Substituents with a positive Hammet substituent constant are electron withdrawing. Examples include CF3.

Substituents with a negative Hammet substituent constant are electron donating. Examples include R groups and OH groups.

The Hammet substituent constant shows the electron donating or accepting nature of the molecule as well as its chemical reactivity. It is often used in the QSAR study of drugs.

Iodo-

Yes. When the aliphatic alkanes have a ring structure containing 2 substituent groups, the cis-isomer occurs when both substituent groups are on the same side of the plane while the trans-isomers occurs when the substituent groups are on different side of the plane.