Question

Beryllium halides are covalent whereas magnesium halides are ionic.
Why?

Answer 1

halogens are non-metals and beryllium is also a non-metal. Since non-metals always form covalent bonds with each other due to almost similar I.P and E.N therefore,beryllium hallides are covalent .Magnesium is a metal and metals mostly form ionic bonds with non-metals due to vast difference in their I.P and E.N,therefore magnesium hallides are always ionic.
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Answer 2

Beryllium halides are covalent whereas magnesium halides are ionic because of difference in their electron-negativeness  .

Explanation:

Elements : are the substances that cant be broken into more simpler substances .

They are either :

• Metals
• Non metals
• Metalloids

Metals

They have tendency to loose electron and by doing that they form electro positive ions .

They form ionic bonds with non metals .

Non metals

They have tendency to gain electrons and by doing so they form electro negative ions .

They form ionic bonds with metals and covalent bonds when combine with other non metal .

Now , as asked above the reason behind Beryllium halides covalent nature is the same :

Be and Mg both belongs to group 2 but there is a difference in their halides nature because :

1. Beryllium is placed on top in second group that is it has smallest size .
2. Due to this it has highest  electro-negativity compared with the rest of the Group.  
3. Because of its electron-negativity it attracts the bonding pair of electrons towards itself more strongly than magnesium and the rest do.
4. If it has to form ionic bond , it has to loose electrons which it doesn't prefer due to its  electron-negative nature .