Question

An element X (atomic number 17) reacts with an element Y (atomic number 20) to form a divalent halide. Where in the periodic table is an element, X placed?
A. Group 17 and 3rd period
B. Group 2 and 4th period
C. Group 17 and 4th period
D. Group 2 and 5th period

Answer 1

Period of the periodic table means the rows which are horizontal of the plane of your page or board. the group of the periodic table means the columns which are vertical to the rows. The elements of a definite row contain the same number of orbits. The elements of the same column have the same chemical behaviors.

Complete step by step answer:
To answer this question the periodic properties of the elements from the different groups should be known. The periodic properties have opposite trends between left to right and top to bottom.
In the periodic table, all the elements are arranged by the atomic numbers. Generally, on the left side of the periodic table, elements are metallic in nature. Moving from left to right the metallic character decreases and the non-metallic character increases. At the extreme left elements are non-metallic in nature.
As we move from left to right in the periodic table, the atomic size of the atom goes on decreasing. This is because more and more electrons are added to the same valence shell. This causes a greater nuclear charge attracting more and more electrons. This increases the net nuclear force of attraction as we move from left to right in the period.
Another factor that must be taken into consideration is that once the valence orbital is half-filled, beyond that the number of half-filled orbitals goes on decreasing. This factor also contributes to the atoms wanting to accept more electrons to satisfy their octet structure.
Now according to the questions, the atomic number of X is 17. That is the number of electrons is greater than neon which is an inert gas of group 18 and 2nd period. Therefore, X should be in the 3rd period. Now the nearest inert gas of X is argon with the atomic number 18
Now, the electronic configuration of x is,
$\left[ {Ne} \right]3{s^2}3{p^5}$ . From this electronic configuration it is clear that the highest principal quantum number of X is 3. So, it is the 3rd-period element and the total number electron in the valence shell is 7. That is, it is an element of group 17.

Note: To find out the position of an element in the periodic table, at first, write down the electronic configuration. Then the principal quantum number of the valence shell will be the period of that element and when the number of the electron in the valence shell is less than 3 then the number of the electron in the valence shell would be its group. But if it is 3 or more than 3 then the group would be 10+ number of valence shell electron.