Overview
Periodic trends are patterns, not rigid rules. Effective nuclear charge, shielding and electron configuration help explain why properties often change systematically across periods and down groups.
The table is a landscape, not a spreadsheet
The periodic table becomes much more interesting once you stop reading it as 118 separate boxes. Move one square to the right and the nucleus has gained a proton; move down a row and an entire electron shell has entered the story. Those small structural changes accumulate into visible patterns in size, bonding and reactivity.
The pleasure—and the trap—is that nature does not draw perfectly smooth curves. Chromium, copper and many heavier atoms remind us that electron energies are close enough for subtle rearrangements to matter. A trend is therefore a map of what usually happens, with the exceptions showing where the physics gets especially interesting.
What changes across the table
Periodic trends are recurring changes in measurable properties as atomic number increases. Across a period, effective nuclear attraction generally rises while electrons are added to the same principal shell; this tends to reduce atomic radius and increase ionization energy. Down a group, additional electron shells usually increase atomic size and modify shielding.
These are trends, not rigid laws. Electron subshell structure, pairing, relativistic effects and the stability of particular configurations create important exceptions. The most useful way to read a trend is therefore to combine the broad direction with the electronic structure of the element being examined.