Question

an element X belongs to group 17 and 3rd period of periodic table write electronic configuration of the element what is its valency it its its nature whether it is a metal or nonmetal give the formula of the compound formed when it combines with an element by having a valency 3

Answer 1

the element X is chlorine 
atomic number of chlorine is 17
electronic configuration is 2,8,7
valency  = 1
non metal as it will gain an electron to full fill it's eight electron inert gas structure

Answer 2

Since, the element is in group 17

So, it is a halogen.

Halogens start from the second period.

Counting the periods and matching it with the periods  of that of halogens, we find that period 3 has Chlorine.

So, element is Chlorine.

Chlorine has an atomic number equal to 17.

So, no. of electrons=17

Hence, Electronic configuration = 2  8  7   [Shell/orbit-wise]

                                                        K  L  M

                                                1S²   2S² 2P⁶  3S² 3P⁵  [Sub-shell-wise]

Now, Valency of this element = 8 - (No. of valence electrons)

                                                 = 8 - 7  = 1

Actually, it has an electrovalency of -1, since it has the tendency to gain one electron.

Clearly, it is a non-metal.

An element with valency 3 could be Gallium (Ga).

So, the formula of the compound is GaCl₃.

(You could have also taken Aluminium as suggested in comments of previous answer).

Please mark my answer as brainliest.

Thanks.