Question

An element ${ X }$ (atomic number 17 ) reacts with an element ${ Y }$ (atomic number 12 ) to form a divalent halide. Where ${ X }$ and ${ Y }$ are placed in the modern periodic table? (mention the group and period of each)

Answer 1

Hint: The electronic configuration of an element X = ${ 1s }^{ 2 }{ 2s }^{ 2 }{ 2p }^{ 6 }{ 3s }^{ 2 }{ 3p }^{ 5 }$
The electronic configuration of an element Y = ${ 1s }^{ 2 }{ 2s }^{ 2 }{ 2p }^{ 6 }{ 3s }^{ 2 }$

Complete step-by-step answer:

X having atomic number ${ 17 }$ (2, 8, 7) The electronic configuration of an element ${ X } = { 1s }^{ 2 }{ 2s }^{ 2 }{ 2p }^{ 6 }{ 3s }^{ 2 }{ 3p }^{ 5 }$. This element belongs to group 17 and period 3 of the modern periodic table.
So, the compound X is Chlorine.

Y having atomic number ${ 12 }$ (2, 8, 2). The electronic configuration of an element ${ Y } = { 1s }^{ 2 }{ 2s }^{ 2 }{ 2p }^{ 6 }{ 3s }^{ 2 }$.
This element belongs to the group ${ 2 }$ and period ${ 3 }$ of the modern periodic table.
So, the compound ${ Y }$ is Magnesium.

Additional Information:
The period of an element corresponds to the principal quantum number of the valence shell.
The block of an element corresponds to the type of orbital which receives the last electron.
The group of an element is predicted by the number of electrons in the valence shell or penultimate shell which is given below;
For s-block elements, group number = no. of valence electrons.
For p-block elements, group number = ${ 10 }$ + number of electrons in the valence shell.
For d-block elements, group number = number of electrons in a ${ (n-1) }$ d sub shell + number of electrons in valence shell.

Note: The possibility of the mistake is that you may confuse between groups and periods. In the modern periodic table, groups are columns and periods are rows.