Cl
Mendeleev example
Chlorine: one electron short of a full shell
Chlorine often gains one electron in simple ionic models, forming Cl⁻.
Mendeleev
Start with the particle story: which protons, electrons or nuclei are changing? Then connect that story to the property you observe.
1
Identify the particles or structure involved.
2
Predict the direction of the effect.
3
Check the explanation against the periodic table or a measured property.