Periodic Trends Explorer
Colour the first 36 elements by size, by how tightly they hold their electrons, or by how strongly they pull on shared electrons, and watch the patterns repeat.
Two forces in a tug of war
Across a period, the nuclear charge grows but the new electrons go into the same shell, so the outer electrons feel a stronger pull (a higher effective nuclear charge): atoms shrink and hold electrons more tightly. Down a group, each new shell sits farther out and is shielded by the inner shells, so atoms grow and let go of electrons more easily.
Look for the exceptions
Trends are not perfectly smooth. Ionisation enthalpy drops from Be to B and from N to O. Click those elements and read their electron configurations to find out why: a lone p electron, or a paired p electron, is easier to remove. Chromium and copper break the usual filling order too.
Listen: clicking an element plays a note. Higher pitch = bigger value. Data: CRC Handbook (ionisation enthalpy), Pauling electronegativity, covalent radii of Cordero et al. (2008, low-spin values for Mn, Fe and Co).