Question

The element present in the III-A group and 3rd period in the periodic table is:
A) Carbon
B) Phosphorous
C) Cobalt
D) Aluminium

Answer 1

The periodic table is a regular arrangement of elements. The period is a horizontal arrangement of elements .In the third period, the electron enters in the third shell. In group III-A, all elements have a common electronic configuration as$\text{ n}{{\text{s}}^{\text{2}}}\text{n}{{\text{p}}^{\text{1}}}\text{ }$.
Complete step by step answer:
The periodic table is a systematic arrangement of the elements according to their atomic number.
The periodic table is composed of:

1. Vertical column OR Groups
2. Horizontal rows OR Period
   The groups are arranged in increasing atomic numbers, such that each atom has the same number of valence electrons. The group III-A is called the boron family. The general last shell electronic configuration for the group III-A elements is given as:
   $\text{ n}{{\text{s}}^{\text{2}}}\text{n}{{\text{p}}^{\text{1}}}\text{ }$.The valence shell accommodated three electrons in it.
   The group third has the following elements in it.

Element	Atomic number	Valence shell configuration
$\text{ B }$	5	$\text{ 2}{{\text{s}}^{\text{2}}}\text{2}{{\text{p}}^{\text{1}}}\text{ }$
$\text{ Al }$	13	$\text{ 3}{{\text{s}}^{\text{2}}}\text{3}{{\text{p}}^{\text{1}}}\text{ }$
$\text{ Ga }$	31	$\text{ 4}{{\text{s}}^{\text{2}}}\text{3}{{\text{d}}^{\text{10}}}\text{4}{{\text{p}}^{\text{1}}}\text{ }$
$\text{ In }$	49	$\text{ 5}{{\text{s}}^{\text{2}}}\text{4}{{\text{d}}^{\text{10}}}\text{5}{{\text{p}}^{\text{1}}}\text{ }$
$\text{ Tl }$	81	$\text{ 6}{{\text{s}}^{\text{2}}}\text{4}{{\text{f}}^{\text{14}}}\text{5}{{\text{d}}^{\text{10}}}\text{6}{{\text{p}}^{\text{1}}}\text{ }$

The general last shell configuration of group 3 is considered as,$\text{ n}{{\text{s}}^{\text{2}}}\text{n}{{\text{p}}^{\text{1}}}\text{ }$.
The period is the horizontal arrangement of elements in the periodic table. The elements are arranged based on the principal quantum number. The principal quantum number ‘n’ defines the highest energy electron in an element.
The third period is as follows:

Element	$\text{Na}$	$\text{Mg}$	$\text{Al}$	$\text{Si}$	$\text{P}$	$\text{S}$	$\text{Cl}$	$\text{Ar}$
Atomic number	11	12	13	14	15	16	17	18
Valence shell conf.	$\text{3}{{\text{s}}^{\text{1}}}\text{3}{{\text{p}}^{\text{0}}}$	$\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{\text{0}}}$	$\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{1}}$	$\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{2}}$	$\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{3}}$	$\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{4}}$	$\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{5}}$	$\text{3}{{\text{s}}^{2}}\text{3}{{\text{p}}^{6}}$

For the third period, the principal quantum number is$\text{ n = 3}$. The third shell is further subdivided into s, p, and d subshells.in periods, the atomic number increases from left to right. For the third period, the last electron is accommodated into the third shell. That is, it is filling up the $\text{3p}$ orbital.
We are interested to find out the elements which are in group III-A and third period.
We have the group IIIA element as listed above. Now let's have a look at the third-period elements. We want to find elements in the third period which have a general last shell electronic configuration as$\text{ n}{{\text{s}}^{\text{2}}}\text{n}{{\text{p}}^{\text{1}}}\text{ }$.
The aluminium has the electronic configuration is:
$\text{ Al = }\left[ \text{Ar} \right]\text{ 3}{{\text{s}}^{\text{2}}}\text{ 3}{{\text{p}}^{\text{1}}}\text{ }$
Aluminium is a third-period element (as the$\text{ n = 3}$) and it belongs to group III.
Hence, (D) is the correct option.

Note: The groups are used to club the elements of the same properties together. The elements in a group have similar properties like group 1 elements are good conductors of electricity. The periods do not exhibit similar properties. Each adjacent element is different in the properties. For example, in the third-period elements the sodium is metal but the chlorine is non-metal.