periodic trends
Walk along a single row of the periodic table from left to right and properties shift in a steady, predictable way — atoms get smaller, harder to ionise, and hungrier for electrons. Walk down a column instead and the trend often reverses. These smooth, repeating patterns across the table are the periodic trends, and they let you guess an element's behaviour just from its address.
Periodic trends are the regular variations in properties — atomic radius, ionisation energy, electron affinity, electronegativity, metallic character — that recur as you move across periods and down groups of the periodic table. They arise from two competing factors: the growing pull of more protons in the nucleus (effective nuclear charge) versus the cushioning of inner electrons and the increasing distance of outer ones.
Knowing the trends lets you predict and compare without memorising every element. The caveat is that trends are general tendencies, not unbreakable laws: there are well-known dips and bumps (for example ionisation energy wobbles between groups 2 and 13, and between 15 and 16) caused by the fine details of subshell filling and stability.
Across period 2, atomic radius shrinks steadily from lithium to fluorine even though electrons are being added — because each added proton pulls the whole electron cloud in tighter. Yet go down group 1 from lithium to caesium and atoms balloon in size, as each new element adds a whole extra shell.
Properties repeat in patterns across and down the table — an element's address hints at its behaviour.
Most across-a-period trends and down-a-group trends point in opposite directions, so always note which way you are moving before predicting whether a property rises or falls.