periodic trends explained
The periodic table is a map, not a list to memorize. Where an element sits tells you its properties. This lesson follows three currents, atomic radius and ionization energy and electronegativity. The three gather, in the end, into one cause, the net pull an outer electron actually feels. Go right along a period and the shell stays the same while protons increase and the pull grows. Go down a group and a new shell pushes the electrons outward. The first picture colors the table by property so you can see the current. The seat speaks the property.
Pick one property on the table. Pick radius and the lower left is bright and the upper right is dark. The radius grows as you go left and as you go down. Pick ionization energy or electronegativity and the colors nearly flip, so the upper right brightens. Press an element and the value comes out. The point is that the same table looks like a completely different map depending on the property. Color shows the current.
Why does that map appear? First walk the radius on its own. Step one cell to the right along a period and the number of shells stays the same while one proton is added. So the pull the outer electron feels grows, the electron cloud is drawn inward, and the radius shrinks. Go down a group and a new shell appears, the outer electron sits in a farther shell, and the radius grows. Walk with the shell count and the Zeff mark together and the current comes into view. Shell and pull move the radius.
Ionization energy is the cost of removing electrons one by one. Keep removing electrons from sodium, Na. The first electron is loosely attached in the outer shell and comes off cheaply. From the next one on you have to break an inner shell, and the value jumps. That large jump is the boundary of a shell. The jumps appear after removing 1 and after removing 9, and they lay bare the configuration 2, 8, 1. The jump points to the shell.
Electronegativity is the force with which two atoms pull a bonding pair toward themselves. Move the difference between the two atoms. When the difference is almost nothing, the electron cloud stays in the middle, a nonpolar bond. When the difference grows, the cloud is pulled toward the stronger side, one side takes a partial negative charge and the other a partial positive charge, and that is a polar bond. When the difference is very large the electron essentially crosses over and the bond is ionic. The same principle appears, only the strength differs. The difference splits the kind of bond.
Now gather every current into one. The pull an outer electron actually feels is fixed by Zeff = Z − S. It is the number of protons with the amount the inner electrons screen taken off. Add protons and the pull grows, so the radius shrinks and the ionization energy and the electronegativity grow. Add screening electrons and the pull shrinks and everything moves the other way. Crossing a period only adds protons, and going down a group adds a shell and screening together, so the three currents are explained by this one number. Move the two sliders and the three bars answer together. One number drives the three currents.