Answering periodic trends questions

1. Compare the electron configurations.
When there is an extra electron in the same shell, size difference is due to electron repulsion.
When there is an extra electron shell, size difference is due to electron shielding.

Example:
Br / I / Br-

Br– is larger than Br: Because added electron increases electron – electron repulsion, increasing size of the electron cloud so Br– is larger.

Br is smaller than I because I outer shell electrons are in an extra energy level this electron shell further from the nucleus and electron shielding of outer electrons is greater so I larger.

Br– is larger than I because of the increase in repulsion when e– added to form the ion, this has a greater influence than the energy level difference for the valence e– so Br larger.