An interactive GCSE Chemistry simulation exploring why reactivity increases down Group 1 but decreases down Group 7. Students can compare alkali metals and halogens side by side, examining how increasing atomic radius and electron shielding affect the attraction between the nucleus and outer or incoming electrons. Animated electron loss and gain, adjustable visual overlays and interactive comparisons help students connect atomic structure to reactivity. Includes prediction questions and GCSE explanation-building activities to reinforce understanding. simopsis