Question

Which of the following elements shown as pairs with their atomic numbers belong to the same period?
A. Z=19 and Z = 38
B. Z=12 and Z= 17
C. Z=11 and Z = 21
D. Z=16 and Z = 35

Answer 1

The periodic table contains periods and groups. Elements in the period are arranged based on the principal quantum number. The electrons are added into the same shell until it fills. The elements belonging to the same period can be identified by electronic configuration; the valence electrons are always present in the same shell ‘n’.

Complete step by step answer:
The periodic table is an orderly and systematic arrangement of elements. Here, the elements are arranged based on increasing the atomic number or the number of protons in the element.
The table consists of two main elements.
1.Column or Group
2.Period

The horizontal arrangement of elements is called the period. The period tells about the energy level of the last shell electron.
Each period contains a principal energy level. The valence shell electrons enter in this shell, such that the number of electrons in the highest energy level goes from increasing from the no electron to the filled orbital.

Lets first write down the electronic configuration of for the atomic number 12,
$\text{A}=\text{ 1}{{\text{s}}^{\text{2}}}\text{ , 2}{{\text{s}}^{\text{2}}}\text{ , 2}{{\text{p}}^{\text{6}}}\text{ , 3s}{{\text{ }}^{\text{2}}},\text{ 3}{{\text{p}}^{\text{0}}}$
For the B (atomic number = 17) is:
$\text{B}=\text{ 1}{{\text{s}}^{\text{2}}}\text{ , 2}{{\text{s}}^{\text{2}}}\text{ , 2}{{\text{p}}^{\text{6}}}\text{ , 3s}{{\text{ }}^{\text{2}}},\text{ 3}{{\text{p}}^{5}}$

Here, the highest energy electron of A and B enters the principal energy level $\text{n=3}$. The orbital is subdivided into s, p, and d subshells. The s subshell accommodates 2 electrons, p on 6 and d on 10.
Here, the highest electron of A is in $\text{3s}$ orbital and for B it is in the $\text{3p}$ orbital. Both subshells have the principal quantum number as the 3.Thus they belong to the same shell.
We know that the electrons are added in the same shell along the horizontal direction i.e.from the left to right in the periodic table.
Therefore, the A and B lie in the same period.

Hence, the atomic number 12 and 17 elements belong to the same period. In the periodic table atomic number 12 corresponds to the Magnesium and the 17 to the Chlorine. Magnesium is group II or alkaline earth metal element and Chlorine is a halogen or group 17 element, Both are the elements of the third period. The third period starts from the sodium and ends on argon.
$Z=11$ , where the last shell configuration is $\text{3}{{\text{s}}^{\text{1}}}\text{3}{{\text{p}}^{\text{0}}}$ and ends at the Argon where the third shell is filled $\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{6}}$ .
the atomic number 12 and 17 elements belong to the same period, third period.
So, the correct answer is “Option B”.

Additional Information:
A period in the periodic table is a row of chemical elements all elements in a row have the same number of electron shells. Each next element in a period has one more proton and is less metallic than its predecessor.
Each column in a Periodic Table is called a group. The elements in each group have the same number of electrons in the outer orbital, those outer electrons are also called valence electrons. These are the electrons involved in chemical bonds with other elements.

Note: The elements with the same number of valence electrons are kept together in group and elements with the increasing valence number are in period. Therefore, The elements under the same group exhibit similar properties like all group I elements are metal. But the same is not applicable for the periods.
The elements in the periods are:

Period	Total number of elements
1	2
2	8
3	8
4	18
5	18
6	32
7	32