A dihalide in which the halogens are attached on adjacent carbons is called a dihalide.

Vicinal dihalide

Explanation:

Dihalide in which the halogens are attached on adjacent carbons are called  Vicinal  dihalides.

They are prepared by reaction between halogen and alkyne and alkene. For example,  1,2-dichloroethane which is given by the reaction between ethylene and chlorine.

since the atomic number is the number of protons in the nucleus, as we move to the left on the periodic table, the electrostatic pull between the nucleus and the valence electron shell decreases. as we move down a group (vertical column), the valence shell energy level gets further and further from the nucleus.