Trends

The trend for elements going across a period in the periodic table is from metallic to non-metallic. 

The higher electronegativity of elements at the top of a group means that there is a similar trend for elements of the same group, particularly as one gets farther to the right in the periodic table.

Even in Group 2 there is some evidence for this as is shown by considering the structures of calcium chloride, magnesium chloride and beryllium chloride.

Calcium has the lowest electronegativity of these three metals, and, like the Group 1 chlorides, calcium chloride is an ionic compound that exists as an infinite lattice of calcium ions and chloride ions (green). A portion of this lattice is shown.

Magnesium is above calcium and has a higher electronegativity. Magnesium chloride exists as an infinite lattice, but in this case the structure is described as a layer structure with the magnesium ions surrounded by six chloride ions in the middle of the layers. This can be represented by the cartoon below from wikipedia.com. 
 
Beryllium has the highest electronegativity of the metals in Group 2, and the chlorides of beryllium are more similar to those of nonmetals with significant covalent character in the Be-Cl bond. Beryllium chloride exists as a polymer, a portion of which is shown below. 

 

The boiling point of BeCl₂ is considerably lower (482) than either MgCl₂ (1412 °C) or CaCl₂ (1935 °C).