Z=9 vs Z=17
Place two elements side by side. Start from atomic number and position, then read the differences in the context of periodic trends rather than as isolated facts.
Z=9
Z=9
Deschide elementul →Z=17
Z=17
Deschide elementul →Mendeleev
Do not compare numbers without asking why they differ. Are the atoms in the same group? The same period? Has a new electron shell appeared? That is where the periodic table becomes an explanation rather than a list.