group due to the addition of electron shells.
Atomic radius decreases moving right across a period because added protons increase nuclear charge, exerting a stronger electromagnetic attraction on electrons within the same shell.
Ionic radius expands when gaining electrons due to electron-electron repulsion and shrinks when losing electrons.
For species with identical electron configurations, ionic radius decreases as atomic number increases.

Ionization energy is the energy required to remove an electron from an atom's outermost shell.
Ionization energy increases moving up and to the right on the periodic table, following a trend directly opposite to atomic radius.
Fransium has a low ionization energy due to its large size and single distant valence electron, while helium has a very high ionization energy due to its small radius and full valence shell.
Successive ionization energies increase progressively, with a dramatic jump occurring when attempting to remove an electron from a stable, full noble gas electron configuration.
Deviations in the ionization energy trend occur due to orbital symmetry; for example, nitrogen has a higher ionization energy than oxygen because nitrogen possesses a stable, half-filled $2p$ subshell.

Electron affinity quantifies how readily an atom gains an electron, increasing moving up and to the right (excluding noble gases).
Fluorine possesses the highest electron affinity because gaining one electron achieves a complete noble gas configuration.
Electronegativity measures an atom's ability to hold electrons tightly, increasing up and to the right (excluding noble gases) due to smaller atomic size and higher effective nuclear charge.